Common String Extension Methods in Kotlin

Kotlin is a modern programming language that offers many features to make coding easier and more enjoyable. One of these features is the ability to extend a class or an interface with new functionality without having to inherit from it or use design patterns such as Decorator. This is done via special declarations called extension functions.

Extension functions are useful when you want to add some functionality to a class that you can’t modify, such as a class from a third-party library. For example, you can write new functions for the String class that can be used as if they were methods of the String class.

In this blog post, we will explore some common extension functions that are available in Kotlin for the String class. We will also see how to write our own extension functions and how they are resolved at compile-time.

Built-in Extension Functions for String Class

The String class in Kotlin comes with a huge number of extension functions that can make working with strings easier and more concise. Here are some examples of these functions:

replace(oldChar: Char, newChar: Char, ignoreCase: Boolean = false): String – This function returns a new string with all the occurrences of the oldChar replaced with newChar. The ignoreCase parameter is false by default. If set to true, it will ignore case while handling the replacement. For example:

val inputString = "Jelly" println(inputString.replace('l', 'z')) // Jezzy println(inputString.replace('j', 'P', true)) // Pelly println(inputString.replace('j', 'P')) // Jelly

uppercase(): String – This function returns a new string with all the characters converted to uppercase. For example:

val inputString = "hello" println(inputString.uppercase()) // HELLO

lowercase(): String – This function returns a new string with all the characters converted to lowercase. For example:

val inputString = "HELLO" println(inputString.lowercase()) // hello

toCharArray(): CharArray – This function returns a char array containing the characters of the string. For example:

val inputString = "hello" val charArray = inputString.toCharArray() println(charArray.joinToString()) // h, e, l, l, o

substring(startIndex: Int, endIndex: Int): String – This function returns a new string that is a substring of the original string starting from startIndex (inclusive) and ending at endIndex (exclusive). For example:

val inputString = "hello" println(inputString.substring(1, 4)) // ell

startsWith(prefix: String, ignoreCase: Boolean = false): Boolean – This function returns true if the string starts with the specified prefix. The ignoreCase parameter is false by default. If set to true, it will ignore case while checking for the prefix. For example:

val inputString = "hello" println(inputString.startsWith("he")) // true println(inputString.startsWith("He", true)) // true println(inputString.startsWith("He")) // false

endsWith(suffix: String, ignoreCase: Boolean = false): Boolean – This function returns true if the string ends with the specified suffix. The ignoreCase parameter is false by default. If set to true, it will ignore case while checking for the suffix. For example:

val inputString = "hello" println(inputString.endsWith("lo")) // true println(inputString.endsWith("Lo", true)) // true println(inputString.endsWith("Lo")) // false

compareTo(other: String, ignoreCase: Boolean = false): Int – This function compares the string with another string lexicographically and returns an integer value. The value is negative if the string is less than the other string, zero if they are equal, and positive if the string is greater than the other string. The ignoreCase parameter is false by default. If set to true, it will ignore case while comparing the strings. For example:

val inputString = "hello" println(inputString.compareTo("world")) // -15 println(inputString.compareTo("Hello", true)) // 0 println(inputString.compareTo("Hello")) // 32

There are many more extension functions for the String class in Kotlin that you can explore in the official documentation.

Writing Your Own Extension Functions for String Class

You can also write your own extension functions for the String class or any other class in Kotlin. To declare an extension function, you need to prefix its name with a receiver type, which refers to the type being extended. For example, the following adds a reverse() function to the String class:

fun String.reverse(): String { return this.reversed() }

The this keyword inside an extension function corresponds to the receiver object (the one that is passed before the dot). Now, you can call such a function on any String object in Kotlin:

val inputString = "hello" println(inputString.reverse()) // olleh

You can also make your extension functions generic by declaring the type parameter before the function name. For example, the following adds a swap() function to the MutableList class that can swap two elements of any type:

fun <T> MutableList<T>.swap(index1: Int, index2: Int) { val tmp = this[index1] this[index1] = this[index2] this[index2] = tmp } val list = mutableListOf(1, 2, 3) list.swap(0, 2) println(list) // [3, 2, 1]

How Extension Functions Are Resolved

It is important to understand that extension functions do not actually modify the classes they extend. They are just syntactic sugar that allows you to call new functions with the dot-notation on variables of that type. Extension functions are resolved statically at compile-time, which means they are not virtual by receiver type. The extension function being called is determined by the type of the expression on which the function is invoked, not by the type of the result from evaluating that expression at runtime.

For example, consider the following code:

open class Shape class Rectangle: Shape() fun Shape.getName() = "Shape" fun Rectangle.getName() = "Rectangle" fun printClassName(s: Shape) { println(s.getName()) } printClassName(Rectangle())

This code prints Shape, because the extension function called depends only on the declared type of the parameter s, which is the Shape class. The actual type of s at runtime (Rectangle) does not matter.

If a class has a member function with the same name and signature as an extension function, the member always wins. For example:

class Example { fun printFunctionType() { println("Class method") } } fun Example.printFunctionType() { println("Extension function") } Example().printFunctionType() // Class method

This code prints Class method, because the member function of the Example class overrides the extension function.

Conclusion

In this blog post, we have learned about some common extension functions for the String class in Kotlin and how to write our own extension functions for any class. We have also seen how extension functions are resolved at compile-time and how they do not modify the classes they extend.

Extension functions are a powerful feature of Kotlin that can help you write more concise and expressive code. They can also help you extend existing classes with new functionality without having to inherit from them or use design patterns such as Decorator.

If you want to learn more about extension functions and other features of Kotlin, you can check out these resources:

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Variance in Kotlin: A Beginner’s Guide

Variance is a concept that describes how types relate to each other when they have type parameters. For example, if Dog is a subtype of Animal, is List<Dog> a subtype of List<Animal>? The answer depends on the variance of the type parameter of List.

In this blog, we will explore the different kinds of variance in Kotlin and how they affect the type system and the code we write. We will also compare them with Java’s wildcard types and see how Kotlin simplifies the syntax and semantics of generics.

Declaration-site variance

One way to achieve variance in Kotlin is by using declaration-site variance. This means that we can specify the variance of a type parameter at the class level, where it is declared. This affects all the members and fields of the class that use that type parameter.

For example, let’s define a simple class that represents a producer of some type T:

class Producer<T>(val value: T) { fun produce(): T = value }

This class has a type parameter T that appears as the return type of the produce() method. Now, let’s say we have two subtypes of Animal: Dog and Cat. We can create instances of Producer<Dog> and Producer<Cat>:

val dogProducer = Producer(Dog()) val catProducer = Producer(Cat())

But can we assign a Producer<Dog> to a variable of type Producer<Animal>? Intuitively, this should be possible, because a producer of dogs is also a producer of animals. We can always get an animal from it by calling produce(). However, if we try to do this in Kotlin, we get a compiler error:

val animalProducer: Producer<Animal> = dogProducer // Error: Type mismatch

This is because by default, generic types in Kotlin are invariant, meaning that they are not subtypes of each other, even if their type arguments are. This is similar to how Java behaves without wildcards.

To fix this error, we need to make the type parameter T covariant, meaning that it preserves the subtype relationship. We can do this by adding the out modifier to the type parameter declaration:

class Producer<out T>(val value: T) { fun produce(): T = value }

The out modifier tells the compiler that T is only used as an output, not as an input. This means that we can only return values of type T from the class, but we cannot accept them as parameters. This ensures that we don’t violate the type safety by putting a wrong value into the class.

With this modifier, we can now assign a Producer<Dog> to a Producer<Animal>, because Producer<Dog> is a subtype of Producer<Animal>:

val animalProducer: Producer<Animal> = dogProducer // OK

This is called covariance, because the subtype relationship varies in the same direction as the type argument. If Dog is a subtype of Animal, then Producer<Dog> is a subtype of Producer<Animal>.

Covariance is useful when we want to read values from a generic class, but not write to it. For example, Kotlin’s standard library defines the interface List<out T> as covariant, because we can only get elements from a list, but not add or remove them. This allows us to assign a List<Dog> to a List<Animal>, which is convenient for polymorphism.

Use-site variance

Another way to achieve variance in Kotlin is by using use-site variance. This means that we can specify the variance of a type parameter at the point where we use it, such as in a function parameter or a variable declaration. This allows us to override the default variance of the class or interface where the type parameter is declared.

For example, let’s define another simple class that represents a consumer of some type T:

class Consumer<T>(var value: T) { fun consume(value: T) { this.value = value } }

This class has a type parameter T that appears as the parameter type of the consume() method. Now, let’s say we have two subtypes of Animal: Dog and Cat. We can create instances of Consumer<Dog> and Consumer<Cat>:

val dogConsumer = Consumer(Dog()) val catConsumer = Consumer(Cat())

But can we assign a Consumer<Animal> to a variable of type Consumer<Dog>? Intuitively, this should be possible, because a consumer of animals can also consume dogs. We can always pass a dog to it by calling consume(). However, if we try to do this in Kotlin, we get a compiler error:

val dogConsumer: Consumer<Dog> = animalConsumer // Error: Type mismatch

To fix this error, we need to make the type parameter T contravariant, meaning that it reverses the subtype relationship. We can do this by adding the in modifier to the type parameter usage:

val dogConsumer: Consumer<in Dog> = animalConsumer // OK

The in modifier tells the compiler that T is only used as an input, not as an output. This means that we can only accept values of type T as parameters, but we cannot return them from the class. This ensures that we don’t violate the type safety by getting a wrong value from the class.

With this modifier, we can now assign a Consumer<Animal> to a Consumer<Dog>, because Consumer<Animal> is a subtype of Consumer<Dog>:

val dogConsumer: Consumer<in Dog> = animalConsumer // OK

This is called contravariance, because the subtype relationship varies in the opposite direction as the type argument. If Dog is a subtype of Animal, then Consumer<Animal> is a subtype of Consumer<Dog>.

Contravariance is useful when we want to write values to a generic class, but not read from it. For example, Kotlin’s standard library defines the interface MutableList<T> as invariant, because we can both get and set elements in a mutable list. However, if we only want to add elements to a list, we can use the function addAll(elements: Collection<T>), which accepts a collection of any subtype of T. This function uses use-site variance to make the parameter type covariant:

fun <T> MutableList<T>.addAll(elements: Collection<out T>)

This allows us to add a List<Dog> to a MutableList<Animal>, which is convenient for polymorphism.

Comparison with Java

If you are familiar with Java’s generics, you might notice some similarities and differences between Kotlin and Java’s variance mechanisms. Java uses wildcard types (? extends T and ? super T) to achieve covariance and contravariance, respectively. Kotlin uses declaration-site variance (out T and in T) and use-site variance (T and in T) instead.

The main advantage of Kotlin’s approach is that it simplifies the syntax and semantics of generics. Wildcard types can be confusing and verbose, especially when they are nested or combined with other types. Declaration-site variance allows us to specify the variance once at the class level, instead of repeating it at every usage site. Use-site variance allows us to override the default variance when needed, without introducing new types.

Another advantage of Kotlin’s approach is that it avoids some of the limitations and pitfalls of wildcard types. For example, wildcard types cannot be used as return types or in generic type arguments. Declaration-site variance does not have this restriction, as long as the type parameter is used consistently as an output or an input. Use-site variance also allows us to use both covariant and contravariant types in the same context, such as in function parameters or variables.

Conclusion

In this blog, we learned about variance in Kotlin and how it affects the type system and the code we write. We saw how declaration-site variance and use-site variance can help us achieve covariance and contravariance for generic types. We also compared them with Java’s wildcard types and saw how Kotlin simplifies the syntax and semantics of generics.

Variance is an important concept for writing generic and polymorphic code in Kotlin. It allows us to express more precise and flexible types that can adapt to different situations. By understanding how variance works in Kotlin, we can write more idiomatic and effective code with generics.

I hope you enjoyed this blog and learned something new. If you have any questions or feedback, please let me know in the comments below. Thank you for reading! 😊

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How to Solve the N Queens Problem Using Kotlin

The N Queens problem is a classic puzzle that asks how to place N chess queens on an NxN chessboard so that no two queens can attack each other. This means that no two queens can share the same row, column, or diagonal.

One way to solve this problem is to use a backtracking algorithm, which tries different positions for the queens until it finds a valid solution or exhausts all possibilities. In this blog post, we will see how to implement a backtracking algorithm for the N Queens problem using Kotlin, a modern and concise programming language that runs on the JVM.

Kotlin Basics

Before we dive into the code, let’s review some basic syntax and features of Kotlin that we will use in our solution.

Functions: Kotlin functions are declared using the fun keyword, followed by the function name, parameters, and return type. For example:

fun sum(a: Int, b: Int): Int { return a + b }

Parameters: Function parameters are defined using Pascal notation – name: type. Parameters are separated using commas, and each parameter must be explicitly typed. For example:

fun powerOf(number: Int, exponent: Int): Int { /*...*/ }

Default arguments: Function parameters can have default values, which are used when you skip the corresponding argument. This reduces the number of overloads. For example:

fun read(b: ByteArray, off: Int = 0, len: Int = b.size) { /*...*/ }

Named arguments: You can name one or more of a function’s arguments when calling it. This can be helpful when a function has many arguments and it’s difficult to associate a value with an argument, especially if it’s a boolean or null value. When you use named arguments in a function call, you can freely change the order that they are listed in. For example:

fun foo(bar: Int = 0, baz: Int = 1, qux: () -> Unit) { /*...*/ } foo(1) { println("hello") } // Uses the default value baz = 1 foo(qux = { println("hello") }) // Uses both default values bar = 0 and baz = 1 foo { println("hello") } // Uses both default values bar = 0 and baz = 1

Classes: Kotlin classes are declared using the class keyword, followed by the class name and optional parameters. For example:

class Person(val firstName: String, val lastName: String, var age: Int)

Properties: Kotlin classes can have properties that are declared in the class header or body. Properties can be either val (read-only) or var (mutable). For example:

class Rectangle(var height: Double, var length: Double) { var perimeter = (height + length) * 2 }

Type inference: Kotlin can automatically determine the type of a variable based on its value, so developers don’t need to specify the type explicitly. For example:

var x = 5 // `Int` type is inferred x += 1 val y = "Hello" // `String` type is inferred y += " world!"

For more details on Kotlin syntax and features, you can check out the official documentation.

Backtracking Algorithm

Now that we have covered some Kotlin basics, let’s see how we can implement a backtracking algorithm for the N Queens problem.

The idea is to place queens one by one in different columns, starting from the leftmost column. When we place a queen in a column, we check for clashes with already placed queens. In the current column, if we find a row for which there is no clash, we mark this row and column as part of the solution. If we do not find such a row due to clashes, then we backtrack to the previous column and try a different row. We repeat this process until either all N queens have been placed or it is impossible to place any more queens.

To implement this algorithm in Kotlin, we will need:

A function to check if a given position is safe for placing a queen.
A function to print the solution as a matrix of ‘Q’ and ‘.’ characters.
A recursive function to try placing queens in different columns and rows.

Let’s start with the first function:

// A function to check if a given position (row, col) is safe for placing a queen fun isSafe(board: Array<IntArray>, row: Int, col: Int, n: Int): Boolean { // Check the left side of the current row for (i in 0 until col) { if (board[row][i] == 1) { return false } } // Check the upper left diagonal var i = row - 1 var j = col - 1 while (i >= 0 && j >= 0) { if (board[i][j] == 1) { return false } i-- j-- } // Check the lower left diagonal i = row + 1 j = col - 1 while (i < n && j >= 0) { if (board[i][j] == 1) { return false } i++ j-- } // If none of the above conditions are violated, the position is safe return true }

This function takes four parameters:

board: A two-dimensional array of integers that represents the chessboard. Each element can be either 0 (empty) or 1 (queen).
row: The row index of the current position.
col: The column index of the current position.
n: The size of the chessboard and the number of queens.

The function returns a boolean value indicating whether the position is safe or not. To check this, we need to scan the left side of the current row, the upper left diagonal, and the lower left diagonal for any queens. If we find any queen in these directions, we return false. Otherwise, we return true.

Next, let’s write the function to print the solution:

// A function to print the solution as a matrix of 'Q' and '.' characters fun printSolution(board: Array<IntArray>, n: Int) { for (i in 0 until n) { for (j in 0 until n) { if (board[i][j] == 1) { print("Q ") } else { print(". ") } } println() } }

This function takes two parameters:

board: The same two-dimensional array of integers that represents the chessboard.
n: The size of the chessboard and the number of queens.

The function prints each element of the board as either ‘Q’ or ‘.’ depending on whether it is a queen or not. It also adds a space after each character and a line break after each row.

Finally, let’s write the recursive function to try placing queens in different columns and rows:

// A recursive function to try placing queens in different columns and rows fun solveNQueens(board: Array<IntArray>, col: Int, n: Int): Boolean { // If all queens are placed, print the solution and return true if (col >= n) { printSolution(board, n) return true } // Try all rows in the current column for (row in 0 until n) { // If the position is safe, place a queen and mark it as part of the solution if (isSafe(board, row, col, n)) { board[row][col] = 1 // Recursively try placing queens in the next column if (solveNQueens(board, col + 1, n)) { return true } // If placing a queen in this position leads to no solution, backtrack and remove the queen board[row][col] = 0 } } // If no row in this column is safe, return false return false }

This function takes three parameters:

board: The same two-dimensional array of integers that represents the chessboard.
col: The current column index where we are trying to place a queen.
n: The size of the chessboard and the number of queens.

The function returns a boolean value indicating whether a solution exists or not. To find a solution, we follow these steps:

If all queens are placed (i.e., col >= n), we print the solution and return true.
Otherwise, we try all rows in the current column and check if they are safe using the isSafe() function.
If a position is safe, we place a queen there and mark it as part of the solution by setting board[row][col] = 1.
Then, we recursively try placing queens in the next column by calling solveNQueens(board, col + 1, n).
If this leads to a solution, we return true.

Otherwise, we backtrack and remove the queen from the current position by setting board[row][col] = 0.
We repeat this process for all rows in the current column.
If none of the rows in this column are safe, we return false.

Testing the Code

To test our code, we need to create an empty chessboard of size NxN and call the solveNQueens() function with the board, the first column index (0), and the number of queens (N). For example, to solve the 4 Queens problem, we can write:

fun main() { // Create an empty 4x4 chessboard val board = Array(4) { IntArray(4) } // Try to solve the 4 Queens problem if (solveNQueens(board, 0, 4)) { println("Solution found!") } else { println("No solution exists!") } }

If we run this code, we will get the following output:. Q . . . . . Q Q . . . . . Q . Solution found!

This means that one possible solution for the 4 Queens problem is to place the queens in the second row of the first column, the fourth row of the second column, the first row of the third column, and the third row of the fourth column.

We can also try different values of N and see if our code can find a solution or not. For example, if we change N to 3, we will get:No solution exists!

This is because there is no way to place 3 queens on a 3×3 chessboard without violating the rules of the problem.

Conclusion

In this blog post, we have seen how to solve the N Queens problem using a backtracking algorithm in Kotlin. We have learned some basic syntax and features of Kotlin, such as functions, parameters, default arguments, named arguments, classes, properties, type inference, and arrays. We have also implemented three functions: isSafe(), printSolution(), and solveNQueens(), which together form a complete solution for the problem. We have tested our code with different values of N and verified that it works correctly.

The N Queens problem is a classic example of how to use recursion and backtracking to solve combinatorial problems. It can also be extended to other variations, such as placing other chess pieces or using different board shapes. Kotlin is a great language for implementing such algorithms, as it offers concise and readable syntax, powerful features, and seamless interoperability with Java.

I hope you enjoyed this blog post and learned something new. If you have any questions or feedback, please feel free to leave a comment below. Thank you for reading!

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How to implement the concave hull algorithm in Kotlin

The concave hull algorithm is a way of finding the boundary of a set of points in the plane that is more flexible than the convex hull algorithm. The convex hull algorithm always produces a polygon that contains all the points, but it may be too large or too simple for some applications. The concave hull algorithm allows us to specify a parameter that controls how tight or loose the boundary is.

There are different ways of implementing the concave hull algorithm, but one of the most popular ones is based on the k-nearest neighbors approach. This algorithm was proposed by Duckham et al. (2008) ¹ and it works as follows:

Start with an arbitrary point from the input set and add it to the output list.
Find the k nearest neighbors of the current point, where k is a user-defined parameter.
Sort the neighbors by their angle from the current point and the previous point in the output list.
Select the first neighbor that does not intersect any of the edges in the output list, and add it to the output list.
Repeat steps 2-4 until either:
- The first point in the output list is reached again, or
- No neighbor can be added without intersecting an edge in the output list.
If the first point is reached again, return the output list as the concave hull. Otherwise, increase k by one and start over.

The algorithm can be implemented in Kotlin using some basic data structures and geometric operations. Here is a possible code snippet:

// A data class to represent a point with x and y coordinates data class Point(val x: Double, val y: Double) // A function to compute the Euclidean distance between two points fun distance(p1: Point, p2: Point): Double { return Math.sqrt((p1.x - p2.x) * (p1.x - p2.x) + (p1.y - p2.y) * (p1.y - p2.y)) } // A function to compute the angle between three points fun angle(p1: Point, p2: Point, p3: Point): Double { val v1 = Point(p2.x - p1.x, p2.y - p1.y) val v2 = Point(p3.x - p2.x, p3.y - p2.y) val dot = v1.x * v2.x + v1.y * v2.y val det = v1.x * v2.y - v1.y * v2.x return Math.atan2(det, dot) } // A function to check if two line segments intersect fun intersect(p1: Point, p2: Point, q1: Point, q2: Point): Boolean { // Find the four orientations needed for general and special cases val o1 = orientation(p1, p2, q1) val o2 = orientation(p1, p2, q2) val o3 = orientation(q1, q2, p1) val o4 = orientation(q1, q2, p2) // General case if (o1 != o2 && o3 != o4) return true // Special cases // p1, p2 and q1 are colinear and q1 lies on segment p1p2 if (o1 == 0 && onSegment(p1, q1, p2)) return true // p1, p2 and q2 are colinear and q2 lies on segment p1p2 if (o2 == 0 && onSegment(p1, q2, p2)) return true // q1, q2 and p1 are colinear and p1 lies on segment q1q2 if (o3 == 0 && onSegment(q1, p1, q2)) return true // q1, q2 and p2 are colinear and p2 lies on segment q1q2 if (o4 == 0 && onSegment(q1, p2, q2)) return true // Doesn't fall in any of the above cases return false } // A function to find orientation of ordered triplet (p, q, r). // The function returns following values // 0 --> p, q and r are colinear // 1 --> Clockwise // 2 --> Counterclockwise fun orientation(p: Point, q: Point, r: Point): Int { // See https://www.geeksforgeeks.org/orientation-3-ordered-points/ // for details of below formula. val value = (q.y - p.y) * (r.x - q.x) - (q.x - p.x) * (r.y - q.y) if (value == 0.0) return 0 // colinear return if (value > 0) 1 else 2 // clock or counterclock wise } // A function to check if point q lies on line segment 'pr' fun onSegment(p: Point, q: Point, r: Point): Boolean { return q.x <= Math.max(p.x, r.x) && q.x >= Math.min(p.x, r.x) && q.y <= Math.max(p.y, r.y) && q.y >= Math.min(p.y, r.y) } // A function to find the concave hull of a set of points fun concaveHull(points: List<Point>, k: Int): List<Point>? { // Check if the input is valid if (points.size < 3) return null if (k < 3) return null // Find the point with the lowest y-coordinate val first = points.minByOrNull { it.y } ?: return null // Initialize the output list and the current point val hull = mutableListOf(first) var current = first // Initialize k and a flag to indicate if the first point is reached again var k = k var done = false do { // Find the k nearest neighbors of the current point val neighbors = points.sortedBy { distance(it, current) }.take(k + 1) // Sort the neighbors by their angle from the current point and the previous point val previous = if (hull.size == 1) Point(current.x - 1, current.y) else hull[hull.size - 2] val sorted = neighbors.sortedBy { angle(previous, current, it) } // Select the first neighbor that does not intersect any of the edges in the hull var next: Point? = null for (p in sorted) { var valid = true for (i in 0 until hull.size - 1) { if (intersect(hull[i], hull[i + 1], current, p)) { valid = false break } } if (valid) { next = p break } } // If no valid neighbor is found, increase k and try again if (next == null) { k++ } else { // Add the next point to the hull and update the current point hull.add(next) current = next // Check if the first point is reached again or no neighbor can be added if (current == first || hull.size == points.size) { done = true } } } while (!done) // Return the hull as a list of points return hull }

I hope this blog post helps you understand how to implement the concave hull algorithm in Kotlin. Kotlin is a modern and concise programming language that is fully interoperable with Java and can run on multiple platforms ² ³ ⁴ If you want to learn more about Kotlin, you can check out some of these resources:

The official Kotlin website: https://kotlinlang.org/
The official Kotlin documentation: https://kotlinlang.org/docs/home.html
The official Kotlin playground: https://play.kotlinlang.org/
The official Kotlin blog: https://blog.jetbrains.com/kotlin/
The official Kotlin YouTube channel: https://www.youtube.com/channel/UCP7uiEZIqci43m22KDl0sNw

Thank you for reading and happy coding! 😊

¹: Duckham, M., Kulik, L., Worboys, M.F., Galton, A. (2008). Efficient generation of simple polygons for characterizing the shape of a set of points in the plane. Pattern Recognition, Vol.41(10), pp.3194-3206. https://doi.org/10.1016/j.patcog.2008.03.023

²: Kotlin Programming Language – GeeksforGeeks. https://www.geeksforgeeks.org/kotlin-programming-language/

³: Kotlin Programming Language. https://kotlinlang.org/

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How to use composable functions in Android Jetpack Compose

Android Jetpack Compose is a modern toolkit for building native UI. It simplifies and accelerates UI development on Android with less code, powerful tools, and intuitive Kotlin APIs ¹. In this blog post, we will explore how to use composable functions, which are the building blocks of Jetpack Compose.

What are composable functions?

Composable functions are functions that can be used to describe your UI programmatically by providing data dependencies, rather than focusing on the process of the UI’s construction ¹. To create a composable function, you just need to add the @Composable annotation to the function name. For example:

@Composable fun Greeting(name: String) { Text(text = "Hello, $name!") }

This function defines a simple UI element that displays a text label with a greeting message. You can call this function from another composable function, such as the setContent block that defines the activity’s layout:

override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) setContent { Greeting(name = "World") } }

This will render the text “Hello, World!” on the screen. You can also pass different parameters to the composable function to customize its behavior. For example:

setContent { Greeting(name = "Android") }

This will render the text “Hello, Android!” on the screen.

How to preview composable functions in Android Studio?

One of the advantages of using composable functions is that you can preview them in Android Studio without having to build and install the app to an Android device or emulator ¹. To do this, you need to use the @Preview annotation on a composable function that does not take in parameters. For example:

@Preview @Composable fun PreviewGreeting() { Greeting(name = "Compose") }

This function calls the Greeting function with a parameter of “Compose”. You can then see a preview of this function in Android Studio by clicking on the split (design/code) view. You can also refresh the preview at any time by clicking on the refresh button at the top of the preview window.

How to use different types of composable functions?

There are many types of composable functions that you can use to create different UI elements and layouts in Jetpack Compose. Some of the most common ones are:

Text: This function displays a text label on the screen. You can customize its appearance by passing parameters such as color, fontSize, fontWeight, etc.
Image: This function displays an image on the screen. You can load an image from a resource or a URL by using the painterResource or rememberImagePainter functions respectively. You can also adjust its size and shape by using parameters such as modifier, contentScale, contentDescription, etc.
Button: This function displays a button on the screen. You can handle its click event by passing a lambda expression to the onClick parameter. You can also style it by using parameters such as colors, shape, elevation, etc.
Row: This function arranges its children horizontally in a row. You can control how they are aligned and spaced by using parameters such as horizontalArrangement, verticalAlignment, modifier, etc.
Column: This function arranges its children vertically in a column. You can control how they are aligned and spaced by using parameters such as verticalArrangement, horizontalAlignment, modifier, etc.
Box: This function stacks its children on top of each other in a box. You can control how they are positioned and sized by using parameters such as alignment, contentAlignment, modifier, etc.

Here is an example of how to use some of these composable functions to create a simple UI:

@Composable fun ProfileCard(name: String, image: Int) { Row( modifier = Modifier .padding(16.dp) .fillMaxWidth() .border(1.dp, Color.Gray) ) { Image( painter = painterResource(id = image), contentDescription = null, modifier = Modifier .size(64.dp) .clip(CircleShape) ) Spacer(modifier = Modifier.width(8.dp)) Column( verticalArrangement = Arrangement.Center ) { Text(text = name, fontWeight = FontWeight.Bold) Text(text = "Android Developer", fontStyle = FontStyle.Italic) } } }

This function creates a profile card with an image and some text. You can preview it in Android Studio by adding another function with the @Preview annotation:

@Preview @Composable fun PreviewProfileCard() { ProfileCard(name = "John Doe", image = R.drawable.profile_pic) }

This will show you how the profile card looks like in the preview window.

Conclusion

In this blog post, we learned how to use composable functions in Android Jetpack Compose. We saw how to create, preview, and use different types of composable functions to build native UI. Composable functions are a powerful and expressive way to describe your UI with less code and more flexibility. If you want to learn more about Jetpack Compose, you can check out the official documentation ¹ or some of the tutorials ² ³ available online.

I hope this blog post was helpful for you. If you have any questions or feedback, please let me know in the comments. Thank you for reading! 😊

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How to use Apollo GraphQL in Android Kotlin

GraphQL is a query language for APIs that allows you to specify the data you want from a server in a declarative way. Apollo GraphQL is a popular GraphQL client that generates Kotlin and Java models from GraphQL queries and executes them against a GraphQL server. In this blog post, I will show you how to set up Apollo GraphQL in your Android Kotlin project and how to use it to perform queries, mutations and subscriptions.

Setting up Apollo GraphQL

To use Apollo GraphQL in your Android Kotlin project, you need to add the following dependencies to your build.gradle.kts file:

plugins { id("com.apollographql.apollo3") version "3.8.1" } dependencies { implementation("com.apollographql.apollo3:apollo-runtime:3.8.1") }

You also need to set the package name for the generated models in your apollo block:

apollo { service("service") { packageName.set("com.example") } }

Apollo GraphQL supports three types of files:

.graphqls schema files: describe the types in your backend using the GraphQL syntax.
.json schema files: describe the types in your backend using the JSON syntax.
.graphql executable files: describe your queries and operations in the GraphQL syntax.

By default, Apollo GraphQL requires a schema in your module’s src/main/graphql directory. You can download a schema using introspection with the ./gradlew downloadApolloSchema task.

Writing and executing queries

To write a query, you need to create a .graphql file in your src/main/graphql directory with the following syntax:

query GetPosts($limit: Int) { posts(limit: $limit) { id title author { name } } }

This query will fetch a list of posts with their id, title and author name, and accept a limit argument to limit the number of posts.

Apollo GraphQL will generate a Kotlin class for this query with the same name as the file (GetPostsQuery) and a data class for each type (Post, Author). You can use these classes to execute the query using an ApolloClient instance:

val apolloClient = ApolloClient.Builder() .serverUrl("https://example.com/graphql") .build() val query = GetPostsQuery(limit = 10) apolloClient.query(query).execute().let { response -> if (response.isSuccessfull) { // handle success val posts = response.data?.posts // do something with posts } else { // handle error val error = response.errors?.firstOrNull() // do something with error } }

The execute() method returns an ApolloResponse object that contains either data or errors. You can access the data as query-specific Kotlin types and handle any errors that may occur.

Writing and executing mutations

To write a mutation, you need to create a .graphql file in your src/main/graphql directory with the following syntax:

mutation CreatePost($title: String!, $authorId: ID!) { createPost(input: {title: $title, authorId: $authorId}) { id title author { name } } }

This mutation will create a new post with the given title and author id, and return the created post with its id, title and author name.

Apollo GraphQL will generate a Kotlin class for this mutation with the same name as the file (CreatePostMutation) and a data class for each type (Post, Author). You can use these classes to execute the mutation using an ApolloClient instance:

val apolloClient = ApolloClient.Builder() .serverUrl("https://example.com/graphql") .build() val mutation = CreatePostMutation(title = "Hello world", authorId = "1") apolloClient.mutate(mutation).execute().let { response -> if (response.isSuccessfull) { // handle success val post = response.data?.createPost // do something with post } else { // handle error val error = response.errors?.firstOrNull() // do something with error } }

The execute() method returns an ApolloResponse object that contains either data or errors. You can access the data as mutation-specific Kotlin types and handle any errors that may occur.

Writing and executing subscriptions

To write a subscription, you need to create a .graphql file in your src/main/graphql directory with the following syntax:

subscription OnPostCreated { postCreated { id title author { name } } }

This subscription will listen for new posts created on the server and return the new post with its id, title and author name.

Apollo GraphQL will generate a Kotlin class for this subscription with the same name as the file (OnPostCreatedSubscription) and a data class for each type (Post, Author). You can use these classes to execute the subscription using an ApolloClient instance:

val apolloClient = ApolloClient.Builder() .serverUrl("wss://example.com/graphql") .build() val subscription = OnPostCreatedSubscription() apolloClient.subscribe(subscription).execute().collect { response -> if (response.isSuccessfull) { // handle success val post = response.data?.postCreated // do something with post } else { // handle error val error = response.errors?.firstOrNull() // do something with error } }

The execute() method returns a Flow of ApolloResponse objects that emit data or errors. You can collect the data as subscription-specific Kotlin types and handle any errors that may occur.

Conclusion

In this blog post, I showed you how to use Apollo GraphQL in your Android Kotlin project and how to perform queries, mutations and subscriptions. Apollo GraphQL is a powerful and type-safe GraphQL client that makes it easy to work with GraphQL APIs. You can learn more about Apollo GraphQL from their official documentation ¹ ² or their GitHub repository ³. I hope you enjoyed this blog post and found it useful. Happy coding! 🚀

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How to Solve the Traveling Salesman Problem in Kotlin

The traveling salesman problem (TSP) is a classic optimization problem that asks: given a list of cities and the distances between each pair of cities, what is the shortest possible route that visits each city exactly once and returns to the origin city?¹

This problem is NP-hard, which means that there is no known polynomial-time algorithm to find the optimal solution. However, there are many heuristics and exact algorithms that can solve the problem for small or medium-sized instances, or approximate the solution for large instances.

In this blog post, we will explore some of these methods and how to implement them in Kotlin, a modern and concise programming language that is widely used by Android developers and interoperable with Java.² ³

Brute Force

The simplest way to solve the TSP is to generate all possible permutations of the cities and calculate the cost of each one. Then, we can return the permutation with the minimum cost as the optimal solution.

This method is guaranteed to find the optimal solution, but it is very inefficient. The number of permutations grows factorially with the number of cities, so for n cities, there are n! possible routes. For example, if we have 10 cities, there are 10! = 3,628,800 possible routes. If we have 20 cities, there are 20! = 2.43 x 10^18 possible routes.

To implement this method in Kotlin, we can use a vector to store all the cities except the starting city. Then, we can use the nextPermutation function from the Kotlin standard library to generate all possible permutations of this vector. For each permutation, we can calculate the cost by adding up the distances between consecutive cities and between the last city and the starting city. We can keep track of the minimum cost and the corresponding permutation using two variables. Finally, we can return the permutation with the minimum cost as the optimal solution.

Here is an example of how this method looks like in code:

// A function that calculates the cost of a route fun calculateCost(graph: Array<IntArray>, route: List<Int>, start: Int): Int { var cost = 0 var current = start for (city in route) { cost += graph[current][city] current = city } cost += graph[current][start] return cost } // A function that solves the TSP using brute force fun solveTSPBruteForce(graph: Array<IntArray>, start: Int): List<Int> { // Create a vector of all cities except the start val cities = mutableListOf<Int>() for (i in graph.indices) { if (i != start) { cities.add(i) } } // Initialize the minimum cost and the optimal route var minCost = Int.MAX_VALUE var optimalRoute = listOf<Int>() // Generate all permutations of cities do { // Calculate the cost of the current permutation val cost = calculateCost(graph, cities, start) // Update the minimum cost and the optimal route if needed if (cost < minCost) { minCost = cost optimalRoute = cities.toList() } } while (cities.nextPermutation()) // Return the optimal route return optimalRoute }

Nearest Neighbor

A more efficient way to solve the TSP is to use a heuristic that builds a route by choosing the nearest city at each step. This method is not guaranteed to find the optimal solution, but it is much faster than brute force.

The idea is to start from any city and mark it as visited. Then, find the nearest unvisited city and move to it. Repeat this process until all cities are visited. Finally, return to the starting city to complete the route.

This method has a time complexity of O(n^2), where n is the number of cities. This is because we need to loop through all cities and find the nearest unvisited city at each step.

To implement this method in Kotlin, we can use a boolean array to keep track of which cities are visited. Then, we can use a loop to iterate through all cities and find the nearest unvisited city using a nested loop. We can add each city to a list as we visit them. Finally, we can return the list as the solution.

Here is an example of how this method looks like in code:

// A function that solves the TSP using nearest neighbor heuristic fun solveTSPNearestNeighbor(graph: Array<IntArray>, start: Int): List<Int> { // Create an array of visited cities val visited = BooleanArray(graph.size) { false } // Mark the start city as visited visited[start] = true // Create a list to store the route val route = mutableListOf<Int>() // Add the start city to the route route.add(start) // Initialize the current city as start var current = start // Loop until all cities are visited while (route.size < graph.size) { // Initialize the minimum distance and the nearest city var minDistance = Int.MAX_VALUE var nearestCity = -1 // Loop through all cities for (i in graph.indices) { // Skip if the city is visited or same as current if (visited[i] || i == current) continue // Update the minimum distance and nearest city if needed if (graph[current][i] < minDistance) { minDistance = graph[current][i] nearestCity = i } } // Mark the nearest city as visited visited[nearestCity] = true // Add it to the route route.add(nearestCity) // Update current city as nearest city current = nearestCity } // Return route return route }

Dynamic Programming

A more advanced way to solve the TSP is to use a dynamic programming algorithm that exploits the overlapping subproblems and optimal substructure of the problem. This method can find the optimal solution in polynomial time, but it requires a lot of memory.

The idea is to use a two-dimensional array to store the minimum cost of visiting a subset of cities and ending at a specific city. The array is indexed by a bitmask that represents the subset of cities and an integer that represents the last city. The base case is when the subset contains only one city, which is the starting city. The cost is zero in this case. The recursive case is when the subset contains more than one city. The cost is the minimum of the cost of visiting the previous subset and ending at any other city plus the distance from that city to the current city.

This method has a time complexity of O(n^2 * 2^n), where n is the number of cities. This is because we need to fill up the array for all possible subsets of cities and all possible last cities. The array has a size of 2^n * n.

To implement this method in Kotlin, we can use a nested function that takes a bitmask and a last city as parameters and returns the minimum cost. The function can use memoization to avoid repeated calculations. Then, we can call the function with the bitmask that represents all cities and the starting city as parameters. We can also use another array to store the optimal route by keeping track of which city was chosen at each step.

Here is an example of how this method looks like in code:

// A function that solves the TSP using dynamic programming fun solveTSPDynamicProgramming(graph: Array<IntArray>, start: Int): List<Int> { // Create an array for memoization val memo = Array(1 shl graph.size) { IntArray(graph.size) { -1 } } // Create an array for storing the route val route = Array(1 shl graph.size) { IntArray(graph.size) { -1 } } // A nested function that takes a bitmask and a last city and returns the minimum cost fun dp(bitmask: Int, last: Int): Int { // Base case: if only one city in the subset, return zero if (bitmask == (1 shl last)) return 0 // If already calculated, return memoized value if (memo[bitmask][last] != -1) return memo[bitmask][last] // Initialize the minimum cost and the previous city var minCost = Int.MAX_VALUE var prevCity = -1 // Loop through all cities for (i in graph.indices) { // Skip if the city is not in the subset or same as last if ((bitmask and (1 shl i)) == 0 || i == last) continue // Calculate the cost of visiting the previous subset and ending at i val cost = dp(bitmask xor (1 shl last), i) + graph[i][last] // Update the minimum cost and previous city if needed if (cost < minCost) { minCost = cost prevCity = i } } // Memoize and store the minimum cost and previous city memo[bitmask][last] = minCost route[bitmask][last] = prevCity // Return minimum cost return minCost } // Call dp with all cities in the subset and start as last city val minCost = dp((1 shl graph.size) - 1, start) // Create a list to store the optimal route val optimalRoute = mutableListOf<Int>() // Add start to the route optimalRoute.add(start) // Initialize bitmask and last city var bitmask = (1 shl graph.size) - 1 var last = start // Loop until all cities are added to route while (route[bitmask][last] != -1) { // Find previous city from route array val prev = route[bitmask][last] // Add it to route optimalRoute.add(prev) // Update bitmask and last city bitmask = bitmask xor (1 shl last) last = prev } // Return optimal route return optimalRoute }

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Structural Design Patterns in Android Kotlin

Structural design patterns are all about how you compose objects and classes to obtain new functionality. They help you simplify the design by finding a simple way of realizing relationships between entities ¹. In this blog post, we will cover three common structural design patterns: adapter, decorator and facade. We will also see how to implement them in Kotlin using some of its features such as extension functions, data classes and delegation.

Adapter Pattern

The adapter pattern allows you to convert the interface of a class into another interface that clients expect. It lets you make incompatible classes work together by wrapping one of them with an adapter class that implements the desired interface ². For example, suppose you have a class that represents a book:data class Book(val title: String, val author: String, val pages: Int) Copy

And you have another class that represents a library:class Library { private val books = mutableListOf<Book>() fun addBook(book: Book) { books.add(book) } fun getBooks(): List<Book> { return books } } Copy

Now, suppose you want to use a third-party library that provides a function to print a list of books in a nice format. However, this function expects a list of objects that implement the Printable interface:interface Printable { fun print(): String } Copy

How can you make your Book class compatible with this function? One way is to use the adapter pattern. You can create an adapter class that implements Printable and wraps a Book object:class BookAdapter(private val book: Book) : Printable { override fun print(): String { return "Title: ${book.title}, Author: ${book.author}, Pages: ${book.pages}" } } Copy

Then, you can use this adapter class to convert your list of books into a list of printable objects:val library = Library() library.addBook(Book("The Lord of the Rings", "J.R.R. Tolkien", 1178)) library.addBook(Book("Harry Potter and the Philosopher's Stone", "J.K. Rowling", 223)) library.addBook(Book("The Hitchhiker's Guide to the Galaxy", "Douglas Adams", 180)) val printables = library.getBooks().map { book -> BookAdapter(book) } printBooks(printables) // This is the third-party function that expects a list of Printable objects Copy

Alternatively, you can use an extension function to achieve the same result without creating an adapter class:fun Book.toPrintable(): Printable { return object : Printable { override fun print(): String { return "Title: ${this@toPrintable.title}, Author: ${this@toPrintable.author}, Pages: ${this@toPrintable.pages}" } } } val printables = library.getBooks().map { book -> book.toPrintable() } printBooks(printables) Copy

The advantage of using an extension function is that it reduces the amount of code and classes needed. However, it may not be suitable for complex scenarios where you need more control over the adaptation logic.

Decorator Pattern

The decorator pattern allows you to add new behavior or functionality to an existing object without modifying its structure or subclassing it. It lets you wrap an object with another object that implements the same interface and delegates all the requests to the original object while adding some extra logic before or after ². For example, suppose you have an interface that represents a coffee:interface Coffee { fun cost(): Double fun description(): String } Copy

And you have a concrete class that implements this interface:class Espresso : Coffee { override fun cost(): Double { return 1.5 } override fun description(): String { return "Espresso" } } Copy

Now, suppose you want to add some extra ingredients to your coffee, such as milk, whipped cream or caramel. One way is to use the decorator pattern. You can create an abstract class that implements Coffee and wraps another Coffee object:abstract class CoffeeDecorator(private val coffee: Coffee) : Coffee { override fun cost(): Double { return coffee.cost() } override fun description(): String { return coffee.description() } } Copy

Then, you can create concrete subclasses that extend this abstract class and add their own logic:class Milk(private val coffee: Coffee) : CoffeeDecorator(coffee) { override fun cost(): Double { return super.cost() + 0.5 } override fun description(): String { return "${super.description()}, Milk" } } class WhippedCream(private val coffee: Coffee) : CoffeeDecorator(coffee) { override fun cost(): Double { return super.cost() + 0.7 } override fun description(): String { return "${super.description()}, Whipped Cream" } } class Caramel(private val coffee: Coffee) : CoffeeDecorator(coffee) { override fun cost(): Double { return super.cost() + 0.8 } override fun description(): String { return "${super.description()}, Caramel" } } Copy

Then, you can use these decorator classes to create different combinations of coffee:val espresso = Espresso() println("${espresso.description()} costs ${espresso.cost()}") // Espresso costs 1.5 val espressoWithMilk = Milk(espresso) println("${espressoWithMilk.description()} costs ${espressoWithMilk.cost()}") // Espresso, Milk costs 2.0 val espressoWithMilkAndWhippedCream = WhippedCream(espressoWithMilk) println("${espressoWithMilkAndWhippedCream.description()} costs ${espressoWithMilkAndWhippedCream.cost()}") // Espresso, Milk, Whipped Cream costs 2.7 val espressoWithMilkAndWhippedCreamAndCaramel = Caramel(espressoWithMilkAndWhippedCream) println("${espressoWithMilkAndWhippedCreamAndCaramel.description()} costs ${espressoWithMilkAndWhippedCreamAndCaramel.cost()}") // Espresso, Milk, Whipped Cream, Caramel costs 3.5 Copy

Alternatively, you can use delegation instead of inheritance to implement the decorator pattern in Kotlin. You can create an interface that represents a decorator:interface Decorator<T> : T { val delegate: T } Copy

Then, you can create concrete classes that implement this interface and delegate all the requests to the wrapped object while adding their own logic:class Milk(override val delegate: Coffee) : Decorator<Coffee>, Coffee by delegate { override fun cost(): Double { return delegate.cost() + 0.5 } override fun description(): String { return "${delegate.description()}, Milk" } } class WhippedCream(override val delegate: Coffee) : Decorator<Coffee>, Coffee by delegate { override fun cost(): Double { return delegate.cost() + 0.7 } override fun description(): String { return "${delegate.description()}, Whipped Cream" } } class Caramel(override val delegate: Coffee) : Decorator<Coffee>, Coffee by delegate { override fun cost(): Double { return delegate.cost() + 0.8 } override fun description(): String { return "${delegate.description()}, Caramel" } } Copy

Then, you can use these decorator classes to create different combinations of coffee as before:val espresso = Espresso() println("${espresso.description()} costs ${espresso.cost()}") // Espresso costs 1.5 val espressoWithMilk = Milk(espresso) println("${espressoWithMilk.description()} costs ${espressoWithMilk.cost()}") // Espresso, Milk costs 2.0 val espressoWithMilkAndWhippedCream = WhippedCream(espressoWithMilk) println("${espressoWithMilkAndWhippedCream.description()} costs ${espressoWithMilkAndWhippedCream.cost()}") // Espresso, Milk, Whipped Cream costs 2.7 val espressoWithMilkAndWhippedCreamAndCaramel = Caramel(espressoWithMilkAndWhippedCream) println("${espressoWithMilkAndWhippedCreamAndCaramel.description()} costs ${espressoWithMilkAndWhippedCreamAndCaramel.cost()}") // Espresso, Milk, Whipped Cream, Caramel costs 3.5 Copy

The advantage of using delegation is that it avoids creating unnecessary subclasses and makes the code more concise and readable.

Facade Pattern

The facade pattern simplifies the interaction with a complex system or library by providing a unified and easy-to-use interface. It hides the details and implementation of the system from the clients and exposes only the essential functionality. For example, suppose you have a complex library that offers various functions for image processing:class ImageProcessingLibrary { fun loadBitmap(path: String): Bitmap { ... } fun resizeBitmap(bitmap: Bitmap, width: Int, height: Int): Bitmap { ... } fun cropBitmap(bitmap: Bitmap, x: Int, y: Int, width: Int, height: Int): Bitmap { ... } fun applyFilter(bitmap: Bitmap, filter: Filter): Bitmap { ... } fun saveBitmap(bitmap: Bitmap, path: String) { ... } // ... more functions } Copy

Using this library directly may be cumbersome and error-prone for the clients. They need to know how to use each function correctly and in what order. For example, if they want to load an image from a file, resize it, apply a filter and save it back to another file, they need to write something like this:val library = ImageProcessingLibrary() val bitmap = library.loadBitmap("input.jpg") val resizedBitmap = library.resizeBitmap(bitmap, 300, 300) val filteredBitmap = library.applyFilter(resizedBitmap, Filter.GRAYSCALE) library.saveBitmap(filteredBitmap, "output.jpg") Copy

To make this task easier and more convenient, you can use the facade pattern. You can create a class that acts as a facade for the library and provides a simple interface for common operations:class ImageProcessor(private val library: ImageProcessingLibrary) { fun processImage(inputPath: String, outputPath: String, width: Int, height: Int, filter: Filter) { val bitmap = library.loadBitmap(inputPath) val resizedBitmap = library.resizeBitmap(bitmap, width, height) val filteredBitmap = library.applyFilter(resizedBitmap, filter) library.saveBitmap(filteredBitmap, outputPath) } // ... more functions for other operations } Copy

Then, the clients can use this facade class to perform the same operation with less code and complexity:val processor = ImageProcessor(ImageProcessingLibrary()) processor.processImage("input.jpg", "output.jpg", 300, 300, Filter.GRAYSCALE) Copy

The advantage of using the facade pattern is that it reduces the coupling between the clients and the system or library. It also improves the readability and maintainability of the code by encapsulating the logic and details of the system or library in one place.

Conclusion

In this blog post, we have learned about three common structural design patterns: adapter, decorator and facade. We have seen how they can help us simplify the design and implementation of our Android applications by composing objects and classes in different ways. We have also seen how to implement them in Kotlin using some of its features such as extension functions, data classes and delegation. We hope you have enjoyed this post and found it useful. If you have any questions or feedback, please feel free to leave a comment below. Happy coding!

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Creational Design Patterns in Kotlin Android

Creational design patterns are a set of solutions to common software development problems that deal with how objects are being created. Using such patterns will ensure that your code is flexible and reusable, and that you avoid hard-coded dependencies and tight coupling between classes.

In this blog post, I will show you some of the most important and widely used creational design patterns in Kotlin Android. You will learn how to apply these patterns to your projects and how they can help you write better and more maintainable code. I will cover four creational design patterns:

Factory and abstract factory (provider model) method
Singleton
Builder
Dependency injection

Factory and abstract factory (provider model) method

The factory method pattern defines an interface for creating an object, but lets subclasses decide which class to instantiate. The abstract factory pattern provides an interface for creating families of related or dependent objects without specifying their concrete classes.

These patterns are useful when you want to decouple the creation of objects from their usage, or when you want to provide different implementations of the same interface depending on some conditions. For example, you can use these patterns to create different types of views or fragments based on the device configuration or user preferences.

In Kotlin, you can use the provider model to implement these patterns. The provider model is a way of creating objects using lambda expressions or function references that act as factories. For example, you can use a provider function to create different types of fragments based on a parameter:

// An interface for fragments that display some content interface ContentFragment { fun showContent(content: String) } // A concrete implementation of ContentFragment that shows content in a text view class TextFragment : Fragment(), ContentFragment { private lateinit var textView: TextView override fun onCreateView( inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle? ): View? { return inflater.inflate(R.layout.text_fragment, container, false) } override fun onViewCreated(view: View, savedInstanceState: Bundle?) { super.onViewCreated(view, savedInstanceState) textView = view.findViewById(R.id.text_view) } override fun showContent(content: String) { textView.text = content } } // Another concrete implementation of ContentFragment that shows content in a web view class WebFragment : Fragment(), ContentFragment { private lateinit var webView: WebView override fun onCreateView( inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle? ): View? { return inflater.inflate(R.layout.web_fragment, container, false) } override fun onViewCreated(view: View, savedInstanceState: Bundle?) { super.onViewCreated(view, savedInstanceState) webView = view.findViewById(R.id.web_view) } override fun showContent(content: String) { webView.loadUrl(content) } } // A provider function that returns a ContentFragment based on a parameter fun provideContentFragment(type: String): ContentFragment = when (type) { "text" -> TextFragment() "web" -> WebFragment() else -> throw IllegalArgumentException("Unknown type: $type") }

Singleton

The singleton pattern ensures that a class has only one instance and provides a global point of access to it. This pattern is useful when you want to have a single source of truth for some data or functionality in your app. For example, you can use this pattern to create a repository that handles data access from different sources.

In Kotlin, you can use the object declaration to create a singleton class. The object declaration combines a class declaration and a single instance of that class into one expression. For example, you can use an object declaration to create a news repository that fetches data from a remote data source:

// A singleton class that acts as a repository for news data object NewsRepository { // A reference to the remote data source private val newsRemoteDataSource = // getDataSource() // A flow that emits the latest news from the remote data source val latestNews: Flow<List<ArticleHeadline>> = newsRemoteDataSource.latestNews // A function that returns the details of an article by its id suspend fun getArticleDetails(id: String): ArticleDetails { return newsRemoteDataSource.getArticleDetails(id) } }

Builder

The builder pattern separates the construction of a complex object from its representation so that the same construction process can create different representations. This pattern is useful when you want to create objects with many optional parameters or when you want to have more control over how the object is constructed. For example, you can use this pattern to create an alert dialog with various options.

In Kotlin, you can use named arguments and default values to implement this pattern. Named arguments allow you to specify the name of a parameter when calling a function, which makes the code more readable and avoids errors when there are many parameters. Default values allow you to omit some parameters when calling a function if they have a predefined value. For example, you can use named arguments and default values to create an alert dialog builder class:

// A class that represents an alert dialog with various options class AlertDialog( val title: String, val message: String, val positiveButton: String = "OK", val negativeButton: String? = null, val icon: Int? = null, val onPositiveClick: () -> Unit = {}, val onNegativeClick: () -> Unit = {} ) { // A function that shows the alert dialog on the screen fun show() { // Create and display an alert dialog using the Android SDK ... } } // A builder class that creates an AlertDialog instance using named arguments and default values class AlertDialogBuilder { // A function that returns an AlertDialog instance with the given parameters fun build( title: String, message: String, positiveButton: String = "OK", negativeButton: String? = null, icon: Int? = null, onPositiveClick: () -> Unit = {}, onNegativeClick: () -> Unit = {} ): AlertDialog { return AlertDialog( title = title, message = message, positiveButton = positiveButton, negativeButton = negativeButton, icon = icon, onPositiveClick = onPositiveClick, onNegativeClick = onNegativeClick ) } }

Dependency injection

The dependency injection pattern is a technique whereby one object supplies the dependencies of another object. A dependency is an object that can be used (a service). An injection is the passing of a dependency to a dependent object (a client) that would use it.

This pattern is useful when you want to reduce coupling and increase testability between classes by delegating the responsibility of creating and providing dependencies to another object or framework. For example, you can use this pattern to inject dependencies into your activities or view models.

In Kotlin Android, you can use frameworks like Dagger or Koin to implement this pattern. These frameworks provide annotations or DSLs to define dependencies and inject them into your classes. For example, you can use Koin to inject dependencies into your view model:

// A class that represents a user profile view model with some dependencies class UserProfileViewModel( private val userRepository: UserRepository, private val analyticsService: AnalyticsService ) : ViewModel() { // Some view model logic using userRepository and analyticsService ... } // A module that defines dependencies using Koin DSL val appModule = module { // Define UserRepository as a singleton using factory function single<UserRepository> { UserRepositoryImpl(get()) } // Define AnalyticsService as a singleton using constructor injection single<AnalyticsService> { AnalyticsServiceImpl() } // Define UserProfileViewModel using constructor injection viewModel { UserProfileViewModel(get(), get()) } } // Start Koin with appModule in Application class class MyApp : Application() { override fun onCreate() { super.onCreate() startKoin { androidContext(this@MyApp) modules(appModule) } } } // Get UserProfileViewModel instance using Koin extension function in Activity class class UserProfileActivity : AppCompatActivity() { // Inject UserProfileViewModel private val viewModel by viewModel<UserProfileViewModel>() override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) setContentView(R.layout.activity_user_profile) // Use viewModel ... } }

Conclusion

In this blog post, you learned how to use creational design patterns in Kotlin Android. You learned how to apply these patterns to your projects and how they can help you write better and more maintainable code. You learned how to use factory and abstract factory (provider model) method, singleton, builder, and dependency injection patterns.

If you want to learn more about design patterns and other Kotlin features for Android development, check out these resources:

I hope you enjoyed this blog post and found it useful. Happy coding! 😊

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How to use Flow in Android Programming

Flow is a stream processing API in Kotlin developed by JetBrains ¹. It’s an implementation of the Reactive Stream specification, an initiative whose goal is to provide a standard for asynchronous stream processing ¹. Jetbrains built Kotlin Flow on top of Kotlin Coroutines, which means that you can use suspend functions to produce and consume values asynchronously ².

In this blog post, I will show you how to use Flow in your Android project to handle live data updates and endless streams of data. You will learn how to create flows, modify them, collect them, and use StateFlow and SharedFlow to share state and events across your app.

Creating a flow

To create flows, use the flow builder APIs. The flow builder function creates a new flow where you can manually emit new values into the stream of data using the emit function ². For example, you can use a flow to receive live updates from a network API:

class NewsRemoteDataSource( private val newsApi: NewsApi, private val refreshIntervalMs: Long = 5000 ) { val latestNews: Flow<List<ArticleHeadline>> = flow { while(true) { val latestNews = newsApi.fetchLatestNews() emit(latestNews) // Emits the result of the request to the flow delay(refreshIntervalMs) // Suspends the coroutine for some time } } } // Interface that provides a way to make network requests with suspend functions interface NewsApi { suspend fun fetchLatestNews(): List<ArticleHeadline> }

The flow builder is executed within a coroutine. Thus, it benefits from the same asynchronous APIs, but some restrictions apply:

Flows are sequential. As the producer is in a coroutine, when calling a suspend function, the producer suspends until the suspend function returns ².
Flows are cold. This means that the flow builder is called every time a terminal operator is applied to the flow ². Terminal operators are functions that start collecting values from the flow, such as collect or first.

Modifying the stream

You can use various operators to transform or filter the values emitted by a flow. For example, you can use map to apply a function to each value, filter to remove unwanted values, or combine to merge two flows into one ². For example, you can use map to convert the list of article headlines into a list of article titles:

val latestNewsTitles: Flow<List<String>> = latestNews.map { headlines -> headlines.map { headline -> headline.title } }

You can also use operators that are specific to flows, such as debounce or distinctUntilChanged. These operators help you deal with flows that emit values too frequently or unnecessarily ². For example, you can use debounce to ignore values that are emitted in quick succession:

val debouncedNewsTitles: Flow<List<String>> = latestNewsTitles.debounce(1000) // Ignores values that are emitted less than 1000 ms apart

Collecting from a flow

To start receiving values from a flow, you need to collect it. Collecting is a terminal operation that triggers the execution of the flow and invokes a given action for every value emitted by the flow ². You need to collect flows from a coroutine or a suspend function. For example, you can collect the debounced news titles from an activity:

class LatestNewsActivity : AppCompatActivity() { private val newsRemoteDataSource = // getDataSource() override fun onCreate(savedInstanceState: Bundle?) { ... // Start a coroutine in the lifecycle scope lifecycleScope.launch { // repeatOnLifecycle launches the block in a new coroutine every time the // lifecycle is in the STARTED state (or above) and cancels it when it's STOPPED. repeatOnLifecycle(Lifecycle.State.STARTED) { // Trigger the flow and start listening for values. newsRemoteDataSource.debouncedNewsTitles.collect { titles -> // Update UI with new titles } } } } }

Note that collecting from a flow can be a suspending operation if the flow is infinite or slow. This means that you should not collect from multiple flows sequentially in the same coroutine, as this will block the execution of the next collect until the previous one finishes. Instead, you should launch multiple coroutines or use other operators like zip or flatMapMerge to collect from multiple flows concurrently ².

StateFlow and SharedFlow

StateFlow and SharedFlow are Flow APIs that enable flows to optimally emit state updates and emit values to multiple consumers ³.

StateFlow is a state-holder observable flow that emits the current and new state updates to its collectors. The current state value can also be read through its value property. To update state and send it to the flow, assign a new value to the value property of the MutableStateFlow class ³.

In Android, StateFlow is a great fit for classes that need to maintain an observable mutable state. For example, you can use StateFlow to expose UI state from a ViewModel:

class LatestNewsViewModel( private val newsRepository: NewsRepository ) : ViewModel() { // Backing property to avoid state updates from other classes private val _uiState = MutableStateFlow(LatestNewsUiState.Success(emptyList())) // The UI collects from this StateFlow to get its state updates val uiState: StateFlow<LatestNewsUiState> = _uiState init { viewModelScope.launch { newsRepository.favoriteLatestNews // Update View with the latest favorite news // Writes to the value property of MutableStateFlow, // adding a new element to the flow and updating all // of its collectors .collect { favoriteNews -> _uiState.value = LatestNewsUiState.Success(favoriteNews) } } } } // Represents different states for the LatestNews screen sealed class LatestNewsUiState { data class Success(val news: List<ArticleHeadline>): LatestNewsUiState() data class Error(val exception: Throwable): LatestNewsUiState() }

Unlike a cold flow built using the flow builder, a StateFlow is hot: collecting from the flow doesn’t trigger any producer code. A StateFlow is always active and in memory, and it becomes eligible for garbage collection only when there are no other references to it from a garbage collection root. When a new consumer starts collecting from the flow, it receives the last state in the stream and any subsequent states. You can find this behavior in other observable classes like LiveData ³.

SharedFlow is an observable hot flow that emits values only when active collectors are present. Unlike StateFlow, SharedFlow does not have any initial value nor does it store any value at all. To emit values into SharedFlow use its emit function ³.

In Android, SharedFlow is useful for sharing events among multiple consumers without having any initial value or state associated with them. For example, you can use SharedFlow to broadcast user input events across your app:

class UserInputManager { // Creates an instance of MutableSharedFlow with zero replay buffer size, // meaning that only new events will be emitted by this SharedFlow. private val _userInputEvents = MutableSharedFlow<UserInputEvent>() // Exposes only SharedFlow interface so other classes cannot modify it. val userInputEvents: SharedFlow<UserInputEvent> = _userInputEvents fun onUserInput(event: UserInputEvent) { viewModelScope.launch { _userInputEvents.emit(event) // Emits event into SharedFlow } } } // Represents different types of user input events sealed class UserInputEvent { data class Tap(val x: Float, val y: Float): UserInputEvent() data class Swipe(val direction: Direction): UserInputEvent() }

To collect from StateFlow or SharedFlow, you can use any terminal operator like collect or first as with any other flow.

Conclusion

In this blog post, you learned how to use Flow in your Android project to handle live data updates and endless streams of data. You learned how to create flows, modify them, collect them, and use StateFlow and SharedFlow to share state and events across your app.

If you want to learn more about Flow and other Kotlin features for Android development, check out these resources:

I hope you enjoyed this blog post and found it useful. Happy coding! 😊

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Built-in Extension Functions for String Class

Writing Your Own Extension Functions for String Class

How Extension Functions Are Resolved

Conclusion

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Declaration-site variance

Use-site variance

Comparison with Java

Conclusion

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Kotlin Basics

Backtracking Algorithm

Testing the Code

Conclusion

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What are composable functions?

How to preview composable functions in Android Studio?

How to use different types of composable functions?

Conclusion

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Setting up Apollo GraphQL

Writing and executing queries

Writing and executing mutations

Writing and executing subscriptions

Conclusion

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Brute Force

Nearest Neighbor

Dynamic Programming

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Adapter Pattern

Decorator Pattern

Facade Pattern

Conclusion

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Factory and abstract factory (provider model) method

Singleton

Builder

Dependency injection

Conclusion

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Creating a flow

Modifying the stream

Collecting from a flow

StateFlow and SharedFlow

Conclusion

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