How do I combine multiple scope functions for concise Kotlin code?

By Wayan in Kotlin 0 Comment

In Kotlin, scope functions (like let, apply, also, run, and with) provide a way to execute a block of code within the context of an object. Combining them allows writing concise and readable code. Here’s how you can effectively use and combine them:

Example: Combining let and apply

Suppose you have an object that you need to both configure and process:

val result = myObject.apply {
    // Configure the object
    property1 = "value1"
    property2 = "value2"
}.let {
    // Use the configured object and transform it if needed
    it.someTransformingFunction()
}

Explanation:

  1. apply is used to mutate the object (configuration stage).
  2. let provides access to the configured object and performs further operations.

Example: Combining with and run

If you need to perform several actions on an object, including calculations:

val result = with(myObject) {
    // Perform actions on the object
    doAction1()
    doAction2()

    // Return something for further use
    this
}.run {
    // Execute more actions in the context of the returned object
    performAdditionalTask()
    finalResult()
}

Explanation:

  1. with is used as a concise way to operate on an object without changing its state, returning the object itself or another value.
  2. run is then used for further actions or transformations.

Example: Combining let and also

To perform logging or debugging while processing data:

val result = sourceString.let { input ->
    input.trim()
}.also { trimmed ->
    println("Trimmed string: $trimmed")
}

Explanation:

  1. let is used to create a pipeline where the string is transformed.
  2. also is used for side-effects, such as logging or debugging, without altering the object.

Example: Combining in Nested Chains

For a more complex scenario where multiple scope functions are needed:

val result = myObject.apply {
    property1 = "value1"
    property2 = "value2"
}.let {
    // Transform the configured object
    it.someTransformingFunction()
}.also {
    // Log the transformation
    println("Transformed object: $it")
}

This approach allows you to configure an object, transform it, and log its state in a single elegant chain.

General Guidelines

  • Use apply when you want to configure or initialize an object.
  • Use let when you want to execute a block of code with the object as a parameter and transform/compute something.
  • Use also when you need to perform a side effect (e.g., logging) while keeping the object unchanged.
  • Use run and with for blocks of code where you are mostly operating on the object and possibly returning a value.

By properly combining these scope functions, you achieve concise, clean, and functional Kotlin code!

How do I apply scope functions like let, also, and run for null safety in Kotlin?

By Wayan in Kotlin 0 Comment

In Kotlin, scope functions such as let, also, and run can be used effectively for handling null safety. These functions allow you to operate on nullable objects and execute a block of code only if the object is not null. Here’s how these scope functions can be applied for null safety:

1. let

The let scope function is triggered only if the object is non-null. It allows safe execution of a code block with the object as its context (it).

Usage for Null Safety:

val name: String? = "John"

name?.let {
    // Executes only if name is not null
    println("The name is $it")
}

If name is null, the let block will not be executed.

2. also

The also function is used for performing some side actions while retaining the original object. Use it when you want to ensure the object is non-null before performing the side effects.

Usage for Null Safety:

val name: String? = "John"

name?.also {
    // Executes only if name is not null
    println("Processing name: $it")
}

If name is null, the block inside also will not be executed.

3. run

The run function is used when you want to execute a block of code within the context of the object and return the result. It can also be used on nullable types with the safe call (?.).

Usage for Null Safety:

val name: String? = "John"

val length = name?.run {
    // Executes only if name is not null
    println("Calculating length of $this")
    this.length
}
println("Name length: $length")

If name is null, the run block will not be executed, and length will remain null.

Comparison and Use Cases:

Scope Function Purpose Nullable Handling Context Object
let Transform or execute action ?.let {} for null check it
also Side effect actions ?.also {} for null check it
run Configure or transform ?.run {} for null check this

Example Combining Null Safety with Scope Functions:

fun main() {
    val name: String? = "Alice"

    // Using let for null-safe operation
    name?.let { 
        println("Name is $it")
    }

    // Using also for additional actions
    name?.also { 
        println("Logging name: $it")
    }

    // Using run to transform and calculate length
    val length = name?.run { 
        println("Calculating length of $this")
        length
    }
    println("Length: $length")
}

Key Takeaways:

  1. Use ?.let {} to execute a block only if the object is non-null.
  2. Use ?.also {} to perform side effects without altering the object.
  3. Use ?.run {} to execute transformations or calculations within a null-safe block.

With these scope functions, Kotlin provides a clean and concise way to handle nullable types safely.

How do I start a thread execution?

By Wayan in Core API, Lang Package 0 Comment

To make a thread begin its execution, call the start() method on a Thread or Runnable instance. Then the Java Virtual Machine will calls the run method of this thread.

The snippet below is showing how to create a thread by implementing the Runnable interface.

package org.kodejava.lang;

public class ThreadRun implements Runnable {

    public static void main(String[] args) {
        // Instantiate ThreadRun
        ThreadRun runner = new ThreadRun();

        // Create instance of Thread and passing ThreadRun object
        // as argument.
        Thread thread = new Thread(runner);

        // By passing Runnable object, it tells the
        // thread to use run() of Runnable object.
        thread.start();
    }

    public void run() {
        System.out.println("Running..");
    }
}

The snippet below is showing how to create a thread by extending the Thread class.

package org.kodejava.lang;

public class ThreadStart extends Thread {

    public static void main(String[] args) {
        ThreadStart thread = new ThreadStart();

        // Start this thread
        thread.start();
    }

    /**
     * The run() method will be invoked when the thread is started.
     */
    @Override
    public void run() {
        System.out.println("Running..");
    }
}

How do I create a multithreaded program?

By Wayan in Core API, Lang Package 0 Comment

Java language which was there when the language was created. This multithreading capabilities can be said like running each program on their on CPU, although the machine only has a single CPU installed.

To create a Thread program in Java we can extend the java.lang.Thread class and override the run() method. But as we know that Java class can only extend from a single class, extending Thread class makes our class cannot be inherited from another class. To solve this problem an interface was introduced, the java.lang.Runnable.

Let see the simplified demo class below. In the program below we’ll have three separated thread executions, the main thread, thread-1 and thread-2.

package org.kodejava.lang;

public class ThreadDemo extends Thread {
    public static void main(String[] args) {
        // Creates an instance of this class.
        ThreadDemo thread1 = new ThreadDemo();

        // Creates a runnable object.
        Thread thread2 = new Thread(new Runnable() {
            public void run() {
                for (int i = 0; i < 5; i++) {
                    sayHello();
                }
            }
        });

        // Set thread priority, the normal priority of a thread is 5.
        thread1.setPriority(4);
        thread2.setPriority(6);

        // Start the execution of thread1 and thread2
        thread1.start();
        thread2.start();

        for (int i = 0; i < 5; i++) {
            sayHello();
        }
    }

    public void run() {
        for (int i = 0; i < 5; i++) {
            sayHello();
        }
    }

    /**
     * The synchronized modifier ensure that two threads cannot execute the block
     * at the same time.
     */
    private static synchronized void sayHello() {
        for (int i = 0; i < 10; i++) {
            System.out.println("Thread [" + Thread.currentThread().getName() +  "] ==> Hi...");
        }

        try {
            // Causes the currently executing thread to sleep for a random
            // milliseconds
            Thread.sleep((long) (Math.random() * 1000 + 1000));
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        // Causes the currently executing thread object to temporarily pause
        // and allow other threads to execute.
        Thread.yield();
    }
}

How do I create a thread by implementing Runnable interface?

By Wayan in Core API, Lang Package 0 Comment

Here is the second way for creating a thread. We create an object that implements the java.lang.Runnable interface. For another example see How do I create a thread by extending Thread class?.

package org.kodejava.lang;

import java.text.DateFormat;
import java.text.SimpleDateFormat;
import java.util.Calendar;

public class TimeThread implements Runnable {
    private final DateFormat df = new SimpleDateFormat("hh:mm:ss");

    // The run() method will be invoked when the thread of this runnable object
    // is started.
    @Override
    public void run() {
        while (true) {
            Calendar calendar = Calendar.getInstance();
            System.out.format("Now is: %s.%n", df.format(calendar.getTime()));

            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }

    public static void main(String[] args) {
        TimeThread time = new TimeThread();

        Thread thread = new Thread(time);
        thread.start();
    }
}

An example result of this code are:

Now is: 07:18:39.
Now is: 07:18:40.
Now is: 07:18:41.
Now is: 07:18:42.
Now is: 07:18:43.