Listing

The complete program is:

SwingUtilities.invokeLater {
       val myFrame=JFrame("Hello World")
       myFrame.setSize(300,200)
       myFrame.defaultCloseOperation=JFrame.EXIT_ON_CLOSE
       myFrame.layout=null
       val myButton=JButton("Click Me")
       myButton.bounds= Rectangle(15,50,150,50)
       myButton.addActionListener  { e->
              myButton.text= e.source.toString()
           }
       myFrame.add(myButton)
       myFrame.isVisible=true
   }

This is about as far as we can go with Swing. There is a lot more to learn, but it is essentially more of the same. You can find out about layouts and other components and build up your GUI bit by bit. A good way to create a Swing GUI in Kotlin is to use NetBeans to create a layout using its interactive drag-and-drop editor, and then copy and paste the Java into your project. If you are using IntelliJ it will offer to convert the Java to Kotlin and this works reasonably well.

Using Coroutines With Swing

As Swing is an asynchronous event driven UI, as are most Uis, you might be wondering, if it is possible to use coroutines? It is, but you need to use a dispatcher that runs on the correct thread. To do this you need to add the appropriate UI module:

kotlinx-coroutines-android

Kotlinx-coroutines-javafx

Kotlinx-coroutines-swing

The Swing dispatcher uses the EDT to run coroutines and this is the one we need. To use it add:

dependencies {
 implementation(
  "org.jetbrains.kotlinx:kotlinx-
                          coroutines-core-jvm:1.5.0")
 implementation(
  "org.jetbrains.kotlinx:kotlinx-coroutines-swing:1.5.0")
}

to the build.gradle file.

With this addition, we can simply rewrite the previous program as:

import kotlinx.coroutines.Dispatchers
import kotlinx.coroutines.runBlocking
import kotlinx.coroutines.swing.Swing
import java.awt.Rectangle
import javax.swing.JButton
import javax.swing.JFrame
fun main() {
    runBlocking(context = Dispatchers.Swing) {
        println(Thread.currentThread().name)
        val myFrame = JFrame("Hello World")
        myFrame.setSize(300, 200)
        myFrame.defaultCloseOperation = 
                                JFrame.EXIT_ON_CLOSE
        myFrame.layout = null
        val myButton = JButton("Click Me")
        myButton.bounds = Rectangle(15, 50, 150, 50)
        myButton.addActionListener { e ->
            myButton.text = e.source.toString()
        }
        myFrame.add(myButton)
        myFrame.isVisible = true
    }
    println("main end")
}

If you run this you will see main end printed, but the program will continue to run as the Swing event dispatcher keeps the thread occupied.

You can use coroutines in a Swing program in a very straightforward way as long as you remember to put code which modifies the UI onto the Swing dispatcher and other code on Main or one of the other dispatchers.

IDE Help

Final version in book

Problems With Types

Final version in book

The Kotlin Principle

Final version in book

Summary

• Working with Java from Kotlin is usually essential due to the number of existing Java libraries that applications need to use. Working with Kotlin from Java is just as easy, but not as common.

• Kotlin is a JVM language and it produces code which is very similar, if not identical, to Java. Using Java code from Kotlin is usually a matter of working out how its syntax maps to Java.

• Java’s get and set methods are automatically converted to Kotlin properties. You can use the get/set methods or Kotlin’s syntax to work with them.

• Kotlin can also convert Boolean functions that are named starting with is and have a setter starting with set as properties.

• Swing is still a useful Java GUI library, but like most GUI libraries it runs its UI on a separate thread from the main program – the EDT.

• Running code on the EDT is easy with the help of the SwingUtilities library.

• An Interface with a Single Abstract Method, SAM, is how Java generally implements types of functions such as event handlers. The compiler automatically converts functions passed as SAM parameters to the appropriate interface implementation. This means you can pass lambdas where a SAM is required.

• The SAM conversion means you can use lambdas as event handlers.

• The IntelliJ and Android Studio IDEs provide lots of help in working with Java including lists of methods and properties that have been automatically converted.

• Both IntelliJ and Android Studio can convert Java code files to Kotlin. You can also paste Java code into a Kotlin file with an optional conversion to Kotlin.

• Java types that don’t have exact equivalents in Kotlin are shown in the IDEs as a type followed by !.

• A particular problem with working with Java is its lack of non-nullable types. When working with a Java nullable, the best option is to treat it as a nullable Kotlin type.

• Arrays in Java are primitive data types and Kotlin provides equivalents.

• The Kotlin principle is to make use of good compile-time syntax to generate the same run-time behavior

This article is an extract from: 

Programmer's Guide To Kotlin Second Edition

You can buy it from: Amazon

Contents

1. What makes Kotlin Special
2. The Basics:Variables,Primitive Types and Functions 
3. Control
        Extract: If and When 
4. Strings and Arrays
5. The Class & The Object
6. Inheritance
7. The Type Hierarchy
   
8. Generics
9. Collections, Iterators, Sequences & Ranges
       Extract: Iterators & Sequences 
10. Advanced functions 
11. Anonymous, Lamdas & Inline Functions
    
12. Data classes, enums and destructuring
        Extract: Destructuring 
13. Exceptions, Annotations & Reflection
14. Coroutines
        Extract: Coroutines 
15. Working with Java
        Extract: Using Swing ***NEW!

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