Using Lambda to Implement Functional Interface Method

We have a virtual Button that when clicked will call an abstract method onClick() of a listener interface OnClickListener.
We need to implement the onClick() so that it prints the Button name.

OnClickListener.java

package com.hmkcode;

public interface OnClickListener {
	void onClick(Button button);
}

Button.java

package com.hmkcode;

public class Button {

	private OnClickListener onClickListener;
	private String name;

	// click the button
	public void click(){
		this.onClickListener.onClick(this);
	}
	
	// getters & setters
}

We have three ways to achieve that:

Implement OnClickListener and override onClick() method.
Use OnClickListener as an anonymous class.
Use Lambda.

Our main class is App.java

package com.hmkcode;

public class App
{
    public static void main( String[] args ){
        System.out.println( "Running App..." );
        new App().run(); 
    }

    public void run(){
        
        Button myButton = new Button();
        myButton.setName("MyButton");
        
        // 1. implements onClickListener
        
        // 2. anonymous class

        // 3. lambda
    	
	
    	// click the button
    	myButton.click();
    }
}

1. Implement `OnClickListener` and override onClick() method

public class App implements OnClickListener
{
    public static void main( String[] args ){...}

    public void run(){
        
        Button myButton = new Button();
        myButton.setName("MyButton");
        
        // 1. implements onClickListener
        myButton.setOnClickListener(this);
        
        // click the button
    	myButton.click();
    }
	
	@Override
	public void onClick(Button button) {
		System.out.println(button.getName() +" Clicked! - implements interface");
		
	}
}

2. Use `OnClickListener` as an anonymous class

public class App
{
    public static void main( String[] args ){...}

    public void run(){
        
        Button myButton = new Button();
        myButton.setName("MyButton");
                
        // 2. anonymous class
        myButton.setOnClickListener(new OnClickListener() {
			@Override
			public void onClick(Button button) {
				System.out.println(button.getName() +" Clicked! - anonymous class");
			}
		}); 
        
        // click the button
    	myButton.click();
    }
}

3. Use Lambda

public class App
{
    public static void main( String[] args ){...}

    public void run(){
        
        Button myButton = new Button();
        myButton.setName("MyButton");

        // 3. lambda
    	OnClickListener lambda = button -> { System.out.println(button.getName()+" Clicked! - lambda"); } ;
    	myButton.setOnClickListener(lambda);
		
        // click the button
    	myButton.click();
    } 
}

To run the code use

java-lambda>mvn exec:java