Java – Event Listeners

Event Listeners:

• Event is an action performed on component.
• When action performed, an object will be created called Event object.
• A listener should collect the event object and handle it.
• java.awt.event providing listener interfaces.
• “Listener” interfaces need to be implemented to defined action logic.

interface ActionListener
{
	void actionPerformed(ActionEvent ae);
}

We need to implement above interface and define action logic in actionPerformed method.

import javax.swing.JFrame;
import javax.swing.JButton;
import java.awt.FlowLayout;
import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;
class MyFrame extends JFrame
{
	static JButton b ;
	MyFrame()
	{
		this.setTitle("First Frame");
		this.setSize(400,300);

		FlowLayout flow = new FlowLayout();
		this.setLayout(flow);

		b = new JButton("Click Me");
		this.add(b);
		b.addActionListener(new ActionLogic());
	}
}

class ActionLogic implements ActionListener
{
	public void actionPerformed(ActionEvent ae)
	{
		System.out.println("Button clicked...");
	}
}
class GUI 
{
	public static void main(String[] args) 
	{
		MyFrame frame = new MyFrame();
		frame.setVisible(true); 
	}
}

Implementing ActionListener using anonymous inner class:

• In the above application, we write action logic in different class.
• Defining action logics with different classes increase the class files count in application.
• To avoid, we use anonymous inner class.

import javax.swing.JFrame;
import javax.swing.JButton;
import java.awt.FlowLayout;
import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;

class MyFrame extends JFrame
{
	static JButton b ;
	MyFrame()
	{
		this.setTitle("First Frame");
		this.setSize(400,300);

		FlowLayout flow = new FlowLayout();
		this.setLayout(flow);

		b = new JButton("Click Me");
		this.add(b);
		b.addActionListener(new ActionListener()
		{
			public void actionPerformed(ActionEvent ae)
			{
				System.out.println("Button clicked - anonymous");
			}
		});
	}
}

class GUI 
{
	public static void main(String[] args) 
	{
		MyFrame frame = new MyFrame();
		frame.setVisible(true); 
	}
}