Event and Listener

  • Changing the state of an object is known as an event.


  • Steps to perform Event Handling Following steps are required to perform event handling:


  • Register the component with the Listener


  • Registration Methods For registering the component with the Listener, many classes provide the registration methods. For example:


  • Button


    • public void addActionListener(ActionListener a){}


  • MenuItem


    • public void addActionListener(ActionListener a){}


  • TextField


    • public void addActionListener(ActionListener a){}


    • public void addTextListener(TextListener a){}


  • TextArea


    • public void addTextListener(TextListener a){}


  • Checkbox


    • public void addItemListener(ItemListener a){}


  • Choice


    • public void addItemListener(ItemListener a){}


  • List


    • public void addActionListener(ActionListener a){}


    • public void addItemListener(ItemListener a){}


Java Event Handling Code 

     
import java.awt.*;  
import java.awt.event.*;  
class AEvent extends Frame implements ActionListener{  
TextField tf;  
AEvent(){  
  
//create components  
tf=new TextField();  
tf.setBounds(60,50,170,20);  
Button b=new Button("click me");  
b.setBounds(100,120,80,30);  
  
//register listener  
b.addActionListener(this);//passing current instance  
  
//add components and set size, layout and visibility  
add(b);add(tf);  
setSize(300,300);  
setLayout(null);  
setVisible(true);  
}  
public void actionPerformed(ActionEvent e){  
tf.setText("Welcome");  
}  
public static void main(String args[]){  
new AEvent();  
}  
}  



     
import java.awt.*;  
import java.awt.event.*;  
class AEvent2 extends Frame{  
TextField tf;  
AEvent2(){  
//create components  
tf=new TextField();  
tf.setBounds(60,50,170,20);  
Button b=new Button("click me");  
b.setBounds(100,120,80,30);  
//register listener  
Outer o=new Outer(this);  
b.addActionListener(o);//passing outer class instance  
//add components and set size, layout and visibility  
add(b);add(tf);  
setSize(300,300);  
setLayout(null);  
setVisible(true);  
}  
public static void main(String args[]){  
new AEvent2();  
}  
}  
import java.awt.event.*;  
class Outer implements ActionListener{  
AEvent2 obj;  
Outer(AEvent2 obj){  
this.obj=obj;  
}  
public void actionPerformed(ActionEvent e){  
obj.tf.setText("welcome");  
}  
}  


     
import java.awt.*;  
import java.awt.event.*;  
class AEvent3 extends Frame{  
TextField tf;  
AEvent3(){  
tf=new TextField();  
tf.setBounds(60,50,170,20);  
Button b=new Button("click me");  
b.setBounds(50,120,80,30);  
  
b.addActionListener(new ActionListener(){  
public void actionPerformed(){  
tf.setText("hello");  
}  
});  
add(b);add(tf);  
setSize(300,300);  
setLayout(null);  
setVisible(true);  
}  
public static void main(String args[]){  
new AEvent3();  
}  
}

Java AWT Button

  • The button class is used to create a labeled button that has platform independent implementation. 


    •  
import java.awt.*;  
public class ButtonExample {  
public static void main(String[] args) {  
    Frame f=new Frame("Button Example");  
    Button b=new Button("Click Here");  
    b.setBounds(50,100,80,30);  
    f.add(b);  
    f.setSize(400,400);  
    f.setLayout(null);  
    f.setVisible(true);   
}  
}

  • Java AWT Button Example with ActionListener 


    •  
import java.awt.*;  
import java.awt.event.*;  
public class ButtonExample {  
public static void main(String[] args) {  
    Frame f=new Frame("Button Example");  
    final TextField tf=new TextField();  
    tf.setBounds(50,50, 150,20);  
    Button b=new Button("Click Here");  
    b.setBounds(50,100,60,30);  
    b.addActionListener(new ActionListener(){  
    public void actionPerformed(ActionEvent e){  
            tf.setText("Welcome to Javatpoint.");  
        }  
    });  
    f.add(b);f.add(tf);  
    f.setSize(400,400);  
    f.setLayout(null);  
    f.setVisible(true);   
}  
}