Two ActionListener

[Niyas Ahmed]

Hi,

I am learning the Java thru Head first Java book. Now I am in AWT. I have one doubt in the below pgm.

I have two buttons, each have to do a separate thing. We can't (implement) use the actionPerformed() twice in the same class. So for that, i have two classes for two buttons.

Note: I know there is one better way than this to solve it. That is Inner Class

I just want to do it other than Inner Class, just for exercise.

How could i fix the below pgm. How o give each of the listener classes a reference to the main class.(I think that is the problem)

import java.awt.*;
import java.awt.event.*;
 
public class Awtex1 implements ActionListener 
{
   Frame f; 
   Label l;
   Button b1,b2;
 
   public Awtex1()
   {
      f = new Frame("AWT Example");
      l = new Label("Hello");
      b1 = new Button("Click me");
      b2 = new Button("Click me also");
   }
 
   public void launchFrame()
   {
      f.add(b1,BorderLayout.SOUTH);
      f.add(b2,BorderLayout.EAST);
      f.add(l,BorderLayout.NORTH);
      f.setSize(200,300);
      f.setVisible(true);
      b1.addActionListener(new FirstButton());
      b2.addActionListener(new SecondButton());
   }
 
   public static void main(String args[])
   {
      Awtex1 awt = new Awtex1();
      awt.launchFrame();
   }
}
 
class FirstButton implements ActionListener
{
    public void actionPerformed(ActionEvent e)
    {
        b1.setLabel("It it First Button");
    }
}
 
class SecondButton implements ActionListener
{
    public void actionPerformed(ActionEvent e)
    {
        b2.setLabel("It it Second Button");
    }
}   

Error

Awtex1.java:40: cannot resolve symbol
symbol  : variable b1
location: class FirstButton
        b1.setLabel("It it First Button");
        ^
Awtex1.java:48: cannot resolve symbol
symbol  : variable b2
location: class SecondButton
        b2.setLabel("It it Second Button");
        ^
2 errors

[Joanne Neal]

You have two choices. You can leave your ActionPerformed methods in the button classes and use e.getSource() instead of b1/b2.
i.e.

class FirstButton implements ActionListener{
    public void actionPerformed(ActionEvent e)    
    {
        Button b = (Button ) e.getSource();
        b.setLabel("It it First Button");
    }
}

or you can put the ActionPerformed method in your main class and use e.getSource() to check which button was clicked.i.e.

public void actionPerformed(ActionEvent e)    
{
    Button b = (Button ) e.getSource();
    if (b == b1) {
        b.setLabel("It it First Button");
    } else {
        b.setLabel("It it Second Button");
    }
}

[Niyas Ahmed Sheikh]

Hi,
Thanks for your reply Joanne Neal. It is working.

Method 1:

import java.awt.*;
import java.awt.event.*;
 
public class Awtex1  
{
   Frame f; 
   Label l;
   Button b1,b2;
 
   public Awtex1()
   {
      f = new Frame("AWT Example");
      l = new Label("Hello");
      b1 = new Button("Click me");
      b2 = new Button("Click me also");
   }
 
   public void launchFrame()
   {
      f.add(b1,BorderLayout.SOUTH);
      f.add(b2,BorderLayout.EAST);
      f.add(l,BorderLayout.NORTH);
      f.setSize(200,300);
      f.setVisible(true);
      b1.addActionListener(new FirstButton());
      b2.addActionListener(new SecondButton());
   }
 
   public static void main(String args[])
   {
      Awtex1 awt = new Awtex1();
      awt.launchFrame();
   }
}
 
class FirstButton implements ActionListener
{
    public void actionPerformed(ActionEvent e)
    {
        Button b1 = (Button) e.getSource();
        b1.setLabel("It it First Button");
    }
}
 
class SecondButton implements ActionListener
{
    public void actionPerformed(ActionEvent e)
    {
        Button b2 = (Button) e.getSource();
        b2.setLabel("It it Second Button");
    }
}   

Method 2:

import java.awt.*;
import java.awt.event.*;
 
public class Awtex1  
{
   Frame f; 
   Label l;
   Button b1,b2;
 
   public Awtex1()
   {
      f = new Frame("AWT Example");
      l = new Label("Hello");
      b1 = new Button("Click me");
      b2 = new Button("Click me also");
   }
 
   public void launchFrame()
   {
      f.add(b1,BorderLayout.SOUTH);
      f.add(b2,BorderLayout.EAST);
      f.add(l,BorderLayout.NORTH);
      f.setSize(200,300);
      f.setVisible(true);
      b1.addActionListener(new FirstButton());
      b2.addActionListener(new SecondButton());
   }
 
   public void actionPerformed(ActionEvent e)    
   {    
        Button b1 = (Button ) e.getSource();    
        if (b1 == b2) 
        {        
            b1.setLabel("It it First Button");    
        }
        else 
        {       
            b2.setLabel("It it Second Button");    
        }
   }
   
    public static void main(String args[])
   {
      Awtex1 awt = new Awtex1();
      awt.launchFrame();
   }
}

Questions:

You can leave your ActionPerformed methods in the button classes and use e.getSource() instead of b1/b2.

How it is possible to use only 'b'. I have two button b1 and b2. I just confused with this. however in the code given by you I used b1 and b2(just see the Method 1 and Method 2).

Then what is getSource() method is used for. I tried to put System.out.println(e.getSource()) to know what it's having. But it is not printing anything.

[Joanne Neal]

Originally posted by Niyas Ahmed Sheikh:
How it is possible to use only 'b'. I have two button b1 and b2. I just confused with this. however in the code given by you I used b1 and b2(just see the Method 1 and Method 2).

Then what is getSource() method is used for. I tried to put System.out.println(e.getSource()) to know what it's having. But it is not printing anything.

e.getSource() returns a reference to the object that triggered the Action. So in this case it will be a reference to the button that you clicked. b1 and b2 also reference these buttons, so e.getSource() will be equal to either b1 or b2.

In method 2, the Awtex1 class should implement actionListener and you should pass an instance of the class to the button's addListener method.
i.e.

public class Awtex1 implements ActionListener
{
  // code snipped
  public void launchFrame()
  {
    // code snipped
    b1.addActionListener(this); 
    b2.addActionListener(this);
  }
  // code snipped
}

[Joanne Neal]

Originally posted by Niyas Ahmed Sheikh:

Method 2:

   public void actionPerformed(ActionEvent e)    
   {    
        Button b1 = (Button ) e.getSource();    
        if (b1 == b2) 
        {        
            b1.setLabel("It it First Button");    
        }
        else 
        {       
            b2.setLabel("It it Second Button");    
        }
   }

Just noticed also that this method should be

   public void actionPerformed(ActionEvent e)    
   {    
        Button b = (Button ) e.getSource();    
        if (b == b1) 
        {        
            b1.setLabel("It it First Button");    
        }
        else 
        {       
            b2.setLabel("It it Second Button");    
        }
   }

[Niyas Ahmed Sheikh]

Hi,

I am getting the strange output:

import java.awt.*;
import java.awt.event.*;
 
public class Awtex1  
{
   Frame f; 
   Label l;
   Button b1,b2;
 
   public Awtex1()
   {
      f = new Frame("AWT Example");
      l = new Label("Hello");
      b1 = new Button("Click me");
      b2 = new Button("Click me also");
   }
 
   public void launchFrame()
   {
      f.add(b1,BorderLayout.SOUTH);
      f.add(b2,BorderLayout.EAST);
      f.add(l,BorderLayout.NORTH);
      f.setSize(200,300);
      f.setVisible(true);
      b1.addActionListener(new FirstButton());
      b2.addActionListener(new SecondButton());
   }
 
   public void actionPerformed(ActionEvent e)    
   {    
        Button b = (Button ) e.getSource(); 
 
        if (b == b1) 
        {        
            b1.setLabel("It it First Button");    
        }
        else 
        {       
            b2.setLabel("It it Second Button");    
        }
        System.out.println(e.getSource());
   }
   
    public static void main(String args[])
   {
      Awtex1 awt = new Awtex1();
      awt.launchFrame();
   }
}

The above program is compiling & running without any problem. I am wondering how "new FirstButton()" and "new SecondButton()" is working without having that class.

import java.awt.*;
import java.awt.event.*;
 
public class Awtex1  
{
   Frame f; 
   Label l;
   Button b1,b2;
 
   public Awtex1()
   {
      f = new Frame("AWT Example");
      l = new Label("Hello");
      b1 = new Button("Click me");
      b2 = new Button("Click me also");
   }
 
   public void launchFrame()
   {
      f.add(b1,BorderLayout.SOUTH);
      f.add(b2,BorderLayout.EAST);
      f.add(l,BorderLayout.NORTH);
      f.setSize(200,300);
      f.setVisible(true);
      b1.addActionListener(this);
      b2.addActionListener(this);
   }
 
   public void actionPerformed(ActionEvent e)    
   {    
        Button b = (Button ) e.getSource(); 
 
        if (b == b1) 
        {        
            b1.setLabel("It it First Button");    
        }
        else 
        {       
            b2.setLabel("It it Second Button");    
        }
        System.out.println(e.getSource());
   }
   
    public static void main(String args[])
   {
      Awtex1 awt = new Awtex1();
      awt.launchFrame();
   }
}

Awtex1.java:25: addActionListener(java.awt.event.ActionListener) in java.awt.But
ton cannot be applied to (Awtex1)
b1.addActionListener(this);
^
Awtex1.java:26: addActionListener(java.awt.event.ActionListener) in java.awt.But
ton cannot be applied to (Awtex1)
b2.addActionListener(this);
^
2 errors

[Joanne Neal]

Awtex1 needs to implement ActionListener

[Mark Henryson]

Hi,
Once again thank you very much for your help!!

I want to see the flow of the program step by step, like in Visual Basic 6.0 we having checkpoint.

For java which IDE is best with the above option and other advanced options

[Joanne Neal]

Originally posted by Mark Henryson:
Hi,
Once again thank you very much for your help!!

I want to see the flow of the program step by step, like in Visual Basic 6.0 we having checkpoint.

For java which IDE is best with the above option and other advanced options

I think most IDEs offer a debugging option that allows you to step through the code and add breakpoints. As to which is best, it's a matter of taste. Try a few out and see which you like.