How to add action listener that listens to multiple buttons

Question

I\'m trying to figure out what i am doing wrong with action listeners. I\'m following multiple tutorials and yet netbeans and eclipse are giving me errors when im trying to

Answer 1

There are good answers here but let me address the more global point of adding action listener that listens to multiple buttons.

There are two popular approaches.

Using a Common Action Listener

You can get the source of the action in your actionPerformed(ActionEvent e) implementation:

JButton button1, button2; //your button

@Override
public void actionPerformed(ActionEvent e) {

    JButton actionSource = (JButton) e.getSource();

    if(actionSource.equals(button1)){
        // YOU BUTTON 1 CODE HERE
    } else if (actionSource.equals(button2)) {
        // YOU BUTTON 2 CODE HERE
    }
}

Using ActionCommand

With this approach you setting the actionCommand field of your button which later will allow you to use switch:

button1.setActionCommand("actionName1");
button2.setActionCommand("actionName2");

And later:

@Override
public void actionPerformed(ActionEvent e) {
    String actionCommand = ((JButton) e.getSource()).getActionCommand();

    switch (actionCommand) {
        case "actionName1": 
            // YOU BUTTON 1 CODE HERE
        break;
        case "actionName2": 
            // YOU BUTTON 2 CODE HERE
        break;
    }
}

Check out to learn more about JFrame Buttons, Listeners and Fields.

Answer 2

You've been told how to sort your immediate problem, but I think there are more important problems here.

• stick with conventions. Even for throw-away code. That means initial cases for class names.

• Don't extend classes you don't need to. JFrame should rarely be extended. In fact, you don't create an instance of your derived class!!!

• Don't bundle a bunch of things into one class. In particular, you should generally only subtype at most one main class or interface at a time (things like Comparable not included).

• Always interact, including construct, Swing/AWT GUIs on the AWT Event Dispatch Thread (EDT). It's ugly and verbose, but that's Java for you.

• Checking a source of an event is a bit of hack. Listeners are small, so you can't even claim the lame performance excuse.

So:

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


public class Calc {
    public static void main(String[] args) {
        java.awt.EventQueue.invokeLater(new Runnable() { public void run() {
            runEDT();
        }});
    }
    private static void runEDT() {
        assert java.awt.EventQueue.isDispatchThread();

        JFrame frame = new JFrame();

        frame.setSize(100, 100);

        JButton button1 = new JButton("1");
        button1.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent event) {
                ...
            }
        });

        frame.add(button1);

        frame.setVisible(true);
    }
}

If you need to access any of the variables from the enclosing method within the listener, make them final.

Answer 3

You are declaring button1 in main method so you can not access it in actionPerform. You should make it global in class.

 JButton button1;
 public static void main(String[] args) {

    JFrame calcFrame = new JFrame();

    calcFrame.setSize(100, 100);
    calcFrame.setVisible(true);

    button1 = new JButton("1");
    button1.addActionListener(this);

    calcFrame.add(button1);
}

public void actionPerformed(ActionEvent e) {
    if(e.getSource() == button1)
}

Answer 4

Here is a modified form of the source based on my comment. Note that GUIs should be constructed & updated on the EDT, though I did not go that far.

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

public class Calc {

    public static void main(String[] args) {

        JFrame calcFrame = new JFrame();

        // usually a good idea.
        calcFrame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);

        final JButton button1 = new JButton("1");
        button1.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent ae) {
                JOptionPane.showMessageDialog(
                    button1, "..is the loneliest number");
            }
        });

        calcFrame.add(button1);

        // don't do this..
        // calcFrame.setSize(100, 100);

        // important!
        calcFrame.pack();

        calcFrame.setVisible(true);
    }
}

Answer 5

I'm amazed that nobody has mentioned using an action command. This is a pretty standard way of associating sources and listeners. Its really useful if;

• you have multiple event sources that need to do the same thing (eg if you want the use to be able to press the enter key on a text field as an alternative to clicking a button next to it)
• you don't have a ref to the component generating the event

see;

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

public class DontExtendJFrame implements ActionListener {

  private enum Actions {
    HELLO,
    GOODBYE
  }

  public static void main(String[] args) {

    DontExtendJFrame instance = new DontExtendJFrame();

    JFrame frame = new JFrame("Test");
    frame.setLayout(new FlowLayout());
    frame.setSize(200, 100);

    JButton hello = new JButton("Hello");
    hello.setActionCommand(Actions.HELLO.name());
    hello.addActionListener(instance);
    frame.add(hello);

    JButton goodbye = new JButton("Goodbye");
    goodbye.setActionCommand(Actions.GOODBYE.name());
    goodbye.addActionListener(instance);
    frame.add(goodbye);

    frame.setVisible(true);
  }

  @Override
  public void actionPerformed(ActionEvent evt) {
    if (evt.getActionCommand() == Actions.HELLO.name()) {
      JOptionPane.showMessageDialog(null, "Hello");
    } else if (evt.getActionCommand() == Actions.GOODBYE.name()) {
      JOptionPane.showMessageDialog(null, "Goodbye");
    }
  }
}