Java Background Color Changing Buttons

Question

I'm trying to code a program using BlueJ Java Editor and the point of this experiment is to change the background color of a GUI when a button is pressed. I have all buttons and action methods implemented that I believe I need. However, when I press the button to change the background color of the GUI, a window pops up telling me my program will not work. This is what I have right now:

import java.awt.Color;

import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JButton;

import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;

public class SimpleGui extends JFrame implements ActionListener {

    JPanel panel;
    JButton blue, green, red, yellow;

    SimpleGui() {

        super("Testing");

        init();
        this.setExtendedState(JFrame.MAXIMIZED_BOTH);
        this.setVisible(true);

    }//end SimpleGui

    public void init() {

        JPanel panel = new JPanel();

        blue = new JButton("Blue");
        green = new JButton("Green");
        red = new JButton("Red");
        yellow = new JButton("Yellow");

        blue.addActionListener(this);
        green.addActionListener(this);
        red.addActionListener(this);
        yellow.addActionListener(this);

        panel.add(blue);
        panel.add(green);
        panel.add(red);
        panel.add(yellow);

        this.add(panel);

    } //end void

    @Override
    public void actionPerformed(ActionEvent event) {

        if(event.getSource() == blue) {

            panel.setBackground(Color.BLUE);

        } else if(event.getSource() == green) {

            panel.setBackground(Color.GREEN);

        } else if(event.getSource() == red) {

            panel.setBackground(Color.RED);

        } else {

            panel.setBackground(Color.YELLOW);

        }

    }
}

I have another class that is used to execute the program and opens the GUI. Now when I execute the program I get a console full of different errors and the program does not work as expected. How can I fix this and why doesn't this work?

Errors:

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at SimpleGUI.SimpleGui.actionPerformed(SimpleGui.java:59)
    at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2022)
    at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2346)
    at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:402)
    at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:259)
    at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:252)
    at java.awt.Component.processMouseEvent(Component.java:6525)
    at javax.swing.JComponent.processMouseEvent(JComponent.java:3324)
    at java.awt.Component.processEvent(Component.java:6290)
    at java.awt.Container.processEvent(Container.java:2234)
    at java.awt.Component.dispatchEventImpl(Component.java:4881)
    at java.awt.Container.dispatchEventImpl(Container.java:2292)
    at java.awt.Component.dispatchEvent(Component.java:4703)
    at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4898)
    at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4533)
    at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4462)
    at java.awt.Container.dispatchEventImpl(Container.java:2278)
    at java.awt.Window.dispatchEventImpl(Window.java:2750)
    at java.awt.Component.dispatchEvent(Component.java:4703)
    at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:751)
    at java.awt.EventQueue.access$500(EventQueue.java:97)
    at java.awt.EventQueue$3.run(EventQueue.java:702)
    at java.awt.EventQueue$3.run(EventQueue.java:696)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:75)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:86)
    at java.awt.EventQueue$4.run(EventQueue.java:724)
    at java.awt.EventQueue$4.run(EventQueue.java:722)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:75)
    at java.awt.EventQueue.dispatchEvent(EventQueue.java:721)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:201)
    at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:116)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:105)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:93)
    at java.awt.EventDispatchThread.run(EventDispatchThread.java:82)

Post the error your getting. Also remove the android tag on your question. — ziondreamt, Aug 06 '16 at 07:38

score 2 · Accepted Answer · answered Aug 06 '16 at 18:06

2

You're never initializing the class member JPanel panel;, instead you are creating a new local variable with JPanel panel = new JPanel();.

This causes the line panel.setBackground(..) to be executed with panel being null.

answered Aug 06 '16 at 18:06

Kiskae

24,655
2
77
74

Thank you very much for your help :) – Reddish Aug 06 '16 at 18:27

score -1 · Answer 2 · answered Aug 06 '16 at 18:15

-1

Have you tried Separating the events for each button in a separate class .. Each implementing ActionListner

answered Aug 06 '16 at 18:15

Marvin Jude

37
7

score -1 · Answer 3 · answered Aug 06 '16 at 18:21

-1

Also Create an instance of that class when you wanna add actionListner to Each Component(button...)

answered Aug 06 '16 at 18:21

Marvin Jude

37
7

Java Background Color Changing Buttons

3 Answers3