ActionListener上的java.lang.NullPointerException | Java秋千

Question

I do apologize in advance for creating this question, I do .NET so I know "how it feel" to ask this question, but I did search and tried to debug my program in Eclipse and still could not figure-out how to fix it, since I am totally new to Java GUI and first time using Eclipse, so ...

I have small program Java swing GUI program, where you have 2 buttons, one of them is hidden (visible set to false) when click on a button, it will shows the hidden button (set Visible to true):

import java.awt.Container;
import java.awt.FlowLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.WindowAdapter;
import java.awt.event.WindowEvent;

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

public class TestButton extends JPanel {

    private JFrame mainFrame;
    private JButton btnShow ;
    private JButton btnNew;

    public TestButton() {
        mainFrame = new JFrame("Test Button");

        JButton btnShow = new JButton("Show New Button");
        JButton btnNew = new JButton("This is New Button");

        Container c = mainFrame.getContentPane();
        c.setLayout(new FlowLayout());

        c.add(btnShow);
        c.add(btnNew);
        btnNew.setVisible(false);

        btnShow.setMnemonic('G');
        btnNew.setMnemonic('N');

        mainFrame.addWindowListener(new WindowAdapter() {
            public void windowClosing(WindowEvent e) {
                System.exit(0);
            }
        });

        ShowButtonHandler ghandler = new ShowButtonHandler();
        btnShow.addActionListener(ghandler);

        mainFrame.setSize(250, 150);
        mainFrame.setLocationRelativeTo(null);
        mainFrame.setResizable(false);
        mainFrame.setVisible(true);
    }

    class ShowButtonHandler implements ActionListener {
        public void actionPerformed(ActionEvent e) {
            btnNew.setVisible(true);
        }
    }

    public static void main(String args[]) {
        TestButton app = new TestButton();
    }
}

But when I click that button, it shows Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException

The error was at this line: btnNew.setVisible(true);

Here is the full trace:

 Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at TestButton$ShowButtonHandler.actionPerformed(TestButton.java:51)
    at javax.swing.AbstractButton.fireActionPerformed(Unknown Source)
    at javax.swing.AbstractButton$Handler.actionPerformed(Unknown Source)
    at javax.swing.DefaultButtonModel.fireActionPerformed(Unknown Source)
    at javax.swing.DefaultButtonModel.setPressed(Unknown Source)
    at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(Unknown Source)
    at java.awt.Component.processMouseEvent(Unknown Source)
    at javax.swing.JComponent.processMouseEvent(Unknown Source)
    at java.awt.Component.processEvent(Unknown Source)
    at java.awt.Container.processEvent(Unknown Source)
    at java.awt.Component.dispatchEventImpl(Unknown Source)
    at java.awt.Container.dispatchEventImpl(Unknown Source)
    at java.awt.Component.dispatchEvent(Unknown Source)
    at java.awt.LightweightDispatcher.retargetMouseEvent(Unknown Source)
    at java.awt.LightweightDispatcher.processMouseEvent(Unknown Source)
    at java.awt.LightweightDispatcher.dispatchEvent(Unknown Source)
    at java.awt.Container.dispatchEventImpl(Unknown Source)
    at java.awt.Window.dispatchEventImpl(Unknown Source)
    at java.awt.Component.dispatchEvent(Unknown Source)
    at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
    at java.awt.EventQueue.access$400(Unknown Source)
    at java.awt.EventQueue$3.run(Unknown Source)
    at java.awt.EventQueue$3.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
    at java.awt.EventQueue$4.run(Unknown Source)
    at java.awt.EventQueue$4.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
    at java.awt.EventQueue.dispatchEvent(Unknown Source)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
    at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.run(Unknown Source)

So normally Null Exception happens when object was not initialized, but I did initialized the btnNew when declaring it, didn't I? Where is the problem here?

Also, I want to add 1 more button called "Run again" where you click on it, it closes current windows and open the new one (Basically I want re run program again) , is it possible that way or how to I archive it?

Answer 1

Your local variable is hiding the instance variable

Change

JButton btnNew = new JButton("This is New Button");

to

btnNew = new JButton("This is New Button");

EDIT

According to your question as a comment... but it would be better to ask a new question or post your code at https://codereview.stackexchange.com/ .

What I mean is that your inner class ShowButtonHandler depends on the outer class TestButton , because it uses the outer class's field btnNew .

class ShowButtonHandler implements ActionListener {
    public void actionPerformed(ActionEvent e) {
        btnNew.setVisible(true);
    }
}

But this dependency is not necessary. The ShowButtonHandler only needs a reference to a JButton that it must set visible when an action is performed.

So in a first step you can break the dependency to the outer class by simply pass the button as a constructor argument.

class ShowButtonHandler implements ActionListener {

    private JButton btnNew;

    public ShowButtonHandler(JButton btnNew){
        this.btnNew = btnNew;
    }

    public void actionPerformed(ActionEvent e) {
        btnNew.setVisible(true);
    }
}

Now you realize that the ShowButtonHandler could be more flexible to allow reuse. You take a look at the class hierarchy and you see that setVisible can be done for any JComponent . So you can make the class more general.

class ShowComponentHandler implements ActionListener {

    private JComponent component;

    public ShowComponentHandler(JComponent component){
        this.component = component;
    }

    public void actionPerformed(ActionEvent e) {
        component.setVisible(true);
    }
}

Since the ShowButtonHandler is now independent and has a more general API it can be placed in an own compilation unit (java file) and be re-used.

In your TestButton class you can still use it

ActionListener showComponentAction = new ShowComponentHandler(btnNew);
btnShow.addActionListener(showComponentAction);