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Thread: Swing Help

  1. #1
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    Default Swing Help

    package gui;
     
    import java.awt.*;
    import java.awt.event.*;
     
    import javax.swing.*;
     
    public class javaFP extends JApplet implements ActionListener{
    	JButton b;
    	JButton b1;
    	JButton b2;
    	JTextField field;
    	public String text;
    	public void init(){
    		try{
    		SwingUtilities.invokeAndWait(new Runnable(){
    			public void run(){
    				swingGUI();
    			}
    		});
    	}
    		catch(Exception e){
    			System.out.println(e);
    		}
    	}
    	public void swingGUI(){
     
    		/*b= new JButton("Button 1"); 
    		b.addActionListener(
    				new ActionListener() {
    					public void actionPerformed(ActionEvent e) {
    						field.setText("Button 1");
    					}
    				}
    				);
    		b1= new JButton("Button 2"); 
    		b1.addActionListener(
    				new ActionListener() {
    					public void actionPerformed(ActionEvent e) {
    						field.setText("Button 2");
    					}
    				}
    				);*/
    		JButton b = new JButton(new AbstractAction ("Button 1") {
    			 public void actionPerformed( ActionEvent e ) {
    				 field.setText("Button 1");
    			 }
    		});
     
    		JButton b1 = new JButton(new AbstractAction ("Button 2") {
    			 public void actionPerformed( ActionEvent e ) {
    				 field.setText("Button 2");
    			 }
    		});
     
    		JButton b2 = new JButton(new AbstractAction ("Button 3") {
    			 public void actionPerformed( ActionEvent e ) {
    				 field.setText("Button 3");
    			 }
    		});
     
    		JTextField field = new JTextField(20);
    		setLayout(new FlowLayout());
    		setSize(300, 250);
    		setVisible(true);
    		add(b);
    		add(b1);
    		add(b2);
    		add(field);
    		field.setEditable(true);
    		b.addActionListener(this);
    		b1.addActionListener(this);
    		b2.addActionListener(this);
     
    	}
    	@Override
    	public void actionPerformed(ActionEvent e) {
    		// TODO Auto-generated method stub
     
    	}
    }


    Using this code I keep getting an error. The way this is supposed to work is that when I click on any button, 1 2 or 3, it puts the name of the button into the TextField. When I click on a button all I get is this:

    Exception in thread "AWT-EventQueue-1" java.lang.NullPointerException
    at gui.javaFP$2.actionPerformed(javaFP.java:46)
    at javax.swing.AbstractButton.fireActionPerformed(Abs tractButton.java:2018)
    at javax.swing.AbstractButton$Handler.actionPerformed (AbstractButton.java:2341)
    at javax.swing.DefaultButtonModel.fireActionPerformed (DefaultButtonModel.java:402)
    at javax.swing.DefaultButtonModel.setPressed(DefaultB uttonModel.java:259)
    at javax.swing.plaf.basic.BasicButtonListener.mouseRe leased(BasicButtonListener.java:252)
    at java.awt.Component.processMouseEvent(Component.jav a:6505)
    at javax.swing.JComponent.processMouseEvent(JComponen t.java:3321)
    at java.awt.Component.processEvent(Component.java:627 0)
    at java.awt.Container.processEvent(Container.java:222 9)
    at java.awt.Component.dispatchEventImpl(Component.jav a:4861)
    at java.awt.Container.dispatchEventImpl(Container.jav a:2287)
    at java.awt.Component.dispatchEvent(Component.java:46 87)
    at java.awt.LightweightDispatcher.retargetMouseEvent( Container.java:4832)
    at java.awt.LightweightDispatcher.processMouseEvent(C ontainer.java:4492)
    at java.awt.LightweightDispatcher.dispatchEvent(Conta iner.java:4422)
    at java.awt.Container.dispatchEventImpl(Container.jav a:2273)
    at java.awt.Component.dispatchEvent(Component.java:46 87)
    at java.awt.EventQueue.dispatchEventImpl(EventQueue.j ava:703)
    at java.awt.EventQueue.access$000(EventQueue.java:102 )
    at java.awt.EventQueue$3.run(EventQueue.java:662)
    at java.awt.EventQueue$3.run(EventQueue.java:660)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$1.doIntersectionPri vilege(ProtectionDomain.java:76)
    at java.security.ProtectionDomain$1.doIntersectionPri vilege(ProtectionDomain.java:87)
    at java.awt.EventQueue$4.run(EventQueue.java:676)
    at java.awt.EventQueue$4.run(EventQueue.java:674)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$1.doIntersectionPri vilege(ProtectionDomain.java:76)
    at java.awt.EventQueue.dispatchEvent(EventQueue.java: 673)
    at java.awt.EventDispatchThread.pumpOneEventForFilter s(EventDispatchThread.java:244)
    at java.awt.EventDispatchThread.pumpEventsForFilter(E ventDispatchThread.java:163)
    at java.awt.EventDispatchThread.pumpEventsForHierarch y(EventDispatchThread.java:151)
    at java.awt.EventDispatchThread.pumpEvents(EventDispa tchThread.java:147)
    at java.awt.EventDispatchThread.pumpEvents(EventDispa tchThread.java:139)
    at java.awt.EventDispatchThread.run(EventDispatchThre ad.java:97)
    Last edited by ByteSized; June 20th, 2012 at 04:02 PM.


  2. #2
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    Default Re: Swing Help

    Put this line before your actionListeners are coded.

    JTextField field = new JTextField(20);

    Right now field is null when it's going into them.

  3. #3
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    Default Re: Swing Help

    package gui;
     
    import java.awt.*;
    import java.awt.event.*;
     
    import javax.swing.*;
     
    public class javaFP extends JApplet implements ActionListener{
    	JButton b;
    	JButton b1;
    	JButton b2;
    	public String text;
    	public void init(){
    		try{
    		SwingUtilities.invokeAndWait(new Runnable(){
    			public void run(){
    				swingGUI();
    			}
    		});
    	}
    		catch(Exception e){
    			System.out.println(e);
    		}
    	}
    	public void swingGUI(){
    		JTextField field = new JTextField(20);
    		JButton b = new JButton(new AbstractAction ("Button 1") {
    			 public void actionPerformed( ActionEvent e ) {
    				 JTextField field = new JTextField(20);
    				 field.setText("Button 1");
    			 }
    		});
     
    		JButton b1 = new JButton(new AbstractAction ("Button 2") {
    			 public void actionPerformed( ActionEvent e ) {
    				 JTextField field = new JTextField(20);
    				 field.setText("Button 2");
    			 }
    		});
     
    		JButton b2 = new JButton(new AbstractAction ("Button 3") {
    			 public void actionPerformed( ActionEvent e ) {
    				 JTextField field = new JTextField(20);
    				 field.setText("Button 3");
    			 }
    		});
     
    		setLayout(new FlowLayout());
    		setSize(300, 250);
    		setVisible(true);
    		add(b);
    		add(b1);
    		add(b2);
    		add(field);
    		field.setEditable(true);
    		b.addActionListener(this);
    		b1.addActionListener(this);
    		b2.addActionListener(this);
     
    	}
    	@Override
    	public void actionPerformed(ActionEvent e) {
    		// TODO Auto-generated method stub
     
    	}
    }

    Now when I click on my buttons nothing is added into the field.

  4. #4
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    Default Re: Swing Help

    JTextField field = new JTextField(20);
    field.setText("Button 1");

    That is because you are referring to the field object you created that only has the scope of the ActionListener.

    Also, either make field either a class variable or final as it will say that field needs to be declared final or something like that if it's being declared inside that method.

  5. #5
    Super Moderator Norm's Avatar
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    Default Re: Swing Help

    You have defined several variables with the same name local to methods that ALL shadow the variable that you want to assign the value to. Remove the definitions from all the methods and move the definition to the class level.
    If you don't understand my answer, don't ignore it, ask a question.

  6. #6
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    Default Re: Swing Help

    Ah okay. Thanks.

  7. #7
    Administrator copeg's Avatar
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    Default Re: Swing Help

    This thread has been cross posted here:

    http://www.java-forums.org/new-java/60845-settext-problems.html

    Although cross posting is allowed, for everyone's benefit, please read:

    Java Programming Forums Cross Posting Rules

    The Problems With Cross Posting


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