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Thread: need corection on java code

  1. #1
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    Default need corection on java code

    Hi :

    I m working on java eclipse I want to move one selected item from list to texTarea but when I execute this code I have got following error :



    PHP Code:

    Exception in thread 
    "AWT-EventQueue-0" java.lang.NullPointerException
        at listtest
    .testlist$3.actionPerformed(testlist.java:84)
        
    at javax.swing.AbstractButton.fireActionPerformed(Unknown Source
    here the whole code :

     
    package listtest;
     
    import java.awt.EventQueue;
     
    import javax.swing.JFrame;
    import javax.swing.JPanel;
    import javax.swing.JList;
    import javax.swing.JScrollPane;
    import javax.swing.ScrollPaneConstants;
    import javax.swing.AbstractListModel;
    import javax.swing.JButton;
    import javax.swing.JTextArea;
    import java.awt.event.ActionListener;
    import java.awt.event.ActionEvent;
     
    public class testlist {
     
    	private JFrame frame;
     
     
    	/**
    	 * Launch the application.
    	 */
    	public static void main(String[] args) {
    		EventQueue.invokeLater(new Runnable() {
    			public void run() {
    				try {
    					testlist window = new testlist();
    					window.frame.setVisible(true);
    				} catch (Exception e) {
    					e.printStackTrace();
    				}
    			}
    		});
    	}
     
    	/**
    	 * Create the application.
    	 */
    	public testlist() {
    		initialize();
    	}
     
    	/**
    	 * Initialize the contents of the frame.
    	 */
    	private void initialize() {
    		frame = new JFrame();
    		frame.setBounds(100, 100, 490, 422);
    		frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    		frame.getContentPane().setLayout(null);
     
    		JPanel panel = new JPanel();
    		panel.setBounds(0, 0, 464, 384);
    		frame.getContentPane().add(panel);
    		panel.setLayout(null);
     
    		JScrollPane scrollPane = new JScrollPane();
    		scrollPane.setVerticalScrollBarPolicy(ScrollPaneConstants.VERTICAL_SCROLLBAR_ALWAYS);
    		scrollPane.setHorizontalScrollBarPolicy(ScrollPaneConstants.HORIZONTAL_SCROLLBAR_ALWAYS);
    		scrollPane.setBounds(121, 69, 77, 120);
    		panel.add(scrollPane);
     
    		final JList list = new JList();
    		list.setModel(new AbstractListModel() {
    			String[] values = new String[] {"1", "2", "3"};
    			public int getSize() {
    				return values.length;
    			}
    			public Object getElementAt(int index) {
    				return values[index];
    			}
    		});
    		scrollPane.setViewportView(list);
     
    		JButton btnNewButton = new JButton("New button");
    		btnNewButton.addActionListener(new ActionListener() {
     
     
    			private JTextArea textArea;
     
    			public void actionPerformed(ActionEvent arg0) {
     
    				textArea.append((String) list.getSelectedValue());
    			}
    		});
    		btnNewButton.setBounds(115, 216, 89, 23);
    		panel.add(btnNewButton);
     
    		JScrollPane scrollPane_1 = new JScrollPane();
    		scrollPane_1.setVerticalScrollBarPolicy(ScrollPaneConstants.VERTICAL_SCROLLBAR_ALWAYS);
    		scrollPane_1.setHorizontalScrollBarPolicy(ScrollPaneConstants.HORIZONTAL_SCROLLBAR_ALWAYS);
    		scrollPane_1.setBounds(128, 265, 73, 108);
    		panel.add(scrollPane_1);
     
    		JTextArea textArea = new JTextArea();
    		scrollPane_1.setViewportView(textArea);
    	}
    }

    thanks to correct me .

  2. #2
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    Default Re: need corection on java code

    Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at listtest.testlist$3.actionPerformed(testlist.java: 84)
    There is a null value in a variable when line 84 is executed. Look at line 84, find the variable with the null value and then backtrack in the code to see why that variable does not have a valid value when line 84 is executed.

  3. #3
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    Default Re: need corection on java code

    hi:

    may be the problem in this lines :


    final JList<?> list = new JList();
    list.setModel(new AbstractListModel() {
    String[] values = new String[] {"1", "2", "3"};
    public int getSize() {
    return values.length;
    }
    public Object getElementAt(int index) {
    return values[index];
    }
    });

  4. #4
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    Default Re: need corection on java code

    What variable has the null value? Add a print statement just before line 84 that prints the contents of all the variables used on line 84 so you can see where the null value is.

  5. #5
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    Default Re: need corection on java code

    I have added bellow code but it always point to line 84 .

    [QUOTE]

    String data = "";

    data = (String) list.getSelectedValue();

    System.out.println(data);


    [QUOTE]

    in console mode only message error is printed
    Attached Images Attached Images

  6. #6
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    Default Re: need corection on java code

    Hi :

    if I remove this line it works but I need textarea

    I can't find a solution

    jlisttest2.PNG

    thanks

  7. #7
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    Default Re: need corection on java code

    Did you try printing the values of the variables used on line 84 to find which one had the null value?

    You need to find that variable so the code can be changed so the variable has a valid value.

  8. #8
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    Default Re: need corection on java code

    hi:

    I'm working on eclipse MARS.1

    can you an example from your side .

    the error still pending

    thanks

  9. #9
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    Default Re: need corection on java code

    Here's an example of what you need to do:
      System.out.println("var1="+var1 +", var2="+var2); //  print the variables used on the next line
      var1.xyz =  var2.y();  // two variables could cause NPE

    Find line 84, get the names of the variables used there and print their values as shown above.

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