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Thread: Trouble with buttonhandler - Exception in thread "AWT-EventQueue-0"

  1. #1
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    Post Trouble with buttonhandler - Exception in thread "AWT-EventQueue-0"

    I get this Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    when i click the button Start, below is it's handler

    Can't understand why...Please help me!!!!!!!!!!!!!!!!!

    private void StartActionPerformed(java.awt.event.ActionEvent evt) {

    int numberOfButtons = a.length+ a2.length+2;
    System.out.println(numberOfButtons);

    jPanel1.setVisible(true);
    jPanel1.setEnabled(true);
    jPanel1.setBackground(Color.LIGHT_GRAY);
    jPanel1.setLayout(new FlowLayout(FlowLayout.CENTER));
    for(int i=0; i<numberOfButtons; i++)
    {
    JPanel p[] = new JPanel[numberOfButtons];
    JButton b[] = new JButton[numberOfButtons];
    p[i] = new JPanel();
    b[i] = new JButton();
    b[i].setEnabled(true);
    b[i].setVisible(true);
    b[i].setText("gf");
    b[i].setSize(50, 30);



    jPanel1.setLayout(new FlowLayout(FlowLayout.TRAILING));

    jPanel1.add(b[0]);
    if(i==1){
    b[1].setBounds(30,50,40,50); //1
    jPanel1.setBounds(300, 300, 100, 100);
    jPanel1.add(b[1]);

    b[i].addActionListener(new ActionListener() {

    public void actionPerformed(ActionEvent e)
    {
    JDialog dwindow = new JDialog();
    dwindow.setEnabled(true);
    dwindow.setVisible(true);
    dwindow.setSize(200, 120);
    }

    });



    }//end of for

    }


  2. #2
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    Default Re: Trouble with buttonhandler - Exception in thread "AWT-EventQueue-0"

    What line is the error on? What is happening on that line? Something must be null there- where do you initialize each of the variables you're dereferencing?
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    Default Re: Trouble with buttonhandler - Exception in thread "AWT-EventQueue-0"

    in this line is the error :jPanel1.add(b[0]);
    i want to put a button into jPanel1 ...when I use it outside the for there is no problem
    Somehow there is a problem in the for loop

  4. #4
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    Default Re: Trouble with buttonhandler - Exception in thread "AWT-EventQueue-0"

    You still haven't posted the actual error- what is the actual Exception? You also haven't used highlight tags, which makes your code pretty hard to read.

    Hint: What is the value of each element of the array when you're trying to use it? Use some print statements to check your assumptions.
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    Default Re: Trouble with buttonhandler - Exception in thread "AWT-EventQueue-0"

    this the error : Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    in this line : jPanel1.add(b[0]);
    the line in blue

    start is a button and bellow is it's actionPerformed event

    private void StartActionPerformed(java.awt.event.ActionEvent evt)
    {

    int numberOfButtons = a2;
    System.out.println(numberOfButtons);

    jPanel1.setVisible(true);
    jPanel1.setEnabled(true);
    jPanel1.setBackground(Color.LIGHT_GRAY);
    jPanel1.setLayout(new FlowLayout(FlowLayout.CENTER));

    for(int i=0; i<numberOfButtons; i++)
    {
    JPanel p[] = new JPanel[numberOfButtons];
    JButton b[] = new JButton[numberOfButtons];
    p[i] = new JPanel();
    b[i] = new JButton();
    b[i].setEnabled(true);
    b[i].setVisible(true);
    b[i].setText("gf");
    b[i].setSize(50, 30);

    jPanel1.add(b[0]);


    b[i].addActionListener(new ActionListener() {

    public void actionPerformed(ActionEvent e)
    {
    JDialog dwindow = new JDialog();
    dwindow.setEnabled(true);
    dwindow.setVisible(true);
    dwindow.setSize(200, 120);
    }

    });



    }//end of for

    }

  6. #6
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    Default Re: Trouble with buttonhandler - Exception in thread "AWT-EventQueue-0"

    Ah, so it's a NullPointerException. Now your job is to figure out what's null in that line. It could either be jPanel1 or b[0], right? So what is the value of each? Are you sure? Step through it with a debugger or add some print statements to test your assumptions.
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    kathaki88 (October 10th, 2012)

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    Default Re: Trouble with buttonhandler - Exception in thread "AWT-EventQueue-0"

    Thanks KevinWorkman!!
    Yes as I checked the b[0] is null. But why?? I create the buttons
    " JButton b[] = new JButton[numberOfButtons];

    b[i] = new JButton();
    b[i].setEnabled(true);
    b[i].setVisible(true);
    b[i].setText("gf");
    b[i].setSize(50, 30);

    "

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    Default Re: Trouble with buttonhandler - Exception in thread "AWT-EventQueue-0"

    You seem to be declaring your array inside the for loop that is supposed to fill it? Don't do that, but instead declare it *before* the for loop. Don't try to manipulate b[0] from within the for loop either.

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    Default Re: Trouble with buttonhandler - Exception in thread "AWT-EventQueue-0"

    Quote Originally Posted by curmudgeon View Post
    You seem to be declaring your array inside the for loop that is supposed to fill it? Don't do that, but instead declare it *before* the for loop. Don't try to manipulate b[0] from within the for loop either.
    That's what I was trying to get him to find for himself by stepping through it with a debugger or adding print statements. Ah well.
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  11. #10
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    Default Re: Trouble with buttonhandler - Exception in thread "AWT-EventQueue-0"

    Oops, sorry, KW!

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