Anonymous ActionListener Class Exception "NullPointerException"

Question

In a for loop, I want to

initialize JButtons [] bts .
add the buttons to a panel pan3 .
add an ActionListener to each JButton .

here what I've got so far :

for(int i = 0 ;i < bts1.length ; i++){
        bts1[i] = new JButton(""+i);
        pan3.add(bts1[i]);
        //The NullPointerException happens after this line . 
        bts[i].addActionListener(new ActionListener(){
            public void actionPerformed(ActionEvent ae){
                int j = 0 ;
                screen.setText(screen.getText()+bts[j].getText());
                j++;
            }
        });
    }

My question is how to solve the NullPointerException ? note for the j I add it because when I tried to use the i , a compiler error occurs ,

j will always be zero for the setText stuff. What are you trying to do with it? — user949300, Nov 01 '13 at 06:53
Then it will immediately fall out of scope and be forgotten. And, the next time in it gets reset to 0. — user949300, Nov 01 '13 at 15:23

score 1 · Accepted Answer · answered Nov 01 '13 at 07:07

1

You initialized JButton as bts1 and calling it as bts. I think that caused the error.
Also,

Declare int i=0; as global scope and for(i = 0 ;i < bts1.length ; i++) then you can use i instead of j.
Or else use as following:

for(int i = 0 ;i < bts1.length ; i++){
        bts1[i] = new JButton(""+i);
        pan3.add(bts1[i]);
        final int j=i; 
        bts1[i].addActionListener(new ActionListener(){
            public void actionPerformed(ActionEvent ae){                   
                screen.setText(screen.getText()+bts1[j].getText());
            }
        });
    }

answered Nov 01 '13 at 07:07

Nizam

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using the `final` modifier so he can use `i` is a good point :) – Stefan Freitag Nov 01 '13 at 07:09
Thanks . It works now , would you clarify why did you use the final modifier here ? – iShaalan Nov 01 '13 at 07:21
1

You can find the answer to your question here: http://stackoverflow.com/questions/3910324/why-inner-classes-require-final-outer-instance-variables-java – Stefan Freitag Nov 01 '13 at 08:11

score 1 · Answer 2 · answered Nov 01 '13 at 07:07

You are using two different arrays of JButtons, bts and bts1.

In your loop you are creating a JButton and assign it to bts1[i].
In the ActionListener part you are accessing bts[j].

Have you initialized also bts[j] somewhere? If not you will run into a NPE.

StanislavL · Answer 3 · 2013-11-01T07:18:59.007

1

Try the event source instead

screen.setText(screen.getText()+((JButton)ae.getSource()).getText());

edited Nov 01 '13 at 07:18

answered Nov 01 '13 at 07:07

StanislavL

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I would understand it better if you add more explanation of what is the differences between these two – iShaalan Nov 01 '13 at 07:17
You can use the event source - the clicked button instance. – StanislavL Nov 01 '13 at 07:18

score 0 · Answer 4 · answered Nov 01 '13 at 06:54

0

try this:
for(int i = 0 ;i < bts1.length-1 ; i++){
   //rest of your code
}

As the array of 10 elements has elements with indexes 0 through 9, not 0 through 10.

answered Nov 01 '13 at 06:54

Tony Jorginson

21
4

score 0 · Answer 5 · answered Nov 01 '13 at 06:54

Beforr you add the button to Panels. You need add Listener for button first.

Have a try with the following code first.

    //The NullPointerException happens after this line . 
    bts[i].addActionListener(new ActionListener(){
        public void actionPerformed(ActionEvent ae){
            int j = 0 ;
            screen.setText(screen.getText()+bts[j].getText());
            j++;
        }
    });

    **pan3.add(bts1[i]);**

Anonymous ActionListener Class Exception "NullPointerException"

5 Answers5