Re: JFrame disaperes after creation.

Lew <>
Fri, 15 May 2009 21:20:34 -0400
<gul4d4$f2b$> wrote:

Hello guys. what am I doing wrong here ?? Im trying to create a
JFrame from a controler class with

 View v = new view(this);

You need to do all GUI work, including creation, on the Event Dispatch Thread
(EDT), which you did not do.

    MFrame[] frames = new MFrame[2];

        for (MFrame mF : frames) {

            mF = new MFrame();

This is what Arne pointed out is wrong. You reassigned 'mF'; you did not
assign a pointer to the array element here. The 'for' expression takes 'mF'
and assigns it each pointer of 'frames' in turn, and at this point every
pointer in 'frames' is 'null' because you haven't assigned the array elements
any other values. Inside the body of the loop, you assign a non-null value to
the pointer 'mF', then throw it away at the end of the body.

            mF.setLayout(new BoxLayout(mF, BoxLayout.PAGE_AXIS));

All of this work is happening in the controller, yes? That is not on the EDT,
and therefore will be buggy.

            mF.buttons.setLayout(new BoxLayout(mF.buttons,
            int i = 0;
            for (JButton jB : mF.sSP) {
                jB = new JButton(type[i]);

                // ImageIcon iI = new ImageIcon();

then when i [sic] try to call


I get a nullpointerexception. Now why is that. I just dont get it.

Are you sure you didn't get a NullPointerException?

Arne Vajh?j wrote:

Try run this for a hint:

public class ForLoops {
    public static void main(String[] args) {
        String[] sa = new String[2];
        try {
            for(String s : sa) {
                s = new String("ABC");
            String s2 = sa[0].substring(0, 1);

What Arne is telling you is that assigning a new pointer to 's' inside the
loop has no effect on the array 'sa'. The new pointer is thrown away at the
end of the loop body with each iteration. You are misusing the for-each

What you want is more like
   for ( int ix = 0; ix < sa.length; ++ix )
     sa [ix] = new String( "ABC" );

            System.out.println("No exception");
        } catch(NullPointerException npe) {
        try {
            for(int i = 0; i < sa.length; i++) {
                sa[i] = new String("ABC");
            String s2 = sa[0].substring(0, 1);
            System.out.println("No exception");
        } catch(NullPointerException npe) {

Variables are pointers.


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