自定义 JFrame 类没有出现

Question

This compiles fine but nothing is visible when it executes the code. What do I need to change?

I've removed my Swing/AWT imports so I could post the question. The code is short and so is my question but apparently that's not good enough...

class Exercise1 {
    public static void main(String[] args) {
        new MyFrame("Test");
    }
}

class MyFrame extends JFrame{
    public MyFrame(String title){
        super(title);
        int i = 2;
        int j = 2;
        JPanel[][] panelHolder = new JPanel[i][j];    
        setLayout(new GridLayout(i,j));
        for(int m = 0; m < i; m++) {
            for(int n = 0; n < j; n++) {
                panelHolder[m][n] = new JPanel();
                add(panelHolder[m][n]);
            }
        }
        JTextField t1 = new JTextField();
        JTextField t2 = new JTextField();
        JTextField t3 = new JTextField();
        panelHolder[2][0].add(t1);
        panelHolder[2][1].add(t2);
        panelHolder[2][2].add(t3);
        setVisible(true);
    }
}

Answer 1

This compiles fine but nothing is visible when it executes the code.

Because you get a run time Exception.

Array indexes are 0 based.

    int i = 2;
    int j = 2;
    JPanel[][] panelHolder = new JPanel[i][j];   

You create a 2D array with room for 2 values in each of your 2 rows.

    panelHolder[2][0].add(t1);
    panelHolder[2][1].add(t2);
    panelHolder[2][2].add(t3);

But you try to add components to the third row/value, which causes the Exception.

Change you array sizes to 3, or only use indexes 0 and 1.

Also, when you create a JTextField use:

new JTextField(10);

so the text fields has a reasonable size.