如何在数组中创建JPanels并向其中添加Jlabels？

Question

I've looked at many websites. Without the panels, the labels appear correctly, with the panels it give the the error:

Exception in thread "main" java.lang.NullPointerException   

so what can I do to fix this?

here is the source code:

JLabel button[] = new JLabel[100];
JPanel[] panel = new JPanel[100];

    for (int i = 0; i < button.length; i++) {
        a = a + 50;

        if (a > 549) {
            b = b + 50;
            a = 50;
        }
        button[i] = new JLabel("hi");
        frame.add(button[i]);    //is this necessary? 
        button[i].setVisible(true); // is this necessary?
        button[i].setSize(50,50);
        panel[i].add(button[i]);
        panel[i].setVisible(true);
        panel[i].setBounds(a, b, 50, 50);
        frame.add(panel[i]);

    }

Whats wrong with this, how can I fix it? just so you know, it should have 100 labels that say hi in a 10 by 10 array. this is what it looks like:

Answer 1

Creating an array of JPanel only creates the array. It doesn't create any JPanel to fill the array. The arrays is thus filled with null s. You must create a JPanel for each element of the array:

panel[i] = new JPanel();
panel[i].add(button[i]);

Moreover, a component may only have one ancestor. The button must be added to the frame or to the panel, but not both. If you want the button in the panel, it must be added to the panel.

Components are visible by default (except top-level ones like frames or dialogs which must be made visible). You don't need to call button.setVisible(true) .

You should definitely learn to use layout managers rather than setting the size and bounds of your components explicitely. That's the only way to have good-looking, portable GUI apps. Read http://docs.oracle.com/javase/tutorial/uiswing/layout/using.html

Answer 2

don't use frame.setLayout(null); use frame.setLayout(new GridLayout(10,10,10,10)); instead, for example

import java.awt.*;
import javax.swing.*;

public class CustomComponent1 extends JFrame {

    private static final long serialVersionUID = 1L;

    public CustomComponent1() {
        setTitle("Custom Component Test / GridLayout");
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    }

    public void display() {
        setLayout(new GridLayout(10, 10, 10, 10));
        for (int row = 0; row < 100; row++) {
            add(new CustomComponents1());
        }
        //pack();
        // enforces the minimum size of both frame and component
        setMinimumSize(getMinimumSize());
        setPreferredSize(getPreferredSize());
        setVisible(true);
    }

    public static void main(String[] args) {
        Runnable r = new Runnable() {

            @Override
            public void run() {
                CustomComponent1 main = new CustomComponent1();
                main.display();
            }
        };
        javax.swing.SwingUtilities.invokeLater(r);
    }
}

class CustomComponents1 extends JLabel {

    private static final long serialVersionUID = 1L;

    @Override
    public Dimension getMinimumSize() {
        return new Dimension(20, 20);
    }

    @Override
    public Dimension getPreferredSize() {
        return new Dimension(20, 20);
    }

    @Override
    public void paintComponent(Graphics g) {
        int margin = 10;
        Dimension dim = getSize();
        super.paintComponent(g);
        g.setColor(Color.red);
        g.fillRect(margin, margin, dim.width - margin * 2, dim.height - margin * 2);
    }
}