java swing nullpointer exeption whilst adding buttons [duplicate]

solution

So i was trying to do a java-swing-gui for a school project and therefor I have to add 72 Buttons (idk) to a JPanel element. I tried using a for-loop:

for (JButton btn : btns) {
    panel.add(btn);
}

but it didnt really work out and threw a nullPointerExeption. Any suggestions?

Here is the full code:

import javax.swing.*;
import javax.swing.plaf.DimensionUIResource;

import java.awt.*;

public class mädn extends JFrame{
    static JPanel panel = new JPanel();
    static JFrame frame = new JFrame();
    static JButton[] fields = new JButton[72];

    public static void main(String[] args) {
        new mädn();
    }

    public mädn() {
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setSize(new DimensionUIResource(500, 500));
        frame.setTitle("Mensch Ärger Dich Nicht");

        panel.setLayout(new GridLayout(11, 11));
        panel.setBackground(Color.blue);
        
        for (JButton field : fields) {
            field.setSize(20, 20);
            panel.add(field);
        }

        frame.add(panel);
        frame.setVisible(true);
    }
}

Answer

When you create an array like this:

static JButton[] fields = new JButton[72];

that array is empty. In other words, even though that array has the capacity to keep 72 buttons, it doesn’t have those buttons yet, you have to add them manually, like this:

for (int i = 0; i < 72; i++) {
    fields[i] = new JButton();
}

But if you don’t, fields[i] will be null by default, and so when you try to do field.setSize(20, 20), that field is null and will cause a NullPointerException.

Primitive arrays also have default values. For example elements of an array of int are all 0, and elements of an array of boolean are all false. The same is true for non-primitive arrays (like JButton), the elements are all by default null.

Your final code would look something like this:

for (int i = 0; i < 72; i++) {
    fields[i] = new JButton();
    fields[i].setSize(20, 20);
    panel.add(fields[i]);
}

or:

for (int i = 0; i < 72; i++) {
    JButton field = new JButton();
    fields[i] = field;
    field.setSize(20, 20);
    panel.add(field);
}

or even shorter:

for (int i = 0; i < 72; i++) {
    var field = fields[i] = new JButton();
    field.setSize(20, 20);
    panel.add(field);
}

Also consider turning that 72 into a constant (a static final field).