JCheckBox[x] is null?

Question

I made a new project with just the code, that contains the error:

package benutzerschnittstelle;

import java.awt.BorderLayout;
import java.awt.EventQueue;

import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JTextField;
import javax.swing.border.EmptyBorder;
import javax.swing.JLabel;
import javax.swing.JCheckBox;

public class versuch extends JFrame {
  private static JPanel contentPane;
  static JCheckBox[] CheckBox;

  public static void main(String[] args) {
    EventQueue.invokeLater(new Runnable() {
      public void run() {
        try {
          versuch frame = new versuch();
          frame.setVisible(true);

          for (int x = 0; x < 3; x++) {
            CheckBox = new JCheckBox[3];
            CheckBox[x] = new JCheckBox("Prüfungsfach");
            CheckBox[x].setBounds(180, (60 + (25 * x)), 120, 21);
            contentPane.add(CheckBox[x]);
          }

          for (int x = 0; x < 3; x++) {
            try {
              System.out.println("checkbox " + x
                      + " : " + CheckBox[x].isSelected());
            } catch (Exception e) {
              System.out.println("error at checkbox " + x
                      + " : " + e.getLocalizedMessage());
            }
          }
        } catch (Exception e) {
          e.printStackTrace();
        }
      }
    });
  }

  public versuch() {
    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    setBounds(100, 100, 450, 300);
    contentPane = new JPanel();
    contentPane.setBorder(new EmptyBorder(5, 5, 5, 5));
    setContentPane(contentPane);
    contentPane.setLayout(null);
  }
}

Since the checkboxes get checked before I even have the opportunity to select them, the output should be:

checkbox 0: false
checkbox 1: false
checkbox 2: false

but the output is:

error at checkbox 0 : Cannot invoke "javax.swing.JCheckBox.isSelected()" because "benutzerschnittstelle.versuch.CheckBoxPruefungsfach[x]" is null
error at checkbox 1 : Cannot invoke "javax.swing.JCheckBox.isSelected()" because "benutzerschnittstelle.versuch.CheckBoxPruefungsfach[x]" is null
checkbox 2 : false

If I vary the length of the array, the last checkbox always works as intended and the rest causes errors... why is that? And how do I fix it?

score 1 · Answer 1 · answered Jun 01 '21 at 10:16

In this loop:

for(int x = 0; x < 3; x++)
{
    CheckBox = new JCheckBox[3];
    CheckBox[x] = new JCheckBox("Prüfungsfach");
    CheckBox[x].setBounds(180, (60 + (25 * x)), 120, 21);
    contentPane.add(CheckBox[x]);
}

You are reassigning the array CheckBox inside the loop. This will override all the values before the current x. So in the last iteration, you’ll have reset the boxes at index 0 and 1 to null.

Move the initialisation of the array before the loop to fix the issue.

CheckBox is also a fairly poor name for an array of check boxes. Use plural names for collections (and by convention variable names in Java start with a lower-case letter: camelCase).

Ok, thanks it's working now – Pascal Rockenstiehl Jun 01 '21 at 10:21 — Pascal Rockenstiehl, Jun 01 '21 at 10:21

score 0 · Answer 2 · answered Jun 01 '21 at 10:16

The problem is that you're creating a new array every time the loop runs, as you assign it inside the loop's body

for(int x = 0; x < 3; x++) {
    CheckBox = new JCheckBox[3];
    CheckBox[x] = new JCheckBox("Prüfungsfach");
    CheckBox[x].setBounds(180, (60 + (25 * x)), 120, 21);
    contentPane.add(CheckBox[x]);
}

Moving CheckBox = new JCheckBox[3]; outside of the loop will prevent you from overwriting this value upon each loop run.

Also, to better distinguish class names from variables, it's good practice to start variables with a lowercase letter.

Thanks, it's working now – Pascal Rockenstiehl Jun 01 '21 at 10:21 — Pascal Rockenstiehl, Jun 01 '21 at 10:21

JCheckBox[x] is null?

2 Answers2