JAVA The following program contains exactly three 3 errors P

JAVA

The following program contains exactly three (3) errors. Please identify them.

import java.awt.Color;
import java.awt.GridLayout;
import java.awt.event.*;
import javax.swing.ImageIcon;
import javax.swing.JButton;
import javax.swing.JFrame;

class Cell extends JButton {

private static final int NUM_COLOURS = 3;

private ImageIcon[] icons;
private int type;

public Cell() {
ImageIcon[] icons;

icons = new ImageIcon[NUM_COLOURS];
for (int i=0; i<NUM_COLOURS; i++) {
icons[i] = new ImageIcon(\"data/ball-\" + Integer.toString(type) + \".png\");
}

this.type = 0;
setBackground(Color.WHITE);
setIcon();
setBorderPainted(false);
}

private void setIcon() {
setIcon(icons[type]);
}

public void update() {
type = type + 1;
setIcon();
}
  
}

public class App extends JFrame implements ActionListener {

private Cell myCell;
  
public App() {
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
myCell = new Cell();
myCell.addActionListener(this);
add(myCell);
pack();
setVisible(true);
}

public void actionPerformed(ActionEvent e) {
myCell.update();
}
  
public static void main(String[] args) {
new App();
}
  
}

Solution

Basically, all the errors are displayed because of 1 thing : NullPointerException.

This is happening because there are icons [ ] arrays that are defined 2 times. In the public Cell() method, there is a local initialization of ImageIcon icons[ ] that is hiding the class element icons[ ] .

setIcon() method operates on class variable icons[ ] which is never initialized as in Cell() constructor, local icons[ ] array is defined and used but never assigned to class element.

So, you can solve this in 2 ways:

1. remove icons[ ] in Cell constructor i.e. 1^st line of that constructor. That means the constructor directly operates on class variable.

2. In the Cell() constructor, add 1 line: this.icons = icons;

Here, this.icons represent class variable and the right hand side icons is local variable defined in 1^st line of the same method.

Use either of these, try to use 1^st one. This solves the problem and the code is running.

Do comment if there is any query. Thank you. :)