So far I have created a frame at adds my ControlPanel class, the class takes an int, and that int is used to fill an array with JButton objects which are the added to a Panel and displayed.
However I am getting a Null Pointer error when I try to add JButtons to the array. I'm not sure why (since I thought null pointers were from when you try to reference something that isn't in the spot in the array?)
Any help on this would be great appreciated.
Error Message:
Exception in thread "main" java.lang.NullPointerException
at elevator.ControlPanel.<init>(ControlPanel.java:22)
at elevator.Elevator_Simulator.main(Elevator_Simulator.java:27)
Main Class
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import javax.swing.event.*;
public class Elevator_Simulator extends JFrame {
public static void main(String[] args) {
JFrame frame = new JFrame ("Elevator Simulation");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.getContentPane().add(new ControlPanel(8));
frame.pack();
frame.setVisible(true);
}
}
Control Panel Class:
import java.awt.Color;
import java.awt.event.*;
import javax.swing.*;
public class ControlPanel extends JPanel implements ActionListener {
public JButton[] floorButton; // an array of floor buttons.
public int[] floorIn; // state of button. 0 == not click >= 1 means clicked (how many times).
public ControlPanel(int x) {
JPanel floors = new JPanel();
for (int i = 0; i < x; i++) { // interates through user input and creates that many JButtons.
floorButton[i].add(new JButton("F" + Integer.toString(i))); // adds a button to button array. Sets text to F(floorNo).
floors.add(floorButton[i]); // adds buttons to the floor panel.
floorButton[i].addActionListener(this); // adds action listener to the buttons.
}
}
public void actionPerformed(ActionEvent e) {
for (int i = 0; i < floorButton.length; i++) {
if (e.getSource() == floorButton[i]) { // checks which button the event occured on.
floorButton[i].setBackground(Color.red); // sets button background to red.
floorButton[i].setText(floorButton[i].getText() + "(1)"); // marks button as clicked.
}
}
}
}
floorButton
is not initalised to anything...
public JButton[] floorButton; // I'm null
public ControlPanel(int x) {
//...
for (int i = 0; i < x; i++) { // in
// Still null
floorButton[i].add(new JButton("F" + Integer.toString(i)));
Initalise the array to reflect what you need, also, don't use floorButton[i].add
, you don't want to add the button to (what is currently a null
element) button, you want to assign it to the position of the array...
public ControlPanel(int x) {
//...
floorButton = new JButton[x];
for (int i = 0; i < x; i++) { // in
floorButton[i] = new JButton("F" + Integer.toString(i));
I'm guessing you'll want to do the same with floorIn
...