null pointer?

[Syamsul Hussin]

hi. i cant get this applet running. here is the code:-

  
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
public class colorValue extends JApplet implements ActionListener{
    
    int first;
    int second;
    int third;
    JLabel Red;
    JLabel Green;
    JLabel Blue;
    JTextField redField;
    JTextField greenField;
    JTextField blueField;
    Canvas canvas;
    Graphics g;
    
    JPanel panel;
    Container pane;
    
    public void init() {
       
       panel = new JPanel();
       panel.setLayout(new GridLayout(1,4));
       
       pane = getContentPane();
       pane.setLayout(new FlowLayout());
       
       canvas = new Canvas();
       
       Red = new JLabel("red");
       Green = new JLabel("green");
       Blue = new JLabel("blue");
       
       redField = new JTextField(3);
       greenField = new JTextField(3);
       blueField = new JTextField(3);
       
       redField.addActionListener(this);
       greenField.addActionListener(this);
       blueField.addActionListener(this);
  
       
       
       panel.add(Red);
       panel.add(redField);
       
       panel.add(Green);
       panel.add(greenField);
       
       panel.add(Blue);
       panel.add(blueField);
       
       pane.add(panel);
       
       canvas.setBackground(Color.white);
       g = canvas.getGraphics();
       canvas.setSize(400,400);
       pane.add(canvas);

    }
    
    public void actionPerformed(ActionEvent e) {
        
        first = Integer.parseInt(redField.getText());
        second = Integer.parseInt(greenField.getText());
        third = Integer.parseInt(blueField.getText());
        
        System.out.println("first: " + first + " second: " + second + " third: " + third);
        
        g.setColor(new Color(first,second,third));
        g.fillRect(100,100,100,100);
        
    }
}

this applet have 3 JTextField which represents red, green and blue. when a user input 3 values ranging between 0 to 255 and press enter, the applet will draw a fill rectangle(100,100,100,100) on a canvas (with white background). the color of the rectangle corresponds to the value input from the user. the problem is, when enter is pressed the following exception is throwed:-

  
java.lang.NullPointerException
        at colorValue.actionPerformed(colorValue.java:75)
        at javax.swing.JTextField.fireActionPerformed(JTextField.java:489)
        at javax.swing.JTextField.postActionEvent(JTextField.java:670)
        at javax.swing.JTextField$NotifyAction.actionPerformed(JTextField.java:7
84)
        at javax.swing.SwingUtilities.notifyAction(SwingUtilities.java:1502)
        at javax.swing.JComponent.processKeyBinding(JComponent.java:2422)
        at javax.swing.JComponent.processKeyBindings(JComponent.java:2457)
        at javax.swing.JComponent.processKeyEvent(JComponent.java:2385)
        at java.awt.Component.processEvent(Component.java:4830)
        at java.awt.Container.processEvent(Container.java:1380)
        at java.awt.Component.dispatchEventImpl(Component.java:3526)
        at java.awt.Container.dispatchEventImpl(Container.java:1437)
        at java.awt.Component.dispatchEvent(Component.java:3367)
        at java.awt.KeyboardFocusManager.redispatchEvent(KeyboardFocusManager.ja
va:1700)
        at java.awt.DefaultKeyboardFocusManager.dispatchKeyEvent(DefaultKeyboard
FocusManager.java:568)
        at java.awt.DefaultKeyboardFocusManager.preDispatchKeyEvent(DefaultKeybo
ardFocusManager.java:739)
        at java.awt.DefaultKeyboardFocusManager.typeAheadAssertions(DefaultKeybo
ardFocusManager.java:673)
        at java.awt.DefaultKeyboardFocusManager.dispatchEvent(DefaultKeyboardFoc
usManager.java:534)
        at java.awt.Component.dispatchEventImpl(Component.java:3396)
        at java.awt.Container.dispatchEventImpl(Container.java:1437)
        at java.awt.Component.dispatchEvent(Component.java:3367)
        at java.awt.EventQueue.dispatchEvent(EventQueue.java:445)
        at java.awt.EventDispatchThread.pumpOneEventForHierarchy(EventDispatchTh
read.java:190)
        at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThre
ad.java:144)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:138)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:130)
        at java.awt.EventDispatchThread.run(EventDispatchThread.java:98)

[Michael Morris]

Hi Syamsul,
The problem is that your Graphics reference is null. In your init() method, if you will change this:

...
       g = canvas.getGraphics();
       canvas.setSize(400,400);
       pane.add(canvas);
...

to this:

...
       canvas.setSize(400,400);
       pane.add(canvas);
       g = canvas.getGraphics();
...

then it should work. The problem here is that if you call canvas.getGraphics() before adding it to the applet's content pane, then it's graphics has not been set.
Hope this helps,
Michael Morris

[Syamsul Hussin]

thanks!