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PONull pointers and private members, basic Java GUI with swing
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  1. PONull pointers and private members, basic Java GUI with swing
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    copied!<p>When I try to build the program below, I get the following error:</p> <pre><code>Exception in thread "main" java.lang.NullPointerException at java.awt.Container.addImpl(Unknown Source) at java.awt.Container.add(Unknown Source) at miniCADFrame$CanvasPanel.&lt;init&gt;(miniCADFrame.java:71) at miniCADFrame.&lt;init&gt;(miniCADFrame.java:17) at miniCAD.&lt;init&gt;(miniCAD.java:12) at miniCAD.main(miniCAD.java:20) </code></pre> <p>Clearly I've got some NULL pointer issues, but I'm not sure where they are! The code that I think is the source of the problem is below (if I avoid using the classes in the code below, the program will run without problems). I've added in the line numbers for the problem spots below.</p> <p>Once finished, the program will allow the user to press a button to draw various shapes.</p> <pre><code>import java.awt.BorderLayout; import java.awt.GridLayout; import java.awt.event.ActionEvent; import java.awt.event.ActionListener; import javax.swing.JButton; import javax.swing.JLabel; import javax.swing.JPanel; public class miniCADFrame extends JPanel { private CanvasPanel canvas = new CanvasPanel(); //LINE 17 private ButtonPanel buttons = new ButtonPanel(); public miniCADFrame() { //Constructor, creates the mother panel this.setLayout(new BorderLayout()); this.add (canvas,BorderLayout.CENTER); } private class ButtonListener implements ActionListener { @Override public void actionPerformed(ActionEvent event) { //Paint the figure associated with the button click canvas.add(new FigurePanel(FigurePanel.OVAL), BorderLayout.CENTER); canvas.repaint(); } } class ButtonPanel extends JPanel { private JButton[] Jbuttons = new JButton[11]; //Constructor ButtonPanel() { setLayout(new GridLayout(4, 4)); // Create buttons to attach to the buttons panel Jbuttons[0] = new JButton("Change Colour"); Jbuttons[1] = new JButton("Up"); Jbuttons[2] = new JButton("Text"); Jbuttons[3] = new JButton("Left"); Jbuttons[4] = new JButton("Enlarge"); Jbuttons[5] = new JButton("Right"); Jbuttons[6] = new JButton("Rectangle"); Jbuttons[7] = new JButton("Down"); Jbuttons[8] = new JButton("Circle"); Jbuttons[9] = new JButton("Save"); Jbuttons[10] = new JButton("Load"); //Add the buttons to the buttons panel for (int i=0; i&lt;11; i++) { Jbuttons[i].addActionListener(new ButtonListener()); buttons.add(Jbuttons[i]); } } } class CanvasPanel extends JPanel { //Constructor CanvasPanel() { // Create "canvas" to hold a label for the buttons panel along with the button panel itself this.setLayout(new BorderLayout()); this.add(new JLabel("CONTROL PANEL"),BorderLayout.NORTH); this.add(buttons, BorderLayout.WEST); //LINE 71 add the button panel to the canvas panel //test this.add(new FigurePanel(FigurePanel.RECTANGLE), BorderLayout.CENTER); } } } </code></pre>
 

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