Thread: Null Pointer Exception Help!!

Hi, I'm trying to build a two classes:
1) One to add the panels
2) The panel class

The code compiles fine, but when i run it, i get a null pointer exception on line 52 of TestFrame (the class)

 import java.awt.BorderLayout;
 import java.awt.Font;
 import java.awt.GridLayout;
 import java.awt.event.ActionEvent;
 import java.awt.event.ActionListener;
 import javax.swing.ButtonGroup;
 import javax.swing.JButton;
 import javax.swing.JCheckBox;
 import javax.swing.JComboBox;
 import javax.swing.JFrame;
 import javax.swing.JLabel;
 import javax.swing.JPanel;
 import javax.swing.JRadioButton;
 import javax.swing.border.EtchedBorder;
 import javax.swing.border.TitledBorder;
 
 /**
    This frame contains a text field and a control panel
    to change the font of the text.
 */
 public class TestFrame extends JFrame
 {
    /**
       Constructs the frame.
    */
    public TestFrame()
    {  
       // Construct text sample
       sampleField = new JLabel("Big Java");
       add(sampleField, BorderLayout.CENTER);
 
       // This listener is shared among all components
       class ChoiceListener implements ActionListener
       {  
          public void actionPerformed(ActionEvent event)
          {  
             setSampleFont();
          }
       }
       listener = new ChoiceListener();
 
       createControlPanel();
       setSampleFont();
       setSize(FRAME_WIDTH, FRAME_HEIGHT);
    }
 
    /**
       Creates the control panel to change the font.
    */
    public void createControlPanel()
    {
       JPanel facenamePanel = comboBox.createComboBox();
 
       // Line up component panels
 
       JPanel controlPanel = new JPanel();
       controlPanel.setLayout(new GridLayout(3, 1));
       controlPanel.add(facenamePanel);
 
       // Add panels to content pane
 
       add(controlPanel, BorderLayout.SOUTH);
    }
 
     /**
       Gets user choice for font name, style, and size
       and sets the font of the text sample.
    */
    public void setSampleFont()
    {  
       // Get font name   
       String facename 
             = (String) facenameCombo.getSelectedItem();
 
       // Set font of text field
 
       sampleField.setFont(new Font(facename,Font.BOLD,32));      
       sampleField.repaint();
    }
 
    private JLabel sampleField;
    private JComboBox facenameCombo;
    private ActionListener listener;
    private static final int FRAME_WIDTH = 300;
    private static final int FRAME_HEIGHT = 400;
 
	 private ComboBox comboBox;
 }

 import java.awt.event.ActionEvent;
 import java.awt.event.ActionListener;
 import javax.swing.JComboBox;
 import javax.swing.JPanel;
 
 /**
    This frame contains a text field and a control panel
    to change the font of the text.
 */
 public class ComboBox extends JPanel
 {
 
    /**
       Creates the combo box with the font style choices.
       @return the panel containing the combo box
    */
    public JPanel createComboBox()
    {
       facenameCombo = new JComboBox();
       facenameCombo.addItem("Serif");
       facenameCombo.addItem("SansSerif");
       facenameCombo.addItem("Monospaced");
       facenameCombo.setEditable(true);
       facenameCombo.addActionListener(listener);
 
       JPanel panel = new JPanel();
       panel.add(facenameCombo);
       return panel;
    }
 
    private JComboBox facenameCombo;
    private ActionListener listener;
 }

For future reference, since there is no line numbering available, it helps to somehow flag or write out the line where the exception occurs.
In your case, it happens here:

JPanel facenamePanel = comboBox.createComboBox();

I do not see where the comboBox was instantiated.

Sorry about the line numbers...

I'm sort of new to Java Programming, but what do you mean by instantiating comboBox?

Do I add this?

ComboBox comboBox = new ComboBox();


(again..sorry if i'm asking a question that seems obvious..)
but then i get a Nullpointer exception for line 74...

so that my code is now:

 1  import java.awt.BorderLayout;
 2  import java.awt.Font;
 3  import java.awt.GridLayout;
 4  import java.awt.event.ActionEvent;
 5  import java.awt.event.ActionListener;
 6  import javax.swing.ButtonGroup;
 7  import javax.swing.JButton;
 8  import javax.swing.JCheckBox;
 9  import javax.swing.JComboBox;
10  import javax.swing.JFrame;
11  import javax.swing.JLabel;
12  import javax.swing.JPanel;
13  import javax.swing.JRadioButton;
14  import javax.swing.border.EtchedBorder;
15  import javax.swing.border.TitledBorder;
16  
17  /**
18     This frame contains a text field and a control panel
19     to change the font of the text.
20  */
21  public class TestFrame extends JFrame
22  {
23     /**
24        Constructs the frame.
25     */
26     public TestFrame()
27     {  
28        // Construct text sample
29        sampleField = new JLabel("Big Java");
30        add(sampleField, BorderLayout.CENTER);
31  
32        // This listener is shared among all components
33        class ChoiceListener implements ActionListener
34        {  
35           public void actionPerformed(ActionEvent event)
36           {  
37              setSampleFont();
38           }
39        }
40        listener = new ChoiceListener();
41  
42        createControlPanel();
43        setSampleFont();
44        setSize(FRAME_WIDTH, FRAME_HEIGHT);
45     }
46  
47     /**
48        Creates the control panel to change the font.
49     */
50     public void createControlPanel()
51     {
[COLOR="Blue"]52        ComboBox comboBox = new ComboBox();[/COLOR]
53 
54        JPanel facenamePanel = comboBox.createComboBox();
55  
56        // Line up component panels
57  
58        JPanel controlPanel = new JPanel();
59        controlPanel.setLayout(new GridLayout(3, 1));
60        controlPanel.add(facenamePanel);
61  
62        // Add panels to content pane
63  
64        add(controlPanel, BorderLayout.SOUTH);
65     }
66   
67      /**
68        Gets user choice for font name, style, and size
69        and sets the font of the text sample.
70     */
71     public void setSampleFont()
72     {  
73        // Get font name   
[COLOR="Blue"]74        String facename 
75              = (String) facenameCombo.getSelectedItem();[/COLOR]
76        
77        // Set font of text field
78        
79        sampleField.setFont(new Font(facename,Font.BOLD,32));      
80        sampleField.repaint();
81     }
82     
83     private JLabel sampleField;
84     private JComboBox facenameCombo;
85     private ActionListener listener;
86     private static final int FRAME_WIDTH = 300;
87     private static final int FRAME_HEIGHT = 400;
88     
89     private ComboBox comboBox;
90  }

You don't necessarily have to add line numbers, just highlight the line the exception is being thrown on. It helps tremendously in locating the error.

Do I add this?

ComboBox comboBox = new ComboBox();

Kind of...you want to instantiate the instance variable, when you do the above you create a local variable that looses scope as soon as the createControlPanel() exits. Same goes for the other facenameCombo variable. You want to instantiate the instance variable:

comboBox = new ComboBox();

Thanks for the info!

My program runs fine after I edit the code, but now my font does not change when i try to select a different font.

 import java.awt.BorderLayout;
 import java.awt.Font;
 import java.awt.GridLayout;
 import java.awt.event.ActionEvent;
 import java.awt.event.ActionListener;
 import javax.swing.ButtonGroup;
 import javax.swing.JButton;
 import javax.swing.JCheckBox;
 import javax.swing.JComboBox;
 import javax.swing.JFrame;
 import javax.swing.JLabel;
 import javax.swing.JPanel;
 import javax.swing.JRadioButton;
 import javax.swing.border.EtchedBorder;
 import javax.swing.border.TitledBorder;
 
 /**
    This frame contains a text field and a control panel
    to change the font of the text.
 */
 public class TestFrame extends JFrame
 {
    /**
       Constructs the frame.
    */
    public TestFrame()
    {  
       // Construct text sample
       sampleField = new JLabel("Big Java");
       add(sampleField, BorderLayout.CENTER);
 
       // This listener is shared among all components
       class ChoiceListener implements ActionListener
       {  
          public void actionPerformed(ActionEvent event)
          {  
             setSampleFont();
          }
       }
       listener = new ChoiceListener();
 
		 comboBox = new ComboBox();
  	 	 facenameCombo = new JComboBox();
 
       createControlPanel();
       setSampleFont();
       setSize(FRAME_WIDTH, FRAME_HEIGHT);
    }
 
    /**
       Creates the control panel to change the font.
    */
    public void createControlPanel()
    {
 
       JPanel facenamePanel = comboBox.createComboBox();
 
       // Line up component panels
 
       JPanel controlPanel = new JPanel();
       controlPanel.setLayout(new GridLayout(3, 1));
       controlPanel.add(facenamePanel);
 
       // Add panels to content pane
 
       add(controlPanel, BorderLayout.SOUTH);
    }
 
     /**
       Gets user choice for font name, style, and size
       and sets the font of the text sample.
    */
    public void setSampleFont()
    {  
       // Get font name   
       String facename 
             = (String) facenameCombo.getSelectedItem();
 
       // Set font of text field
 
       sampleField.setFont(new Font(facename,Font.BOLD,32));      
       sampleField.repaint();
    }
 
    private JLabel sampleField;
    private JComboBox facenameCombo;
    private ActionListener listener;
    private static final int FRAME_WIDTH = 300;
    private static final int FRAME_HEIGHT = 400;	 
    private ComboBox comboBox;
 }

this is the test code:

 import javax.swing.JFrame;
 
 /**
    This program allows the user to view font effects.
 */
 public class TestViewer
 {  
    public static void main(String[] args)
    {  
       JFrame frame = new TestFrame();
       frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
       frame.setTitle("FontViewer");
       frame.setVisible(true);      
    }
 }

Did you ever try adding an actionListener to something and having the actionListener call the method that sets it to bold or whatever?