一个内部类的问题
源程序如下：
[code=Java][/code]
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;


public class Text1
{
public static void main(String[] args)
{
TextFrame frame = new TextFrame();
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setVisible(true);
}


}

class TextFrame extends JFrame
{
public TextFrame()
{
setTitle("TextFrame");
setSize(WIDTH,HEIGHT);


panel = new JPanel();

panel.setLayout(new GridLayout(5,2));
panel.add(new JLabel("Input:"));
textfield1 = new JTextField(10); //增加一个Input文本域
panel.add(textfield1);
textfield1.addActionListener(listener1);

panel.add(new JLabel("output:"));
textfield2 = new JTextField(10); //增加一个Onput文本域
textfield2.setEditable(false);
panel.add(textfield2);

group = new ButtonGroup();

addRadioButton("A",false);
addRadioButton("B",false);
addRadioButton("C",false); //增加四个单选按钮
addRadioButton("D",false);

choice = new Choice(); //增加一个choice组件
panel.add(choice);

add(panel,BorderLayout.NORTH);

}

public void addRadioButton(String name,boolean selected)
{
JRadioButton button = new JRadioButton(name,selected);
group.add(button);
panel.add(button);
button.addActionListener(listener2);
}


ActionListener listener1 = new ActionListener()
{
public void actionPerformed(ActionEvent evt)
{

String s = textfield1.getText();
pass(s);
String text = textfield2.getText() + " " + textfield1.getText();

textfield2.setText(text);

if(evt.getSource() == textfield1)
{
textfield1.setText("");

}

}

};

ActionListener listener2 = new ActionListener()
{
public void actionPerformed(ActionEvent evt)
{
String s1 = evt.getActionCommand();
String s2 = s1.toLowerCase();
find(s2);

}

};

int i = 0;
public void pass(String s)
{

str[i] = s;
i++;
}

public void find(String s)
{
for(int i = 0; i < str.length; i++)
{
if(str[i].startsWith(s))
choice.add(str[i]);
}
}



public static final int WIDTH = 400;
public static final int HEIGHT = 300;
private JTextField textfield1;
private JTextField textfield2;
private JPanel panel;
private ButtonGroup group;
private Choice choice;
private String[] str;
}

编译没问题，但在Input文本域中输入单词按回车后就有异常了。想请教高手这是怎么回事？
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at TextFrame.pass(Text1.java:101)
at TextFrame$1.actionPerformed(Text1.java:69)
at javax.swing.JTextField.fireActionPerformed(Unknown Source)
at javax.swing.JTextField.postActionEvent(Unknown Source)
at javax.swing.JTextField$NotifyAction.actionPerformed(Unknown Source)
at javax.swing.SwingUtilities.notifyAction(Unknown Source)
at javax.swing.JComponent.processKeyBinding(Unknown Source)
at javax.swing.JComponent.processKeyBindings(Unknown Source)
at javax.swing.JComponent.processKeyEvent(Unknown Source)

at java.awt.Component.processEvent(Unknown Source)
at java.awt.Container.processEvent(Unknown Source)
at java.awt.Component.dispatchEventImpl(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.KeyboardFocusManager.redispatchEvent(Unknown Source)
at java.awt.DefaultKeyboardFocusManager.dispatchKeyEvent(Unknown Source)
at java.awt.DefaultKeyboardFocusManager.preDispatchKeyEvent(Unknown Source)
at java.awt.DefaultKeyboardFocusManager.typeAheadAssertions(Unknown Source)
at java.awt.DefaultKeyboardFocusManager.dispatchEvent(Unknown Source)
at java.awt.Component.dispatchEventImpl(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Window.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.EventQueue.dispatchEvent(Unknown Source)
at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.run(Unknown Source)



[解决办法]
public void pass(String s) {

str[i] = s;
i++;
}

public void find(String s) {
for (int i = 0; i < str.length; i++) {
if (str[i].startsWith(s))
choice.add(str[i]);
}
}
[解决办法]
public void find(String s)
{
if(str != null && str.length !=0) {
for(int i = 0; i < str.length; i++)
{
if(str[i] != null) {
if(str[i].startsWith(s))
choice.add(str[i]);
}
}

}

}