源代码：
import java.awt.*;
import javax.swing.*;
import java.awt.event.ActionListener;
import java.util.EventListener;
import java.awt.event.ActionEvent;
import ChatProject.MyMain;
public class MyClient extends JFrame implements ActionListener {
JTextField username; //声明文本框，用来输入用户名
JTextField password; //用来输入用户密码
public MyClient(String title) {
super(title);
this.setSize(300, 200);
this.setLocation(500, 250);
Container con = this.getContentPane();
con.setLayout(new GridLayout(2, 1));
JPanel p1 = new JPanel();
JPanel p2 = new JPanel();
con.add(p1);
con.add(p2);
JLabel l1 = new JLabel("用户名：");
JLabel l2 = new JLabel("密码：");
JTextField username = new JTextField(20);
JTextField password = new JTextField(15);
JButton bok = new JButton("登录");
JButton bcancel = new JButton("取消");
p1.add(l1);
p1.add(username);
p1.add(l2);
p1.add(password);
p2.add(bok);
p2.add(bcancel);
bok.addActionListener(this);
bcancel.addActionListener(this);
}
public static void main(String[] args) {
MyClient M = new MyClient("登录窗口");
M.show();
}
public void actionPerformed(ActionEvent e) {
System.out.println(e.getActionCommand());
String uid = username.getText();
String pwd = password.getText();
if (e.getActionCommand().equals("登录"))
{
if (uid.equals("123") && pwd.equals("123456"))
{
MyMain MM = new MyMain(uid);
MM.show();
this.hide();
}
} else if (e.getActionCommand().equals("取消"))
System.exit(0);
}
}
}
运行之后，如下报错，求高手指教！谢谢了！
run：
登录
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at ChatProject.MyClient.actionPerformed(MyClient.java:52)
at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:1995)
at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2318)
at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:387)
at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:242)
at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:236)
at java.awt.Component.processMouseEvent(Component.java:6263)
at javax.swing.JComponent.processMouseEvent(JComponent.java:3267)
at java.awt.Component.processEvent(Component.java:6028)
at java.awt.Container.processEvent(Container.java:2041)
at java.awt.Component.dispatchEventImpl(Component.java:4630)
at java.awt.Container.dispatchEventImpl(Container.java:2099)
at java.awt.Component.dispatchEvent(Component.java:4460)
at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4574)
at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4238)
at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4168)
at java.awt.Container.dispatchEventImpl(Container.java:2085)