try，catch（异常作业）用string实现

package ch03;

public class TryCatchDemo {
public static void main(String[] args)
{
//定义一个String变量值为null
String str=null;
try
{
if(str.equals("hello"));
{
System.out.println("hello java");
}

}
catch(NullPointerException e)
{
System.out.println("空指针异常");
}
}
}

package ch03;
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
//定义一个界面 可以输入一串字符串
//该面板还有一个确定
//点击确定可以弹出对话框
//输入不正确弹出空字符串
public class TryCatchDemo extends JFrame
{
private JPanel p;
private  JTextField txtResult;

private JButton btnConfirm;
private JMenu menuManger;

public TryCatchDemo()
{
super("比较异常输出对话框");
p=new JPanel();
menuManger=new JMenu("比较异常管理框");//总框
txtResult=new JTextField(20);//指定列数的空的输入表
//注册监听
btnConfirm=new JButton("确定");//当点击确定按钮时  应弹出对话框  显示“字符创不存在”
btnConfirm.addMouseListener(new MouseAction());
p.add(txtResult);
p.add(btnConfirm);

this.add(p);
this.setSize(300,150);
this.setLocation(100,100);
this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
this.setVisible(true);
}
private class MouseAction extends MouseAdapter
{public void mouseClicked(MouseEvent e)
{	//获取事件渊
Object source=e.getSource();
if(source==btnConfirm)
{
//如果消息是从btnConfirm中发出的

JOptionPane.showMessageDialog(btnConfirm,"字符串不存在");

}

}

}
public static void main(String[] args)
{
TryCatchDemo t=new TryCatchDemo();
t.setVisible(true);
}
}