大家帮下!

[wangju466511977]

1 import java.awt.*;
2 import javax.swing.*;
3 public class Study {
4 private JLabel lb1=new JLabel("姓名:");
5 private JLabel lb2=new JLabel("班级:");
6 private JTextField tf1=new JTextField(10);
7 private JTextField tf2=new JTextField(10);
8 private JFrame f=new JFrame();
9 private JPanel panel=new JPanel();
10 public Study(){
11 f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
12 Container cp=f.getContentPane();
13 panel.setLayout(new GridLayout(2,2,10,10));
14 panel.add(lb1);
15 panel.add(tf1);
16 Graphics g=panel.getGraphics();
17 Graphics2D g2=(Graphics2D)g;
18 g2.setPaint(Color.RED);
19 g2.drawLine(0,100,panel.getWidth(),100);
20 panel.add(lb2);
21 panel.add(tf2);
22 cp.add(panel);
23 f.setSize(200,200);
24 f.setVisible(true);
25 }
26 public static void main(String args[]){
27 new Study();
}
}

为什么上面的程序会发生下面的错误?
Exception in thread "main" java.lang.NullPointerException
at Study.<init>(Study.java:18)
at Study.main(Study.java:27)

[cangbai217]

18 g2.setPaint(Color.RED);set了一个空，先判断下Color.RED是不是为空

[zhangyu_fox]

new study()不是静态方法，不能放在main中

[cangbai217]

Graphics g=panel.getGraphics(); 为空

[wangju466511977]

那应该如何改呀?

[yibunengjing]

16 Graphics g=panel.getGraphics();
是空值，这个Graphics 有用吗？就画一条线

[wuxo84]

new study()不是静态方法? 这里是在创建实例！

setPaint方法放到Paint方法中，也就是说重写Paint方法，但不要调用这个方法

import java.awt.*; 
import javax.swing.*; 

public class Test5 extends JFrame
{
	private JLabel lb1=new JLabel("姓名:"); 
	private JLabel lb2=new JLabel("班级:"); 
	private JTextField tf1=new JTextField(10); 
	private JTextField tf2=new JTextField(10); 
	private JFrame f=new JFrame(); 
	private JPanel panel=new JPanel(); 
	public Test5()
	{ 
		f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); 
		Container cp=f.getContentPane(); 
		panel.setLayout(new GridLayout(2,2,10,10)); 
		panel.add(lb1); 
		panel.add(tf1); 
		Graphics g=panel.getGraphics(); 
		Graphics2D g2=(Graphics2D)g; 
		panel.add(lb2); 
		panel.add(tf2); 
		cp.add(panel); 
		f.setSize(200,200); 
		f.setVisible(true); 
	}
	
	public void paint(Graphics2D g2)
	{
		g2.setPaint(Color.RED); 
		g2.drawLine(0,100,panel.getWidth(),100); 
	}   
	  
	public static void main(String args[])
	{
		new Test5();
	}
}