调用JPanel.getGraphics()出现java.lang.NullPointerException

2011-12-15 10:28:36 339 阅读 AI 写同款 · GEO 优化AI 选题数据监测一键分发 ›

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各位大大，我想画个网格，但是
Graphics graphics = panel.getGraphics();
这条语句却报出java.lang.NullPointerException？
不知什么原因，求解，谢谢.



package test;



import java.awt.*;

import javax.swing.*;



public class TestGrid extends JFrame{



	public static void main(String[] args) {  

        TestGrid game = new TestGrid();  

        game.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);  

        game.setVisible(true);  

    } 

	

	public TestGrid(){

		super();

		this.setTitle("just a test!");

		this.setSize(600, 400);

		this.getContentPane().setLayout(new BorderLayout());



		DrawGrid(resultPanel);

		this.add(resultPanel,BorderLayout.CENTER);

	}

	



	private void DrawGrid(JPanel panel) {

		width = panel.getWidth();

		height = panel.getHeight();



		section_width = width / 20;

		section_height = height / 20;



		remainder_width = width - section_width * 20;

		remainder_height = height - section_height * 20;



		Graphics graphics = panel.getGraphics();

		graphics.clearRect(0, 0, width, height);

				

		for (int i = 0; i <= 20; i++) {

			graphics.drawLine(remainder_width / 2 + section_width * i,

					remainder_height / 2, 

					remainder_width / 2 + section_width * i,

					height - remainder_height / 2);

		}



		for (int i = 0; i <= 20; i++) {

			graphics.drawLine(remainder_width / 2, 

					remainder_height / 2 + section_height * i,

					width - remainder_width / 2,

					remainder_height / 2 + section_height * i);

		}

	}

	

	private int width ;

	private int height ;

	private int section_width;

	private int section_height;

	private int remainder_width;

	private int remainder_height;

	private JPanel resultPanel = new JPanel();

}