getGraphics() 上的空指针异常 [英] Null Pointer Exception on getGraphics()

问题描述

我的应用程序看起来像这样，我在 draw() 方法中收到了空指针异常，确切地说是在 g.drawImage(img, 0, 0, null) 处

my application looks like that, i am getting a null pointer exception at the draw() method, to be exact at g.drawImage(img, 0, 0, null)

package com.ochs.game;

import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.image.BufferedImage;

import javax.swing.JFrame;
import javax.swing.JPanel;

public class Game extends JPanel implements Runnable{
private static final long serialVersionUID = 8229934361462702491L;

public static final int WIDTH = 320;
public static final int HEIGHT = 240;
public static final int SCALE = 2;

public boolean isRunning;

private BufferedImage img;
private Graphics2D g2d;

public Game() {
    setFocusable(true);
    requestFocus();
    start();
}

public void start() {
    isRunning = true;
    new Thread(this).start();
}

public void stop() {
    isRunning = false;
}

public void run() {
    long start;
    init();
    while(isRunning) {
        start = System.currentTimeMillis();

        update();
        render();
        draw();

        try {
            Thread.sleep(5 - (System.currentTimeMillis() - start));
        } catch (Exception e) {
        }
    }
}

public void init() {
    img = new BufferedImage(WIDTH*SCALE, HEIGHT*SCALE, BufferedImage.TYPE_INT_RGB);
    g2d = (Graphics2D) img.getGraphics();
}

public void update() {

}

public void render() {

}

public void draw() {
    Graphics g = getGraphics();
    g.drawImage(img, 0, 0, null);    // <<<<< getting null pointer here!
}

public static void main(String[] args) {
    Dimension size = new Dimension(WIDTH*SCALE, HEIGHT*SCALE);
    Game gameComponent = new Game();
    JFrame frame = new JFrame();
    frame.setVisible(true);
    frame.setSize(size);
    frame.setLocationRelativeTo(null);
    frame.setResizable(false);
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.add(gameComponent);
}
}

现在我的问题是:为什么在尝试绘制名为 img 的缓冲图像时会出现空指针异常?我也尝试过使用 drawString() 输出一些字符串，但这也给自己一个空指针异常.有没有人有建议?

Now my question is: why do i get a null pointer exception when trying to draw the bufferedimage called img? I also tried just outputting some string by using drawString() but this just gives myself a nullpointerexception, too. does anyone has an advice?

推荐答案

您很可能在 JPanel 被渲染之前尝试通过 getGraphics() 获取 Graphics 上下文，因此该方法返回空值.不要这样做.在组件上使用 getGraphics() 获取 Graphics 上下文存在问题，其中一个是您在上面看到的问题，另一个是获取的 Graphics 上下文不会持久化.有时需要这样做，但通常我们通过 paintComponent(...) 进行被动绘制.通常，Swing Timer 可用于动画循环.

You're likely trying to get the Graphics context via getGraphics() before the JPanel has been rendered, and thus the method returns null. Don't do this. There are problems with using getGraphics() on a component to get the Graphics context, one of which is the problem you're seeing above, and another is that the Graphics context obtained will not persist. There are occasions when this is necessary to do, but usually we do passive drawing via paintComponent(...). Often a Swing Timer can be used for the animation loop.

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