如何避免此 NullPointerException [英] How to avoid this NullPointerException

问题描述

我正在开发一款小型街机视频游戏，我希望使用双缓冲来改进动画效果.我有一个类应该绘制空白图像，另一个类应该绘制一条简单的线条.但是，我在应该绘制线条的线上不断收到 NullPointerException

I'm working on a small arcade video game, and I am looking to double buffer to improve animation. I have one class that's supposed to draw the blank image, and another class that's supposed to draw a simple line. However, I keep getting a NullPointerException on the line where the line is supposed to be drawn

class Render extends JPanel {
    public int dbWidth = 500, dbHeight = 400;
    public Image dbImage = null;
    public Graphics dbg;

    public void gameRender() {

        if( dbImage == null )  
            dbImage = createImage( dbWidth, dbHeight );

        dbg = dbImage.getGraphics();
        dbg.setColor( Color.white );
        dbg.fillRect( 0, 0, dbWidth, dbHeight );
    }
}

class MC extends Render {
    public Render render = new Render();

    public void draw() {
        render.gameRender();
        dbg.drawLine( 100, 100, 200, 200 ); // line where NullPointerException occurs
    }
}

我想是图形变量 dbg 为空，但它在 gameRender(); 中获取了 dbImage.getGraphics(); 的值；我该如何解决这个 NullPointerException?

I suppose it's the Graphics variable dbg that's null, but it gets the value of dbImage.getGraphics(); in gameRender(); How could I fix this NullPointerException?

我也在这样的另一个类中调用 draw() 方法

I am also calling the draw() method in another class like this

    public void run() {

    running = true;

    while( running ) {

        mc.draw();
        try {
            Thread.sleep( 50 );
        }
        catch( Exception e ) {}
    }
}

我在那个类的构造函数中说过 mc = new MC();

I said in that class's constructor that mc = new MC();

推荐答案

您在 this 实例上调用 dbg，而不是 render<的实例/代码>.

You're calling dbg on the this instance, not the instance of render.

你需要把它改成

render.dbg.drawLine(....)

或者，如果您想让 dbg 调用保持不变，您可以调用

Alternatively, if you wanted to leave the dbg call the same, you could call

this.gameRender();

先调用再调用

dbg.drawLine(...);

这篇关于如何避免此 NullPointerException的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！