How to avoid this NullPointerException

问题:

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
    }
}

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?

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 ) {}
    }
}

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

回答1:

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

You need to change it to

render.dbg.drawLine(....)

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

this.gameRender();

first and then call

dbg.drawLine(...);

回答2:

You setup dbg by invoking gameRender on render, but not this.

回答3:

Since MC extends render, the dbg you are referring to belongs to the MC instance that called draw. You can fix it by calling

render.dbg.drawLine( 100, 100, 200, 200 );

or take advantage of the inheritance you implemented

class MC extends Render {
    //MC is a render, so you don't need to create another one
    public void draw() {
        gameRender(); //Call the MC's own gameRender
        dbg.drawLine( 100, 100, 200, 200 );  //Calling gameRender initialized dbg so you won't get a NullPointerException
    }
}