How to avoid this NullPointerException

Question

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 t

Answer 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(...);

Answer 2

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

Answer 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
    }
}