NullPointerException in game

Question

I'm trying to make a space invaders type game in Java, but can't work out why the game won't work. Here's the error

java.lang.NullPointerException
at UFO_Game.checkForHit(UFO_Game.java:131)
at UFO_Game.setup(UFO_Game.java:109)
at UFO_Game.run(UFO_Game.java:44)
at acm.program.Program.runHook(Program.java)
at acm.program.Program.startRun(Program.java)
at acm.program.Program.start(Program.java)
at sun.applet.AppletPanel.run(Unknown Source)
at java.lang.Thread.run(Unknown Source)

and here's the code getting the error.

public void checkForHit(){
    GObject CHECK = getElementAt(Bullet.getX(), Bullet.getY());
    if(CHECK == SHIP){
        removeAll();
    }

By the way, I'm using the ACM stuff like in the Stanford methodology.

go to the line it indicated (131) and check any variables there. Are they equal to null? If so you need to either change it to a value or set it equal to nothing. Like `String myString;` — CodeAddict, Jan 19 '13 at 00:03
Have you instantiated the `UFO_Game` object prior to calling `checkForHit()`? — Porkbutts, Jan 19 '13 at 00:03
-1 It is your job, as a developer, do determine *which* expression is null; from there is is only a small matter to find out *why* it is null. If you cannot deduce as such from a stack-trace, attach a debugger (a debugger is useful anyway). — , Jan 19 '13 at 00:07
Also, *there are many "duplicate" answers that explain what an NPE means and what might cause it in different situations* — , Jan 19 '13 at 00:10
http://stackoverflow.com/questions/218384/what-is-a-null-pointer-exception — , Jan 19 '13 at 00:11

score 3 · Answer 1 · answered Jan 19 '13 at 00:04

I suppose that Bullet is a field in your UFO_Game class. It isn't assigned a valid value (object fields are initialized with null). When you try to invoke getX(), a NullPointerException is thrown. To avoid this, initialize the Bullet field when you construct each UFO_Game object.

By the way, methods and field names in Java should start with a lower case letter. Anyone familiar with Java will read Bullet.getX() as an invocation of a static (class) method.

NullPointerException in game

1 Answers1