Java-空指针异常

Question

I am creating a simple video game and I have been trying to add some basic collision detection. I am doing this out of a entityCollision.class that I have created. The collision detection uses rectangles that are placed outside of the charater of AI. I am trying to create these rectangles by doing this:

Movement Movement;
AIMovement AIMovement;

Rectangle player = new Rectangle(Movement.x + 4, Movement.y, 56, 64);
Rectangle robot1 = new Rectangle(AIMovement.x[0] + 16, AIMovement.y[0], 32, 64);
Rectangle robot2 = new Rectangle(AIMovement.x[2] + 16, AIMovement.y[2], 32, 64);
Rectangle robot3 = new Rectangle(AIMovement.x[3] + 16, AIMovement.y[3], 32, 64);
Rectangle robot4 = new Rectangle(AIMovement.x[4] + 16, AIMovement.y[4], 32, 64);

In the code I first make a non-static way to refrence the class. After that I try to make the variables using the cords from the other classes. This is were it gives me the nullpointerexeption, right on the line of the first rectangle. All three of the classes I am using are created around the same time, this is the code:

Main Main;
Movement Movement;
AIMovement AIMovement;
Bullet Bullet;
entityCollision entityCollision;

public GamePanel() throws FileNotFoundException, InterruptedException{
    Movement = new Movement();
    AIMovement = new AIMovement();
    Bullet = new Bullet();
    entityCollision = new entityCollision();

I get the error right after the code is created like in the code above.

I would like to know how would I make it were the entityCollision class is created after the other classes variables are not null or other ways of getting ride of the error.

Just comment and I will add more code or answer questions

Answer 1

Why do you go from

Rectangle robot1 = new Rectangle(AIMovement.x[0] + 16, AIMovement.y[0], 32, 64);

to

Rectangle robot2 = new Rectangle(AIMovement.x[2] + 16, AIMovement.y[2], 32, 64);?

On your fourth robot, if there is nothing in AIMovement.x[4] and/or the corresponding y , which I am assuming there is not, then you are passing a null pointer to the Rectangle class constructor.

Answer 2

First fix all the cases where you have a variable name that collides with a class name. The best way is to follow the convention of naming each variable starting with a lower case letter and each class starting with an upper case letter.

You have a bunch of these cases, and they make it ambiguous whether eg Movement.x gets the static variable x from the Movement class or the instance variable x from the Movement variable -- which is null so that expression will give a NullPointerException.

Now you need to set those variables before use.

If you're still puzzled on what's causing the exception, step through the code in the debugger and see where the exception happens and what's null at that point. You can split that statement into smaller parts if it has more than one expression that could cause the exception.

Eg new Rectangle(aIMovement.x[0] + 16, aIMovement.y[0], 32, 64) will throw a NPE if aIMovement is null or aIMovement.x is null or aIMovement.y is null.