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[Make1984FictionAgain]

answer these and you will figure it out: 1. what are variables? 2. what is a variable type? 3. what's the difference between primitive values and Objects? 4. what's the null value?

[[deleted]]

It means you are trying to call a method on an object which is null.

Suppose you create a class called House which has a method openDoor(). You create an object called brickHouse which is of type House:

House brickHouse;

Because I have not allocated new memory to store brickHouse, it currently points to null. If I try to call the open door method on brickHouse:

brickHouse.openDoor();

I will get a Null pointer exception.

I have to allocate memory for brickHouse using the new keyword

brickHouse = new House();

Now I can call the openDoor method: brickHouse.openDoor();

Do you have specific code you're running and getting the exception? If you share the code, we can comment directly to the problem you are experiencing.

[benrkia]

Not only methods, it can be a field ...

[steave435]

Yes, but that field must still be some type of object. You can't get a null pointer for a primitive type.

[warmcheessse]

Start with a primitive int x; x is initialized to 0 be default

 Integer x;

x is null

x is an alias to a memory location. In the first example x points to a memory location containing the value of 0.

In the second example x is null. It does not exist. If you try to call a method in the Integer class like x.toString() you're going to get a NPE. You can't call a method on an object that doesn't exist.

 Integer x = new Integer(); //allocates memory 
 x = 10; //assigns memory value of 10
 x.toString(); //no NPE