java stack NullPointerException

Question

I have a class Floor which has a Stack of Blocks and I don't know how to initialize it. I had tried like this:

public class Floor {
    private Stack<Block> stack;
    private static int size;
    public void setStack(Stack<Block> stack) {
        this.stack = stack;
    }
    public void addBlock(Block b){
        stack.push(b);
    }
}

public class InputDevice {
    Block a0=new Block('I',false);
    Floor [] floor=new Floor[5];
    Stack<Block> stack=new Stack<Block>();
    floor[0].setStack(stack);
    floor[0].addBlock(a0);
}

Why are you using the static modifier for the size field?

Hovercraft Full Of Eels
– Hovercraft Full Of Eels

2013-01-12 11:36:26 +00:00
Commented Jan 12, 2013 at 11:36 — Hovercraft Full Of Eels
– Hovercraft Full Of Eels, Commented Jan 12, 2013 at 11:36

Kent · Accepted Answer · 2013-01-12 11:33:47Z

5

Floor [] floor=new Floor[5];

you declared the array, but you didn't init the elements, then :

floor[0].setStack(stack); floor[0] is null, npe!

also I suggest that in your Floor class, addBlock(Block b) method, check if the stack is null, if it is null, otherwise it would have problem(NPE) if someone init Floor, and directly floor.addBlock(b).

answered Jan 12, 2013 at 11:33

Kent

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Hovercraft Full Of Eels · Accepted Answer · 2013-01-12 11:32:32Z

2

You've not initialized any of the Floor objects in the array yet. When you create an array of objects it's like creating an egg carton. You can't use any eggs until you put some in the carton first. You can't use any objects in the array before you've initialized them, which is often done within a for loop. i.e.,

Floor [] floor=new Floor[5];
for (int i = 0; i < floor.length; i++) {
  floor[i] = new Floor();
}

answered Jan 12, 2013 at 11:32

Hovercraft Full Of Eels

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2 Comments

Alexandru Cimpanu Over a year ago

Thank you! I thought that by new Floor[5] I was creating 5 Floor objects when I actually had just a pointer to a 5 Floor memory space.

Hovercraft Full Of Eels Over a year ago

@AlexandruCimpanu: exactly! The other metaphor I like to use is that of a likening an array to a parking lot. You first have to fill it with cars to be able to drive one.

Pratik · Accepted Answer · 2013-01-12 11:35:56Z

0

try this code

public class Floor {
    private Stack<Block> stack;
    private static int size;
    public void setStack(Stack<Block> stack) {
        this.stack = stack;
    }
    public void addBlock(Block b){
        stack.push(b);
    }
}

public class InputDevice {
    Block a0=new Block('I',false);
    Floor [] floor=new Floor[5];
    floor[0] = new Floor();
    Stack<Block> stack=new Stack<Block>();
    floor[0].setStack(stack);
    floor[0].addBlock(a0);
}

answered Jan 12, 2013 at 11:35

Pratik

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