Java generics NullPointerException

Question

I want to make an Universal stack using generics .

public class UniversalStack<E> implements StackInterface<E> {

    private E[] stack;
    private int dim;
    private int index;

    @SuppressWarnings("unused")
    public UniversalStack(int dim)
    {
        this.dim=dim;
        this.index=0;
        @SuppressWarnings("unchecked")
        E[] stack = (E[]) new Object[dim];


    }

    @Override
    public void push(E el) {
        // TODO Auto-generated method stub
        if(index+1<dim)
        {
            stack[index] = el;
            index=index+1;
        }

    }
}

Everything compiles succesfully . The problem comes when I call the following :

UniversalStack<Integer> integerStack = new UniversalStack<>(10);
integerStack.push(new Integer(1));

I get

Exception in thread "main" java.lang.NullPointerException
    at com.java.lab4.UniversalStack.push(UniversalStack.java:41)
    at com.java.lab4.testStack.main(testStack.java:14)

Could you explain me what am I doing wrong ? If I made a stupid mistake don't be harsh on me , I am a beginner so I don't really know much.

James Montagne · Accepted Answer · 2012-02-16 17:37:52Z

7

You're re-declaring stack within your constructor rather than assigning to the outer stack:

E[] stack = (E[]) new Object[dim];

Should be

stack = (E[]) new Object[dim];

therefore stack is null when used in push.

answered Feb 16, 2012 at 17:37

James Montagne

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2 Comments

Teo Over a year ago

Yeah , that was it .. I didn't realize I re-declared the stack member until I read it again after I posted here . Thanks !

Sajan Chandran Over a year ago

I think eclipse tried to warn you, but you added suppresswarnings annotation :-)

dsingleton · Accepted Answer · 2012-02-16 17:40:06Z

-1

Just use the stack class that already exists for Java.

Stack<Integer> stack = new Stack<Integer>();

More documentation is here http://docs.oracle.com/javase/6/docs/api/java/util/Stack.html

answered Feb 16, 2012 at 17:40

dsingleton

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1 Comment

dsingleton Over a year ago

I'm confused, why does this warrant a -1? He's trying to use a stack, and I suggest he used the stack implementation that already exists? Seems like a better idea to use what exists and is tried and true than write your own....

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