If using function that can return null

Question

There is a function get() that return a value if it is there in ArrayDeque otherwise returns null i.e x can be some value or null. If get() returns x then function B() should perform some computations otherwise should not do anything.

T get()
{
    //compute x
    return x;
}

void B()
{
     int z;
     if(y.get()!=null)
     {
          z=y.get(); // gives null pointer exception
          .....
     }
}

The problem is that y.get() already returns the value which is not assigned to any variable, thus gives null pointer exception. If i use something like if((z=y.get()) != 0) it gives exception in cases when x is null. How can i achieve this functionality?

How can you return null and assign it to the int variable z ?? Its type mismatch. — Sandeep Pathak
– Sandeep Pathak, Commented Mar 13, 2012 at 6:19
Neither of the versions you write should throw, unless it's get itself that throws. Are you sure you're looking at the right part of your code? — Mat
– Mat, Commented Mar 13, 2012 at 6:19
So, does get() return T, or an Integer? B() seems to assume that it's an Integer. — Dmitri
– Dmitri, Commented Mar 13, 2012 at 6:20
y.get() will throw a null pointer only if y is null...or may be some logic inside your get() is throwing an exception... — Shashank Kadne
– Shashank Kadne, Commented Mar 13, 2012 at 6:20

Jon Skeet · Accepted Answer · 2012-03-13 06:58:30Z

I suspect this is an ArrayDeque<Integer>, right?

When you have:

int z = y.get();

That's like saying:

int z = ((Integer) y.get()).intValue();

Just use:

Integer z = y.get();

instead. Then you can test whether z is null. On the other hand, if y.get() has already returned a non-null value, I'm surprised if it's then returning a null value - you'd expect it to return the same thing twice, right? Are there other threads involved?

Additionally, it's not clear what you mean by this:

The problem is that y.get() already returns the value which is not assigned to any variable, thus gives null pointer exception.

I don't see where the "thus" in here... it's fine to call a method and not store the return value in a variable. If that's throwing a NullPointerException, it would really suggest that y is null. Of course, all of this would be easier to diagnose if you would post a short but complete program demonstrating the problem.

As an aside, it's not clear why you're calling y.get() twice in the first place. I would restructure the code to:

void B()
{
     Integer z = y.get();
     if (z != null)
     {
         // Use z
     }
}

Do you really want to call it twice?

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