Question

My code is something like:

public class Foo {
    public int a;
    Bar[] bar = new Bar[10];

    a = bar[0].baz;
}

public class Bar {
    public int b;

    public Bar () { //I tried doing away with this constructor, but that didn't
                    //fix anything
        b = 0;
    }

    public int Baz () {
        //do somthing
    }
}

And I get an error message similar to:

Exception in thread "Foo" java.lang.NullPointerException

at whichever line in Foo I try to call any function or value of the class Bar. How do I prevent bar[] from being null?

EDIT: After some fiddling, I finally got it fixed, thanks everyone!
However, I couldn’t call the constructor to fix things up; I had to create another function and call that function from Main (in my case, class Foo is actually class Main, if that really matters). My final result:

public class Foo {
    public int a;
    Bar[] bar = new Bar[10];

    public Foo () { //this constructor wasn't called for some reason... I checked this
                    //by using System.out.println... no message was print onscreen
        for (int a = 0; a < bar.length; a++)
            bar[a] = new Bar();
    }

    public static void initializeFoo () {
        for (int a = 0; a < bar.length; a++)
            bar[a] = new Bar();
    }

    public static void Foo () {
        initializeFoo();
        a = bar[0].baz;
    }
}

Anybody want to help me out with that, or am I supposed to create another question? 🙂

Answer 1

You have allocate memory for the references by writing this Bar[] bar = new Bar[10]; but who will allocate memory for the array elements? Infact arrays of reference types are initialized with null references.

You need to allocate memory to array elements too:

for(int i=0; i<bar.length; ++i)
   bar[i]=new Bar();