Java: Forward declarations of classes in namespaces

Question

How do I do forward declarations in Java?

I have two classes, each needs to call a method in the other and they both reside within different namespaces. For example...

package one;

class A {
    public void foo() {
        B b = new B();
        b.bah();
    }
}

and

package two;

class B {
    public void bah() {
        A a = new A();
        a.foo();
    }
}

UPDATE

In Eclipse when this code is encountered a compile time error would be thrown up "A cycle was detected in the build path...".

Answer 1

Just import them. Java is a lot cleverer than C++ about these things.

Answer 2

Well, you import the classes:

package one;

import two.B;

class A {
    public void foo() {
        new B().bah();
    }
}

or use their full name:

package one;

class A {
    public void foo() {
        new two.B().bah();
    }
}

But, needless to say, that will cause an exception to be thrown (at least, in the snippet you posted...).

Answer 3

Given the comments here (ie you don't need to do explicit forward declarations as the compiler can cope) I have downgraded the Eclipse error to a warning and will have to just ignore it.

To do this go to...

Window > Preferences > Java > Compiler > Building > Build path problems > Circular dependences

Answer 4

In Eclipse when this code is encountered a compile time error would be thrown up "A cycle was detected in the build path...".

I think that Eclipse is complaining because you have a circular dependency between classes in different Eclipse projects in your workspace. Eclipse wants to be able to build projects in a linear sequence.

If you put the mutually dependent classes into the same project, Eclipse would be happy.