Allowing instances of a particular class to be created by only one other class in Java?

Question

Suppose that I have two classes A and B and I want to make it so that instances of B can only be created in A and in B itself. I don't want any other class (including subclasses of A) to be allowed to create instances of B. Is there any way of doing this in Java?

Here is a bit of code if it's not clear what I am trying to do:

public class A {
    B instance;
    public A(){
        // Still allows for subclasses to access B
        instance = B.getInstance((Object)this);
    }
}

Here is the class whose construction I want to limit:

public class B {

    // If I make this public all classes can create it, but
    // if I make it private without any getter methods then
    // no other classes but itself can create it
    private B(){}

    // Problem with this is that subclasses of  A
    // can also create instances of B
    public static B getInstance(Object o){
        if(o instanceof A)
            return new B();
        else
            return null;
    } 
}

I have tried Googling and searching on StackOverflow for possible solutions, but the closest thing that I have found is using a Singleton design pattern with a modified getInstance() method to make sure only a class with a particular type can have access to instances of class B. While this works fairly well, it still enables any subclass which extends A to get instances of B. Is there any way to stop this from occurring or would it ruin the whole point of subclassing if a subclass couldn't do what its superclass could do?

Answer 1

Suppose that I have two classes A and B and I want to make it so that instances of B can only be created in A and in B itself. I don't want any other class (including subclasses of A) to be allowed to create instances of B.

You could make B a private inner class of the A class.

Answer 2

While this works fairly well, it still enables any subclass which extends A to get instances of B. Is there any way to stop this from occurring or would it ruin the whole point of subclassing if a subclass couldn't do what its superclass could do?

If you don't want your class A to be subclassed, you can make class A final, or can have a private constructor for A. Although as suggested in previous answers,, its better to create private inner classes.

public final class A {
    B instance;
    public A(){
        // Still allows for subclasses to access B
        instance = B.getInstance((Object)this);
    }
}

Answer 3

A can present something to B that only A can possess. For example

public class A
{
    public static class Pass
    {
        private Pass(){}

Only A can create an A.Pass object. If such objects are only transmitted from A to B, nobody else can get a hold of them and pretend to be A.

public class B
{
    public static B getInstance(A.Pass token)
    {
        if(pass==null) 
            throw ...      
        else
            caller must be A