How to share an object between Java classes?

Question

I have three java class ( A.java & B.java & C.java ) that all of them located in a common package . I want to make an object of class B ( instanceOfB ) in class A and could use that object in class C . How do that ? Thanks

public class A{
   public B instanceOfB;
}

public class B{ 
}

public class C{ 
}

Just... have a B member in C and set both the A and C member to the same object? What have you tried? — kutschkem, Sep 04 '20 at 07:45

score 1 · Answer 1 · answered Sep 04 '20 at 07:45

1

You could have a getter method to retrieve the instance of class B from the A object and use it in C:

public B getB() {
   return this.B;
}

answered Sep 04 '20 at 07:45

Vladimir Stanciu

1,468
1
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That just kicks the can down the road, now the `C` needs access to an `A` instance. – Joachim Sauer Sep 04 '20 at 07:46

score 0 · Answer 2 · answered Sep 04 '20 at 07:45

Somehow you need to pass the instance of B to C during construction. The simplest way is to have a constructor on C:

public class C {
  private B myInstanceOfB;
 
  public C(B instance) {
    this.myInstanceOfB = instance;
  }
}

Now this would mean that whoever creates a C instance must know to do that and have access to a B instance. It's possible that you want to "hide" that requirement, then you can do things like add a factory method for C into the A class:

public class A {
  private B instanceOfB;

  public C createC() {
    return new C(instanceOfB);
  }
}

If you do this you can also make the C constructor package-private to indicate to potential users that they should not attempt to instantiate it directly (and document in the JavaDoc how to get a C instance).

Whether or not that makes sense depends on what the relation between A and C is.

How to share an object between Java classes?

2 Answers2