Java: how does a component know its owner

问题

Suppose I have a class A and a class B.

public class A {

    private B b;

    public A() {
        this.b = new B();
    }

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

public class B {

    public String getSome() {
        return "Get some!";
    }
}

I know I can get B through A, because A has (or owns) B: new A().getB().
But if I have B, can I get A?

回答1:

Sure, just add routine getA() in you class B, and change the line in your constructor to

public A() {
    this.b = new B(this);
}

This of course assumes your class B has a constructor which accepts an A, e.g.,

public B(A a) {
    this.a = a;
}

回答2:

B needs an explicit reference to its owner:

public class B {
  private final A owner;

  public B(A owner) {
    this.owner = owner;
  }

  public A getOwner() {
    return owner;
  }
}

And in A:

public A() {
  b = new B(this);
}

回答3:

Nope. There is no such thing as an 'owner' in Java. Any object can be referenced by any number of other objects.

回答4:

If you need B to always be bound to an instance of A, make B an inner class of A:

class A {

    B b = new B();

    class B {
        String getSome() {
            // this will refer to the enclosing A
            return A.this.toString();
        }
    }
}

An inner (non-static) class always has an implicit reference to the enclosing instance and cannot exist without it. In order to instantiate B from outside, you need a nasty syntax: B b = new A().new B();

回答5:

No you cannot. B has no reference to A.

回答6:

No.

Class a has reference to class B, but class B has no reference to class A. References are one way only.

回答7:

No, that's not possible. You're looking for backreferences, but we have to create them in the code if needed.

If you want to collect all referencers to B, you could do this with a constructor or with a factory (pattern) that creates B's. I'll show the factory:

public class B {

   private static Set<? extends Object> referencers = new HashSet<? extends Object>();
   private B(){}  // no public constructor
   public static create(Object parent) {
     // cooperative approach, the caller should pass "this"
     referencers.add(parent);
   }
   public static remove(Object parent) {
     referencers.remove(parent);
   }
}

回答8:

you can also use inner classes

package test;

public class A {

B b = null;

public B getB()
{
    return b;
}

public class B {

    public A getA()
    {
        return A.this;
    }
}

public static void main(String[] args) {
    B b = new A().new B();
}

}

来源：https://stackoverflow.com/questions/5022604/java-how-does-a-component-know-its-owner

标签

java

ownership