Java - Having the same reference in multiple class

Question

how to make references to the same instance in multiple objects?

Let say I have Class A :

Class A{
    public A relative = null;
    public int number = 0;
}

and I will create multiple A and those A's relative will be a reference to one of those A.

For example: Let's create four A, so now we have A1, A2, A3, A4.

And the following map will shows (Left Hand Side)'s relative is making reference to (Right Hand Side)

A1 => null,
A2 => A1,
A3 => A1,
A4 => A2

What I want to achieve is: When I modified A1 by (A2.relative.number = 1), and will also "apply" to A3's relative (Which supposed to be a reference to the same instance A1)

So, here are what I have tried:

Using setter:
```
public void relateTo(A relative){
    this.relative = relative;
}
```
This doesn't works, since Java is passing reference by value. Where the relative inside the scope of this method is just a copy. So whenever I make any changes to it from outside, it doesn't make changes to this.
Public the variable and direct access it
```
A2.relative = A1;
```
But nope, this doesn't works.

I have no idea how to achieve this, any ideas?

It would really help if you'd show a short but complete example. If A2 and A3 both refer to A1, then it really should be fine. — Jon Skeet
– Jon Skeet, Commented Dec 6, 2013 at 15:33

christopher · Accepted Answer · 2013-12-06 15:33:38Z

3

When I modified A1 by (A2.relative.number = 1), and will also "apply" to A3's relative (Which supposed to be a reference to the same instance A1)

This principle is simlpy achieved by making two references point at the same object

Example

A a1 = new A();
A a2 = new A();
A a3 = new A();

a2.relative = a1;
a3.relative = a1;
// Now both a2 relative and a3 relative point to the same A object.

Now if I make a change to a2 relative, say

a2.relative.number = 2;

Then a3 relative will have also changed.

Why?

This is because, when you declare an object, you're actually handling the pointer to that object; not the object itself. What the code above does, is create one relative, and make two classes point to it.

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answered Dec 6, 2013 at 15:33

christopher

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Comments

Duncan Jones · Accepted Answer · 2013-12-06 15:33:30Z

1

This doesn't works, since Java is passing reference by value. Where the relative inside the scope of this method is just a copy. So whenever I make any changes to it from outside, it doesn't make changes to this.

Did you try this?

Java passes object references by value; any changes made to those references affect the object in question. Similar to passing pointers in other languages.

answered Dec 6, 2013 at 15:33

Duncan Jones

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