Java version of "Pointer Pointer"

Question

I have a situation I've run into a couple of times, where multiple classes from different parts of the architecture want to be looking at the same data structure, which may be entirely replaced (for instance, when a newer version comes from the network.) If I keep the data structure and update the information that's easy enough (everything can reference the same one, and will get the up-to-date info when they look), but if the reference is re-assigned entirely, that isn't reflected elsewhere.

I had an idea to make a super-generic wrapper like:

public class Wrapper<T>{
    public T _value;
}

With the idea that you would never re-assign the wrapper, just the value inside it. All the different components could then point to the wrapper, and the inner value could be re-assigned cleanly when a "new" version came along. (I think I've seen this done with ** in C.)

But, I'm suspicious that I've never seen this before. Is there a better way to address this problem?

AtomicReference might be a good choice, so you are guaranteed up-to-date reads on multiple threads. — Andy Turner
– Andy Turner, Commented Sep 7, 2016 at 21:24
Additionally, consider adding version field to instance of updatable data structure, so observers can detect if data was updated when they were looking at something else. — Victor Sorokin
– Victor Sorokin, Commented Sep 7, 2016 at 21:27

sidgate · Accepted Answer · 2016-09-07 23:33:50Z

2

Use AtomicReference.

String initialReference = "initial value referenced";

AtomicReference<String> atomicStringReference =
    new AtomicReference<String>(initialReference);

String newReference = "new value referenced";
//directly update
atomicReference.set(newReference);

//compare and set
boolean exchanged = atomicStringReference.compareAndSet(initialReference, newReference);
System.out.println("exchanged: " + exchanged);

exchanged = atomicStringReference.compareAndSet(initialReference, newReference);
System.out.println("exchanged: " + exchanged);

//get value
String reference = atomicReference.get();

Code example from here

answered Sep 7, 2016 at 23:33

sidgate

15.3k12 gold badges76 silver badges132 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

Edward Peters Over a year ago

I went with that, though I feel a little guilty, since the thing I'm representing isn't strictly immutable (sometimes I modify it, sometimes I want to replace it)

Bohemian Over a year ago

@EdwardPeters so what? The pointer pointer part will work. If your object is mutable and you're working with a multi-threaded environment, you'll need to handle concurrency there too. You may consider using AtomicReference for your class's mutable fields too.

Edward Peters Over a year ago

@Bohemian Yeah, I synchronized on it, but the documentation I read for AtomicReference said "Immutable" a lot, so I'm worried I might be breaking a contract somewhere.

Collectives™ on Stack Overflow

Java version of "Pointer Pointer"

1 Answer 1

3 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

3 Comments

Your Answer

Sign up or log in

Post as a guest

Related