Question

In the following code snippet, why is it desirable to return a copy of data[i]. What exactly happen in a multi-threaded environment if copy is not made.

protected Object[] data; 
..
public synchronized Object get(int i) 
throws NoSuchElementException 
{   if (i < 0 || i >= size ) 
      throw new NoSuchElementException();
    return data[i]; 
}

Answer 1

why is it desirable to return a copy of data[i].

You’re returning a copy of the reference at index i, not a copy of the object.

Unless you create a copy of the object through for instance data[i].clone(), you’ll always have a single object and share references to it among your threads. There is nothing wrong with sharing references to a single object among several threads.

What exactly happen in a multi-threaded environment if copy is not made.

Well, unless you synchronize your threads using the synchronized methods, wait/notify or the java.util.concurrent-classes you may end up with race-conditions. A race condition is basically a situation where the result of the execution is dependent on the particular scheduling (the order in which the threads may execute).

If you share objects of a certain class among threads, you should design it to be “thread safe”. If your object represents a value object I recommend you to make it immutable.