Synchronization, When to or not to use?

Question

I have started learning concurrency and threads in Java. I know the basics of synchronized (i.e. what it does). Conceptually I understand that it provides mutually exclusive access to a shared resource with multiple threads in Java. But when faced with an example like the one below I am confused about whether it is a good idea to have it synchronized. I know that critical sections of the code should be synchronized and this keyword should not be overused or it effects the performance.

public static synchronized List<AClass> sortA(AClass[] aArray) 
{
    List<AClass> aObj = getList(aArray);

    Collections.sort(aObj, new AComparator());

    return aObj;
}

public static synchronized List<AClass> getList(AClass[] anArray) 
{
    //It converts an array to a list and returns
}

Answer 1

Assuming each thread passes a different array then no synchronization is needed, because the rest of the variables are local.

If instead you fire off a few threads all calling sortA and passing a reference to the same array, you'd be in trouble without synchronized, because they would interfere with eachother.

Beware, that it would seem from the example that the getList method returns a new List from an array, such that even if the threads pass the same array, you get different List objects. This is misleading. For example, using Arrays.asList creates a List backed by the given array, but the javadoc clearly states that Changes to the returned list "write through" to the array. so be careful about this.