[java] Why is Java Vector (and Stack) class considered obsolete or deprecated?

Why is Java Vector considered a legacy class, obsolete or deprecated?

Isn't its use valid when working with concurrency?

And if I don't want to manually synchronize objects and just want to use a thread-safe collection without needing to make fresh copies of the underlying array (as CopyOnWriteArrayList does), then is it fine to use Vector?

What about Stack, which is a subclass of Vector, what should I use instead of it?

You can use the synchronizedCollection/List method in java.util.Collection to get a thread-safe collection from a non-thread-safe one.

Vector was part of 1.0 -- the original implementation had two drawbacks:

1. Naming: vectors are really just lists which can be accessed as arrays, so it should have been called ArrayList (which is the Java 1.2 Collections replacement for Vector).

2. Concurrency: All of the get(), set() methods are synchronized, so you can't have fine grained control over synchronization.

There is not much difference between ArrayList and Vector, but you should use ArrayList.

From the API doc.

As of the Java 2 platform v1.2, this class was retrofitted to implement the List interface, making it a member of the Java Collections Framework. Unlike the new collection implementations, Vector is synchronized.

Besides the already stated answers about using Vector, Vector also has a bunch of methods around enumeration and element retrieval which are different than the List interface, and developers (especially those who learned Java before 1.2) can tend to use them if they are in the code. Although Enumerations are faster, they don't check if the collection was modified during iteration, which can cause issues, and given that Vector might be chosen for its syncronization - with the attendant access from multiple threads, this makes it a particularly pernicious problem. Usage of these methods also couples a lot of code to Vector, such that it won't be easy to replace it with a different List implementation.

java.util.Stack inherits the synchronization overhead of java.util.Vector, which is usually not justified.

It inherits a lot more than that, though. The fact that java.util.Stack extends java.util.Vector is a mistake in object-oriented design. Purists will note that it also offers a lot of methods beyond the operations traditionally associated with a stack (namely: push, pop, peek, size). It's also possible to do search, elementAt, setElementAt, remove, and many other random-access operations. It's basically up to the user to refrain from using the non-stack operations of Stack.

For these performance and OOP design reasons, the JavaDoc for java.util.Stack recommends ArrayDeque as the natural replacement. (A deque is more than a stack, but at least it's restricted to manipulating the two ends, rather than offering random access to everything.)