What's the difference between these two java variable declarations?

Question

public class SomeClass {
    private HashSet<SomeObject> contents = new HashSet<SomeObject>();
    private Set<SomeObject> contents2 = new HashSet<SomeObject>();
}

What's the difference? In the end they are both a HashSet isn't it? The second one looks just wrong to me, but I have seen it frequently used, accepted and working.

Answer 1

Set is an interface, and HashSet is a class that implements the Set interface.

Declaring the variable as type HashSet means that no other implementation of Set may be used. You may want this if you need specific functionality of HashSet.

If you do not need any specific functionality from HashSet, it is better to declare the variable as type Set. This leaves the exact implementation open to change later. You may find that for the data you are using, a different implementation works better. By using the interface, you can make this change later if needed.

You can see more details here: When should I use an interface in java?