Convert Set<Object> to Collection<String>

Question

I have a Set<Object>.

I need to get a Collection<String> from it.

I can think of making a for loop to add and cast all the Objects, but that is ugly and probably also slow.

@Override
public Collection<String> keys()
{
    // props is based on HashMap
    Set<String> keys = new HashSet<>();
    for (Object o : props.keySet()) {
        keys.add((String) o);
    }
    return keys;
}

What is the right way?

Solution

If you know that all the Objects inside the HashSet are strings, you can just cast it:

Collection<String> set = (Collection<String>)(Collection<?>)props.keySet();

Java implements generics with erasure, meaning that the HashSet itself doesn't know at runtime that it's a HashSet<Object>--it just knows it's a HashSet, and the compiler is responsible for helping programmers to avoid doing things that would create runtime exceptions. But if you know what you're doing, the compiler won't prevent you from doing this cast.

Comments

[MightyPork]

Sorry I wasn't clear enough. Check if the answer is still valid for what I really am doing.

[StriplingWarrior]

@MightyPork: Yes, the answer is still valid. Since you don't have to do any specific steps for conversion (you're just casting Objects to Strings), you can achieve the same end by casting the collection itself.

[MightyPork]

No good, I tried return (Set<String>) props.keySet(); but it said I Cannot cast from Set<Object> to Set<String>.

[StriplingWarrior]

@MightyPork: Try my update. I've been away from java for too long.

[MightyPork]

Okay now this at least compiles. I'll have to fix a lot more code before I can try if it actually works. It's "Unchecked" though - is that okay?

[StriplingWarrior]

@MightyPork: I don't know enough about your code to be able to tell you whether this is "okay." I'd personally question why props isn't cast to the right type in the first place--that would give you better type safety, and avoid the need for this kind of hack. Your original code would have the advantage of "failing fast" if any objects in the keySet are not strings, but you're the one who says you know they'll always be strings.

[Stuart Marks]

@MightyPork Add @SuppressWarnings("unchecked") before this line and it will shut off the warning. Of course, only do this if you are sure the contents are all strings.