How to initialize HashSet values by construction?

I need to create a Set with initial values.

Set<String> h = new HashSet<String>();
h.add("a");
h.add("b");

Is there a way to do this in one line of code?

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There is a shorthand that I use that is not very time efficient, but fits on a single line:

Set<String> h = new HashSet<>(Arrays.asList("a", "b"));

Again, this is not time efficient since you are constructing an array, converting to a list and using that list to create a set.

When initializing static final sets I usually write it like this:

public static final String[] SET_VALUES = new String[] { "a", "b" };
public static final Set<String> MY_SET = new HashSet<>(Arrays.asList(SET_VALUES));

Slightly less ugly and efficiency does not matter for the static initialization.

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Collection literals were scheduled for Java 7, but didn't make it in. So nothing automatic yet.

You can use guava's Sets :

Sets.newHashSet("a", "b", "c")

Or you can use the following syntax, which will create an anonymous class, but it's hacky:

Set<String> h = new HashSet<String>() {{
    add("a");
    add("b");
}};

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In Java 8 I would use:

Set<String> set = Stream.of("a", "b").collect(Collectors.toSet());

This gives you a mutable Set pre-initialized with "a" and "b". Note that while in JDK 8 this does return a HashSet , the specification doesn't guarantee it, and this might change in the future. If you specifically want a HashSet , do this instead:

Set<String> set = Stream.of("a", "b")
                        .collect(Collectors.toCollection(HashSet::new));

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