How to initialize HashSet values by construction?

Question

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? For instance, it's useful for a final static field.

Answer 1

If the contained type of the Set is an enumeration then there is java built factory method (since 1.5):

Set<MY_ENUM> MY_SET = EnumSet.of( MY_ENUM.value1, MY_ENUM.value2, ... );

Answer 2

import com.google.common.collect.Sets;
Sets.newHashSet("a", "b");

or

import com.google.common.collect.ImmutableSet;
ImmutableSet.of("a", "b");

Answer 3

This is an elegant solution:

public static final <T> Set<T> makeSet(@SuppressWarnings("unchecked") T... o) {
        return new HashSet<T>() {
            private static final long serialVersionUID = -3634958843858172518L;
            {
                for (T x : o)
                   add(x);
            }
        };
}

Answer 4

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");
}};

Answer 5

With Eclipse Collections there are a few different ways to initialize a Set containing the characters 'a' and 'b' in one statement. Eclipse Collections has containers for both object and primitive types, so I illustrated how you could use a Set<String> or CharSet in addition to mutable, immutable, synchronized and unmodifiable versions of both.

Set<String> set =
    Sets.mutable.with("a", "b");
HashSet<String> hashSet =
    Sets.mutable.with("a", "b").asLazy().into(new HashSet<String>());
Set<String> synchronizedSet =
    Sets.mutable.with("a", "b").asSynchronized();
Set<String> unmodifiableSet =
    Sets.mutable.with("a", "b").asUnmodifiable();

MutableSet<String> mutableSet =
    Sets.mutable.with("a", "b");
MutableSet<String> synchronizedMutableSet =
    Sets.mutable.with("a", "b").asSynchronized();
MutableSet<String> unmodifiableMutableSet =
    Sets.mutable.with("a", "b").asUnmodifiable();

ImmutableSet<String> immutableSet =
    Sets.immutable.with("a", "b");
ImmutableSet<String> immutableSet2 =
    Sets.mutable.with("a", "b").toImmutable();

CharSet charSet =
    CharSets.mutable.with('a', 'b');
CharSet synchronizedCharSet =
    CharSets.mutable.with('a', 'b').asSynchronized();
CharSet unmodifiableCharSet =
    CharSets.mutable.with('a', 'b').asUnmodifiable();
MutableCharSet mutableCharSet =
    CharSets.mutable.with('a', 'b');
ImmutableCharSet immutableCharSet =
    CharSets.immutable.with('a', 'b');
ImmutableCharSet immutableCharSet2 =
    CharSets.mutable.with('a', 'b').toImmutable();

Eclipse Collections is compatible with Java 5 - 8.

Note: I am a committer for Eclipse Collections.

Answer 6

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));