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

You can do it in Java 6:

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

But why? I don't find it to be more readable than explicitly adding elements.

Answer 2

A generalization of coobird's answer's utility function for creating new HashSets:

public static <T> Set<T> newHashSet(T... objs) {
    Set<T> set = new HashSet<T>();
    for (T o : objs) {
        set.add(o);
    }
    return set;
}

Answer 3

I feel the most readable is to simply use google Guava:

Set<String> StringSet = Sets.newSet("a", "b", "c");

Answer 4

The Builder pattern might be of use here. Today I had the same issue. where I needed Set mutating operations to return me a reference of the Set object, so I can pass it to super class constructor so that they too can continue adding to same set by in turn constructing a new StringSetBuilder off of the Set that the child class just built. The builder class I wrote looks like this (in my case it's a static inner class of an outer class, but it can be its own independent class as well):

public interface Builder<T> {
    T build();
}

static class StringSetBuilder implements Builder<Set<String>> {
    private final Set<String> set = new HashSet<>();

    StringSetBuilder add(String pStr) {
        set.add(pStr);
        return this;
    }

    StringSetBuilder addAll(Set<String> pSet) {
        set.addAll(pSet);
        return this;
    }

    @Override
    public Set<String> build() {
        return set;
    }
}

Notice the addAll() and add() methods, which are Set returning counterparts of Set.add() and Set.addAll(). Finally notice the build() method, which returns a reference to the Set that the builder encapsulates. Below illustrates then how to use this Set builder:

class SomeChildClass extends ParentClass {
    public SomeChildClass(String pStr) {
        super(new StringSetBuilder().add(pStr).build());
    }
}

class ParentClass {
    public ParentClass(Set<String> pSet) {
        super(new StringSetBuilder().addAll(pSet).add("my own str").build());
    }
}