Java: Constructing a Map from a single expression?

Question

I wonder why there's no way to construct a map with values in a single expression. Or is it? I'd expect

new HashMap().add(key, value).add(key, value)...;

I can't find anything like that even in Commons Collections.

Did I miss some way in JDK or Commons?

Answer 1

Guava has that with its ImmutableMap:

final Map<Foo, Bar> map = ImmutableMap.of(foo1, bar1, foo2, bar2, etc, etc);

Bonus: ImmutableMap's name is not a lie ;)

Note that there are 5 versions of the .of() method, so up to 5 key/value pairs. A more generic way is to use a builder:

final Map<Foo, Bar> map = ImmutableMap.<Foo, Bar>builder()
    .put(foo1, bar1)
    .put(foo2, bar2)
    .put(foo3, bar3)
    .put(etc, etc)
    .build();

Note, however: this map does not accept null keys or values.

Alternatively, here is a poor man's version of ImmutableMap. It uses a classical builder pattern. Note that it does not check for nulls:

public final class MapBuilder<K, V>
{
    private final Map<K, V> map = new HashMap<K, V>();

    public MapBuilder<K, V> put(final K key, final V value)
    {
        map.put(key, value);
        return this;
    }

    public Map<K, V> build()
    {
        // Return a mutable copy, so that the builder can be reused
        return new HashMap<K, V>(map);
    }

    public Map<K, V> immutable()
    {
        // Return a copy wrapped into Collections.unmodifiableMap()
        return Collections.unmodifiableMap(build());
    }
}

Then you can use:

final Map<K, V> map = new MapBuilder<K, V>().put(...).put(...).immutable();