builder for HashMap

Not quite a builder, but using an initializer:

Map<String, String> map = new HashMap<String, String>() {{
    put("a", "1");
    put("b", "2");
}};

There is no such thing for HashMaps, but you can create an ImmutableMap with a builder:

final Map<String, Integer> m = ImmutableMap.<String, Integer>builder().
      put("a", 1).
      put("b", 2).
      build();

And if you need a mutable map, you can just feed that to the HashMap constructor.

final Map<String, Integer> m = Maps.newHashMap(
    ImmutableMap.<String, Integer>builder().
        put("a", 1).
        put("b", 2).
        build());