Question

If I want an empty map or a one element map, Java Collections has a method for that. Why is there no method for more than one element? What’s the best way to create a static final map with 2 elements in it? I know I can do something like:

private static final Map<String, String> MAP = new HashMap<String, String>() {
    { put("a", "b"); put("c", "d"); }
};

But then Eclipse complains about the serialVersionUID…

Answer 1

The reason Collections has methods for 0 and 1 entry maps is because they are special cases… the empty Map is an immutable singleton for example.

For what you want to do, though, I’d strongly recommend using Guava. Its Immutable* collections (ImmutableMap specifically) are what you want I think:

private static final ImmutableMap<String, String> MAP = ImmutableMap.of(
    "a", "b",
    "c", "d");

You can do the above for small maps, and for bigger maps you can write:

private static final ImmutableMap<String, String> MAP = 
    ImmutableMap.<String, String>builder()
      .put("a", "b")
      .put("b", "c")
      ...
      .put("y", "z")
      .build();

If you don’t use Guava, you’ll still likely want to ensure that this map can’t be changed. This is a lot uglier:

private static final Map<String, String> MAP;

static {
  Map<String, String> temp = new HashMap<String, String>();
  temp.put("a", "b");
  temp.put("b", "c");
  MAP = Collections.unmodifiableMap(temp);
}