ImmutableMap.Builder should have remove method

[dapengzhang0]

Without Builder.remove() method, it will be very inefficient to create a new immutable map from an existing immutable map removing a key:

Map newMapWithoutKey1 = ImmutableMap.builder()
    .putAll(map)
    .remove("key1")
    .build();

[kluever]

In general, we recommend mutating a mutable map type instead of trying to pretend a builder is a mutable map.

So, use a LinkedHashMap or HashMap, perform your additions/removals, and then ImmutableMap.copyOf(mutableMap)

[dapengzhang0]

Then one has to make a mutable copy of an existing map, even the original map is mutable, because we may not want the original map to be mutated.

Map copy = new LinkedHashMap(map);
copy.remove("key1");
Map newMapWithoutKey1 = ImmutableMap.copyOf(copy);

[kluever]

That seems fine to me.

If you're super worried about performance, you can add the entries from the existing mutable map to the ImmutableMap builder in a loop, and just don't add the entry if it has a certain key (the key you want to "remove").

Or, alternatively, use a lazy Maps.filterKeys() and Predicate.not(badKey), wrapped in ImmutableMap.copyOf().

[dapengzhang0]

Thanks. It is fine to me as well.

If I'm supper worried about performance, I have to use ImmutableMap.builderWithExpectedSize() before adding entries in a loop, otherwise the performance is almost the same as making a mutable copy. Is my understanding correct?

[kluever]

That sounds approximately correct, yes.