Java 8 Map merge VS compute, essential difference?

The documentation of Map#compute(K, BiFunction) says:

Attempts to compute a mapping for the specified key and its current mapped value (or null if there is no current mapping). For example, to either create or append a String msg to a value mapping:

map.compute(key, (k, v) -> (v == null) ? msg : v.concat(msg))

(Method merge() is often simpler to use for such purposes.)

If the remapping function returns null, the mapping is removed (or remains absent if initially absent). If the remapping function itself throws an (unchecked) exception, the exception is rethrown, and the current mapping is left unchanged.

The remapping function should not modify this map during computation.

And the documentation of Map#merge(K, V, BiFunction) says:

If the specified key is not already associated with a value or is associated with null, associates it with the given non-null value. Otherwise, replaces the associated value with the results of the given remapping function, or removes if the result is null. This method may be of use when combining multiple mapped values for a key. For example, to either create or append a String msg to a value mapping:

map.merge(key, msg, String::concat)

If the remapping function returns null, the mapping is removed. If the remapping function itself throws an (unchecked) exception, the exception is rethrown, and the current mapping is left unchanged.

The remapping function should not modify this map during computation.

The important differences are:

• For compute(K, BiFunction<? super K, ? super V, ? extends V>):

  • The BiFunction is always invoked.
  • The BiFunction accepts the given key and the current value, if any, as arguments and returns a new value.
  • Meant for taking the key and current value (if any), performing an arbitrary computation, and returning the result. The computation may be a reduction operation (i.e. merge) but it doesn't have to be.

• For merge(K, V, BiFunction<? super V, ? super V, ? extends V>):

  • The BiFunction is invoked only if the given key is already associated with a non-null value.
  • The BiFunction accepts the current value and the given value as arguments and returns a new value. Unlike with compute, the BiFunction is not given the key.
  • Meant for taking two values and reducing them into a single value.