Returns a Collector that accumulates elements into a Map whose keys and values are the result of applying the provided mapping functions to the input elements.

If the mapped keys contains duplicates (according to Object.equals(Object)), the value mapping function is applied to each equal element, and the results are merged using the provided merging function. The Map is created by a provided supplier function.

Parameters:

<T>		the type of the input elements
<K>		the output type of the key mapping function
<U>		the output type of the value mapping function
<M>		the type of the resulting Map
keyMapper		a mapping function to produce keys
valueMapper		a mapping function to produce values
mergeFunction		a merge function, used to resolve collisions between values associated with the same key, as supplied to Map.merge(Object, Object, BiFunction)
mapSupplier		a function which returns a new, empty Map into which the results will be inserted

Returns:  a Collector which collects elements into a Map whose keys are the result of applying a key mapping function to the input elements, and whose values are the result of applying a value mapping function to all input elements equal to the key and combining them using the merge function

See also:
toMap(Function, Function), toMap(Function, Function, BinaryOperator), toConcurrentMap(Function, Function, BinaryOperator, Supplier)

@implNote The returned Collector is not concurrent. For parallel stream pipelines, the combiner function operates by merging the keys from one map into another, which can be an expensive operation. If it is not required that results are merged into the Map in encounter order, using toConcurrentMap(Function, Function, BinaryOperator, Supplier) may offer better parallel performance.