Java streams | groupingby the same element

I have a stream of words that I want to sort according to the appearance of the same elements (= words)

For example: {Hello, world, hello}

to

Map<String,List<String>>

Hello}

World, {world}

What have I got so far:

Map<Object,List<String>> list = streamofWords.collect(Collectors.groupingBy(???));

Problem 1: the stream seems to have lost information about the string it is processing, so the compiler forces me to change the type to object, list

Question 2: I don't know what to put in the gastrointestinal tract and group them in the same way I know I can handle individual elements in a lambda expression, but I don't know how to get to the "outside" of each element to check for equality

thank you

Solution

The keyextractor you are searching for is the identity function:

Map<String,List<String>> list = streamofWords.collect(Collectors.groupingBy(Function.identity()));

Edit supplementary notes:

> Function. Identity () uses a method to return a 'function', which only returns the parameters it obtains. > Collectors. Groupingby (function < s, k > keyextractor) provides a collector that collects all elements of the stream to map < K, list < s > > It uses the keyextractor implementation to check the s class object of the stream and derive the key of type K from it This key is the key of the map, which is used to get (or create) the list in the result map to which the flow element is added