Grouping and Partitioning

Learn about the `groupingBy` and `partitioningBy` method.

Introduction to grouping and partitioning

The groupingBy collector groups elements based on a function we provide. For example:

List<Dragon> dragons = getDragons();
Map<Character,List<Dragon>> map =
        .collect(groupingBy(dragon -> dragon.getName().charAt(0)));

Similarly, the partitioningBy method creates a map with a boolean key. For example:

// Group by whether or not the dragon is green
Map<Boolean,List<Dragon>> map =

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