Chaining many operations with map and reduce

We can chain filter, map, and reduce operations. The following code adds a new getHighestScoreSumForMinPlayersCount method to the MemoryMobileGameRepository class. The code file for the sample is included in the java_9_oop_chapter_12_01 folder, in the example12_19.java file.

public long getHighestScoreSumForMinPlayersCount(int minPlayersCount) {
    return getAll().stream()
        .filter(game -> (game.playersCount >= minPlayersCount))
        .mapToLong(game -> game.highestScore)
        .reduce(0, (sum, highestScore) -> sum + highestScore);
}

The new method performs a filter chained with a mapToLong and finally a reduce operation. The call to filter generates a Stream<MobileGame> with the instance of MobileGame whose playersCount value is equal or greater than the minPlayersCount value received as an argument. The mapToLong method returns a LongStream, that is, a specialized Stream<T> that describes a stream of long primitives...