Combining a map operation with reduce

The following lines show an imperative code version of a for loop that calculates the sum of all the lowestScore values for the mobile games. The code file for the sample is included in the java_9_oop_chapter_12_01 folder, in the example12_13.java file.

int lowestScoreSum = 0;
for (MobileGame mobileGame : repository.getAll()) {
    lowestScoreSum += mobileGame.lowestScore;
}
System.out.println(lowestScoreSum);

The code is very easy to understand. The lowestScoreSum variable has a starting value of 0, and each iteration of the for loop retrieves a MobileGame instance from the List<MobileGame> returned by the repository.getAll() method and increases the value of the lowestScoreSum variable with the value of the mobileGame.lowestScore field.

We can combine the map and reduce operations to create a functional version of the previous imperative code to calculate the sum of all the lowestScore values for the mobile games. The next lines chain a call to...