How can i transform my code using java 8 stream API?

Question

I am writing a simple method to print out statistics of a series of outcomes of games. Every Game has a list of Outcomes in it, which contain enums according to the outcome of a game. My instructor has commented a TODO in my code:

public static void printStatistics(List<Game> games) {
    float win = 0;
    float lose = 0;
    float draw = 0;
    float all = 0;

    //TODO: should be implemented /w stream API
    for (Game g : games) {
        for (Outcome o : g.getOutcomes()) {
            if (o.equals(Outcome.WIN)) {
                win++;
                all++;
            } else if (o.equals(Outcome.LOSE)) {
                lose++;
                all++;
            } else {
                draw++;
                all++;
            }
        }
    }
    DecimalFormat statFormat = new DecimalFormat("##.##");

    System.out.println("Statistics: The team won: " + statFormat.format(win * 100 / all) + " %, lost " + statFormat.format(lose * 100 / all)
            + " %, draw: " + statFormat.format(draw * 100 / all) + " %");

}

I am familiar with lambda expressions. I tried looking online for solutions, but could not find examples of a stream accessing fields of a field of a class. I would be happy if you could give me a solution, or provide me with a relevant tutorial. Thanks.

Why use stream ? Won't be more efficient as you need to compute multiple values not directly related to object you manipulate — azro
– azro, Commented Mar 4, 2019 at 21:51
That was my first question too. There is no way i can get all 3 values that i need with just 1 pipline, right? — Mangos
– Mangos, Commented Mar 4, 2019 at 21:54
If it's just for the floating-point math, you can make the literals floats instead. E.g. win * 100f / all — shmosel
– shmosel, Commented Mar 4, 2019 at 21:57

shmosel · Accepted Answer · 2019-03-04 22:12:06Z

5

You can stream games, flatMap to outcomes, then collect them to a map of counts:

Map<Outcome, Long> counts = games.stream()
        .map(Game::getOutcomes)
        .flatMap(Collection::stream)
        .collecting(Collectors.groupingBy(o -> o, Collectors.counting()));

long win = counts.getOrDefault(Outcome.WIN, 0L);
long lose = counts.getOrDefault(Outcome.LOSE, 0L);
long draw = counts.getOrDefault(Outcome.DRAW, 0L);
long all = games.stream()
        .mapToInt(g -> g.getOutcomes().size())
        .sum();

edited Mar 4, 2019 at 22:12

answered Mar 4, 2019 at 21:55

shmosel

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Ruslan · Accepted Answer · 2019-03-04 22:11:46Z

2

groupingBy fits this case:

Map<Outcome, Long> map = games.stream()
        .flatMap(game -> game.getOutcomes().stream())
        .collect(Collectors.groupingBy(Function.identity(), Collectors.counting()));

And get win, lose count:

long win = map.get(Outcomes.WIN);
long lose = map.get(Outcomes.LOSE);
...

To get all count you need to sum all values from map

long all = map.values().stream()
    .mapToLong(Long::valueOf)
    .sum();

edited Mar 4, 2019 at 22:11

answered Mar 4, 2019 at 21:55

Ruslan

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Comments

awwsmm · Accepted Answer · 2019-03-04 21:58:17Z

To access the fields of the class in a Stream, use map:

games.stream().map(game -> 
  game.getOutcomes().stream().map(outcome -> {
    // do something with each `outcome`
  })
)

The above code assumes that getOutcomes() returns a List, which is not specified in the current version of OP's question.

Note that you cannot simply increment counters within a Stream, as you might hope to, because all variables used within Streams must be final or effectively final. You'll have to dig a bit more into the Stream API to figure out how to increment the way you did in your original solution.

Hint: you want to do something like this, which utilises Collectors.groupingBy().

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