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

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();

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

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();

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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