java8 stream sum multiple

Question

I am curious, how to sum up multiple variables in a java8 stream.

Integer wCPU = 0;
Double wnetwork = 0.0;
Double wMem = 0.0;

this.slaContractList.forEach(sla -> {
    wCPU += sla.getNumberOfCPUs();
    wnetwork += sla.getNetworkBandwith();
    wMem += sla.getMemory();
});

However, this does not compile as the variable in the lambda expression should be final.

Answer 1

Try to use Stream.reduce and Stream.sum:

Double wnetwork = slaContractList.stream()
            .mapToDouble(sla -> sla.getNetworkBandwith())
            .sum();

Double wMem = slaContractList.stream()
            .mapToDouble(sla -> sla.getMemory())
            .sum();

Integer wCPU = slaContractList.stream()
            .mapToInt(sla -> sla.getNumberOfCPUs())
            .sum();

See https://docs.oracle.com/javase/tutorial/collections/streams/reduction.html

The advantage of using stream is option of use parallelStream() instead of stream(). In some situations it can be more efficient than simple loop.

Answer 2

Just to do a sum, i would use the sum operation of streams just like in Łukasz answer, but, for a more general solution for resolving the "final problem" you can use the classes of java.util.concurrent.atomic. It is intented to be used in stream and is thread-safe, so could be used in a parallel stream.

AtomicInteger wCPU = new AtomicInteger();
DoubleAccumulator wnetwork = new DoubleAccumulator(Double::sum,0.d);
DoubleAccumulator wMem = new DoubleAccumulator(Double::sum,0.d);

this.slaContractList.forEach(sla -> {
    wCPU.addAndGet(sla.getNumberOfCPUs());
    wnetwork.accumulate(sla.getNetworkBandwith());
    wMem.accumulate(sla.getMemory());
});

Now you see that there are 2 kind of implementation: the Accumulator and the Atomic, the choice between these 2 is another question:

java 8 : Are LongAdder and LongAccumulator preferred to AtomicLong?

Answer 3

I would do a simple hack like this:

    Integer[] wCPU = new Integer[1];
    Double[] wnetwork = new Double[1];
    Double[] wMem = new Double[1];

    this.slaContractList.forEach(sla -> {
        wCPU[0] += sla.getNumberOfCPUs();
        wnetwork[0] += sla.getNetworkBandwith();
        wMem[0] += sla.getMemory();
    });

It's optional to have the final keyword for both , as in Java 8 they have introduced the effectively final concept. It means, you have assigned only once.