Why doesn't Java Option call Consumer in ifPresent ()?

Question

public class TestSupplier {

Optional<Integer> opt1;

public static void main(String[] args) {
    // TODO Auto-generated method stub

    TestSupplier ts1 = new TestSupplier();

    ts1.opt1 = ts1.average(100,20,30,80);
    Consumer<Integer> cns1 = (x) -> x += 3;
    ts1.opt1.ifPresent(cns1);
    System.out.println(ts1.opt1.get());

}


private Optional<Integer> average(int... n1) {
    if (n1.length == 0) return Optional.empty();
    int sum = 0;
    for(int score: n1) sum += score; 
    return Optional.of(sum/n1.length);

}

}

when I run the code, the result is 57 (that is, the correct result from 100, 20, 30, 80 on average), but I create a Consumer that should increase the result by 3 ... but it does not seem to work.

Can someone help me?

Answer 1

The action is Consumeractually executed, but the body you provided only modifies the local instance, which ultimately gets lost. The method ifPresent()should only be used to perform side effects (actions).

, Optional, map().

ts1.opt1
  .map(x -> x + 3).orElseThrow(...)

get() Optional. , orElse, orElseGet orElseThrow.

Answer 2

Consumer<Integer> cns1 = new Consumer<Integer>() {
    public @Override void accept(Integer x) {
        // x is a local variable
        x += 3; // unboxing, adding, boxing 
        // the local variable has been changed
    }
};

.

-

ts1.opt1.map(x -> x + 3).ifPresent(System.out::println);

(, AtomicInteger):

Consumer<AtomicInteger> cns1 = x -> x.addAndGet(3);

Consumer<AtomicInteger> ( , @pivovarit ).

,

IntStream.of(100, 20, 30, 80).average().ifPresent(System.out::println);

.