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javalambdajava-8option-typeconsumer

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

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 of 100, 20, 30, 80 average) but I create a Consumer that should increment the result by 3... but it seems to not work.

Can someone help me?

Solution

  • The Consumer action is actually being run but the body you provided modifies only a local instance which eventually gets lost. The ifPresent() method should be used for performing side-effects(actions) only.

    If you want to perform a calculation on a value held by an Optional instance, use map() instead.

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

    Remember to be careful when using get() on an Optional instance. Before you decide to use it, have a look at orElse, orElseGet, and orElseThrow.