Why Java's Optional doesn't 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 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?

Answer 1

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.

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
    }
};

It is that case when turning a lambda into an anonymous class perfectly makes this all clear to understand.

The best possible way here is

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

You could use an instance of a mutable class (e.g. the AtomicInteger class):

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

which changes its state after accepting the Consumer<AtomicInteger> (although it is not recommended, look at @pivovarit's answer).

Furthermore, the line

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

might replace all your routine work.