How Consumer andThen method works internally in java functional interfaces

Question

can someone let me know how the below program works internally:

public class Main {
  public static void main(String[] args) {
    Consumer<String> c = (x) -> System.out.println(x.toLowerCase());
    c.andThen(c).andThen(c).accept("Java2s.com");
  }
}

Answer 1

Did you look at the code of andThen?

default Consumer<T> andThen(Consumer<? super T> after) {
    Objects.requireNonNull(after);
    return (T t) -> { accept(t); after.accept(t); };
}

It's creating a new Consumer for each call to andThen, finally at the end invoking the accept method (which is the only abstract one).

How about a different approach:

    Consumer<String> first = x -> System.out.println(x.toLowerCase());
    Consumer<String> second = y -> System.out.println("aaa " + y);

    Consumer<String> result = first.andThen(second);

Running this code is not going to produce anything, since you have not invoked accept anywhere just yet.

On the other hand, you can see what happens when calling accept on each other:

 Consumer<String> result = first.andThen(second);

    first.accept("Java"); // java
    second.accept("Java"); // aaa Java
    System.out.println("---------");
    result.accept("Java"); // java, aaa Java

andThen returns a composition of this Consumer with the next one.