Java 8 stream with filter condition working flow

Learn Hadoop :

In a Java 8 stream with a filter condition, every element in the collection is passed to the filter for checking the condition. Here I am writing two different filter conditions and giving different workflows.

public static void main(String[] args) {

    List<String> asList = Arrays.asList("a", "b", "c", "d", "e", "a", "b", "c");

    //line 1
    asList.stream().map(s -> s).filter(distinctByKey(String::toString)).forEach(System.out::println);

    Predicate<String> strPredicate = (a) -> {
        System.out.println("inside strPredicate method--");
        return a.startsWith("a");
    };

    //line 2
    asList.stream().filter(strPredicate).forEach(System.out::println);
}

public static <T> Predicate<T> distinctByKey(Function<? super T, ?> keyExtractor) {
    System.out.println("inside distinctByKey method...");
    Set<Object> seen = ConcurrentHashMap.newKeySet();
    return t -> seen.add(keyExtractor.apply(t));
}

In the above sample code, the statement line 1 filter condition is executing only once but line 2 is executing for every element in the collection output.

I thought the distinctByKey method would execute for every element in the collection, but it is not the case. Why ?

Also the Set object reference variable seen is executing only once? How is the flow working?

Lino :

distinctByKey is a lambda factory method. It is returning a Predictate<T>.

So when you execute: filter(distinctByKey(String::toString)) you're in fact calling the distinctByKey method first, which then returns a Predicate. That predicate then gets executed for every element. Just the factory function will only be executed once.

When moving the System.out.println inside the returned lambda you'll get the desired print statements:

public static <T> Predicate<T> distinctByKey(Function<? super T, ?> keyExtractor) {
    System.out.println("inside distinctByKey method...");
    Set<Object> seen = ConcurrentHashMap.newKeySet();
    return t -> {
        System.out.println("inside distinctByKey.lambda method... ");
        return seen.add(keyExtractor.apply(t));
    };
}