Java 8 predicates using methods bodies are called only once?

chiperortiz :

I have examined the following snippet:

public static <T> Predicate<T> distinctByKey(Function<? super T, ?> keyExtractor) {
    Map<Object, Boolean> computed = new ConcurrentHashMap<>();/*IS THIS LINE CALLED ONCE ON STREAM->FILTER NOT MATTER HOW LONG THE STREAM IS*/
    return t -> {return computed.putIfAbsent(keyExtractor.apply(t), Boolean.TRUE) == null;};
}

private void test(){
    final long d = Stream.of("JOHN","STEPHEN","ORTIZ","RONDON")
            .filter(distinctByKey(Function.identity()))
            .count();
    System.out.println("d = " + d);
}

This code is not mine. I know that using a ConcurrentMap is not the right choice in this example and I should use ConcurrentMap instead of Map in this case, but this is not my concern now.

I thought that the distinctByKey method is called or interpreted in each iteration of the Stream. I mean the Map being instantiated in each turn, but it's not!

Is the body of the Predicate method called only once?

In the Stream iteration, is this an assertion?

Because when I try the following code:

final Function<String,Integer>a = (name)->name.length();
System.out.println(distinctByKey(a).test("JOHN"));
System.out.println(distinctByKey(a).test("STEPHEN"));
System.out.println(distinctByKey(a).test("ORTIZ"));
System.out.println(distinctByKey(a).test("RONDON"));

I can see that the body of the method is indeed called in each line. What makes the body of the filter to only be called once?

Eran :

When you call .filter(distinctByKey(Function.identity())), the argument passed to filter() is evaluated. That's the only time distinctByKey(Function.identity()) is executed and returns an instance of Predicate<String>.

That Predicate is then evaluated (i.e. it's test() method is executed) multiple times, each time for a different element of the Stream.

To make your last snippet behave similar to the Stream pipeline, it should look like this:

final Function<String,Integer> a = (name)->name.length();
Predicate<String> pred = distinctByKey(a);
System.out.println(pred.test("JOHN"));
System.out.println(pred.test("STEPHEN"));
System.out.println(pred.test("ORTIZ"));
System.out.println(pred.test("RONDON"));