How to test Predicate with method reference inside forEach of java 8

Question

I am trying out method reference inside forEach

private static void printConditionally8(List<Person> people, Predicate<Person> predicate) {
    people.forEach(p-> { if (predicate.test(p)){
        System.out.println("Print here");}
    });
}

Above works fine but I want to make it more short using methods reference however its giving compilation problem.Is there any way to make it happen ?

private static void printConditionally8(List<Person> people, Predicate<Person> predicate) {
    people.forEach({ if (predicate::test){
        System.out.println("Print here");}
     });
}

https://stackoverflow.com/questions/21488056/how-to-negate-a-method-reference-predicate — Ravindra Ranwala, Jan 29 '20 at 06:12

ernest_k · Answer 1 · 2020-01-29T06:13:01.820

6

You should be able to filter the list before running your action:

people.stream().filter(predicate).forEach(p -> System.out.println("Print here"));

You can't use if(predicate::test) because if takes a boolean expression (the type of predicate::test wouldn't even be known here - check lambda expressions' target typing documentation). The only way to make it work would be to invoke the test() method as you did it in your first snippet.

edited Jan 29 '20 at 06:13

answered Jan 29 '20 at 06:08

ernest_k

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that is perfect approach!! More concise is : people.stream().filter(predicate).forEach(System.out::println); – JavaBuilder Jan 29 '20 at 06:11

score 1 · Answer 2 · answered Jan 29 '20 at 06:18

I think you use the method reference concept in a wrong way.

Method reference is a nifty syntax for calling a method as if its a lambda (java 8+ will do all the conversions):

Here is an example:

public class Foo {



   private static void printConditionally8(List<Person>persons, Predicate<Person> f) {
      persons.stream().filter(f).forEach(p -> System.out.print(p + " is here"));
   }
   private static Boolean myFilter(Person p) {
      return p.age > 18; 
   }

   public static void main(String[] args) {
      List<Person> persons = ... // create a list of persons
      printConditionally8(persons, Foo::myFilter); 
   } 

}

Notice how does the main method really call the printConditionally8. It passes the reference to the method myFilter as if is an "anonymous class" - that implements the Predicate interface.

We dont need myFilter(Person p) method explicitly here. You can pass lambda directly printConditionally8(persons, p-> p.age > 18); — JavaBuilder, Jan 29 '20 at 06:37
Of course you can, but it've assumed that the question is about a difficulty to grasp a concept of method inference that's why I've provided an example like this — Mark Bramnik, Jan 29 '20 at 06:43

How to test Predicate with method reference inside forEach of java 8

2 Answers2