Why Java Method Reference of instance method cannot be assigned to Consumer interface

Question

Here is my Code :

public class SearchByLambda {

     private Map<String,Consumer<Person>> searchCritertiaHolder = new HashMap<String,Consumer<Person>>();

     private static final String AGED = "aged";

     public SearchByLambda(){
           searchCritertiaHolder.put(AGED, (Person p)-> {p.filterAgedPerson(p);} );
     }

     private Consumer<Person> getFilter(String personType){
          return searchCritertiaHolder.get(personType);
     }

     public static void main(String[] args) {
          SearchByLambda searchUsage = new SearchByLambda();
          Person p = new Person(59,"shailesh");
          Person p1 = new Person(58,"ganesh");

          searchUsage.getFilter(AGED).accept(p);
          searchUsage.getFilter(AGED).accept(p1);

          Person.printAgedPersons();
     }
 }

 class Person{

       private static List<Person> agedPersons = new ArrayList<>();

       private int age;

       private String name;

       public int getAge() {
              return age;
       }

       public void setAge(int age) {
          this.age = age;
       }

       public String getName() {
            return name;
       }

       public void setName(String name) {
            this.name = name;
       }

       public Person(int age,String name){
           this.age = age;
           this.name = name;
       }

       public void filterAgedPerson(Person person){
          if(person.getAge() > 58){
              agedPersons.add(person);
          }
       }

       public static void printAgedPersons(){
            for(Person person : agedPersons){
                System.out.println(person.getName());
            }
       }
 }

When I replace following Lambda expression

     searchCritertiaHolder.put(AGED, (Person p)-> {p.filterAgedPerson(p);});

with

              searchCritertiaHolder.put(AGED, Person::filterAgedPerson);

it gives me compilation error. I am using java 8 and and compiling through eclipse. Why is this so? Why cannot I assign method reference for instance method of any arbitrary object to consumer functional interface?

Answer 1

Your definition of filterAgedPerson takes a Person as an argument, even though it is not a static method. It doesn't need to, and it shouldn't if you want to use it as a Consumer<Person>. What you are ending up with is something compatible with BiConsumer<Person, Person>.

It might help to think of it this way: method references to non-static methods always take an "extra" argument which is used as this.

The easiest way for you to fix this with your current code structure is to modify the filterAgedPerson method to not take a Person as an argument

   public void filterAgedPerson() {
      if (this.getAge() > 58) {
          agedPersons.add(person);
      }
   }

As an aside, you might want to also consider making your filters Predicate<Person> instead of Consumer<Person> and moving the results handling elsewhere. This will give you more flexibility as things get more complicated.