为什么实例方法的Java方法引用无法分配给Consumer接口

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.