Method and Constructor Reference

Method Reference

1.

• Method Reference is an abbreviated syntax used with lambda expressions for invoking a method.

• Double colon operator (::) is used to represent a method reference.

• The syntax for the same is:

ClassName::MethodName

2. A method reference can be used to point the following types of methods:

• Static method

• Instance method

• Constructors using new operator (TreeSet::new)

eg:

public class newfeatures {

    public static void main(String[] args) {
        Converter<String, Integer> converter = (a) -> Integer.valueOf(a);
        Integer aa = converter.convert("1234");
        System.out.println(aa);
        String bb = "31234";
        System.out.println(Integer.valueOf(bb));
        Converter<String, Integer> test1 = Integer::valueOf;
        Integer cc = test1.convert("0987");
        System.out.println(cc);
        Something st1 = new Something();
       // Converter<String, String> test2 = st1::startsWith;
       Converter<String, String> test2 = st1::endWith;
        String st2 = test2.convert("Java");
        System.out.println(st2);

    }
}

@FunctionalInterface
interface Converter<F, T> {
    T convert(F from);
}

class Something {
    String startsWith(String s) {
        return String.valueOf(s.charAt(0));
    }

    String endWith(String s) {
        return String.valueOf(s.charAt(s.length()-1));
    }
}

Constructor Reference

class Person{
    String firstName;
    String lastName;

    Person(){}


    Person(String firstName, String lastName){
        this.firstName = firstName;
        this.lastName = lastName;
    }
}

interface PersonFactory<P extends Person>{
    P create(String firstName, String lastName);
}


public void main(String[] avgs){

    PersonFactory<Person> pF = Person::new;
    Person p = pF.create("Peter", "Parker");
}

Instead of implementing the factory manually, we glue everything together via constructor reference.

We create a reference to the Person constructor via Person::new. The Java compiler automatically choose the right constructor by matching the signature of PersonFactory.create.