Method and Constructor References

The above example code can be further simplified by utilizing static method references:

  1. Converter<String, Integer> converter = Integer::valueOf;
  2. Integer converted = converter.convert("123");
  3. System.out.println(converted);   // 123

Java 8 enables you to pass references of methods or constructors via the :: keyword. The above example shows how to reference a static method. But we can also reference object methods:

  1. class Something {
  2.     String startsWith(String s) {
  3.         return String.valueOf(s.charAt(0));
  4.     }
  5. }
  1. Something something = new Something();
  2. Converter<String, String> converter = something::startsWith;
  3. String converted = converter.convert("Java");
  4. System.out.println(converted);    // "J"

Let’s see how the :: keyword works for constructors. First we define an example class with different constructors:

  1. class Person {
  2.     String firstName;
  3.     String lastName;
  5.     Person() {}
  7.     Person(String firstName, String lastName) {
  8.         this.firstName = firstName;
  9.         this.lastName = lastName;
  10.     }
  11. }

Next we specify a person factory interface to be used for creating new persons:

  1. interface PersonFactory<P extends Person> {
  2.     P create(String firstName, String lastName);
  3. }

Instead of implementing the factory manually, we glue everything together via constructor references:

  1. PersonFactory<Person> personFactory = Person::new;
  2. Person person = personFactory.create("Peter", "Parker");

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