Method reference is used to refer method of functional interface. It is compact and easy form of lambda expression. Each time when you are using lambda expression to just referring a method, you can replace your lambda expression with method reference.

3 types of method references:

1. Reference to a static method
2. Reference to an instance method
3. Reference to a constructor

Reference to a static method
Syntax

  ClassName::MethodName

import java.util.function.BiFunction;
class Arithmetic 
 {
 public static int add(int a, int b) 
 {
  return a + b;
 }
}
public class MethodReference 
{
 public static void main(String[] args) 
 {
  BiFunction < Integer, Integer, Integer > adder = Arithmetic::add;
  int result = adder.apply(10, 20);
  System.out.println(result);
 }
}

Reference to an instance method
Syntax

Object::methodName

import java.util.function.BiFunction;
class Arithmetic 
 {
 public int add(int a, int b) 
 {
  return a + b;
 }
}
public class MethodReference 
{
 public static void main(String[] args) 
 {
  Arithmetic objArithmetic = new Arithmetic();
  BiFunction < Integer, Integer, Integer > adder = objArithmetic::add;
  int result = adder.apply(10, 20);
  System.out.println(result);
 }
}

Reference to a constructor
Syntax

ClassName::new  

interface Messageable {
 Message getMessage(String msg);
}
class Message {
 Message(String msg) {
  System.out.print(msg);
 }
}
public class ConstructorReference {
 public static void main(String[] args) {
  Messageable hello = Message::new;
  hello.getMessage("Hello");
 }
}