Method Reference in Java 8

In java 8 method reference is introduced as a helper for the outstanding feature, Lambda Expression.  In some cases a lambda expression does nothing but to call an exsting method.  Method reference let you make this kind of lambda expression cleaner and shorter.

Let see an example, suppose we have a stream created from a User list. The class User has a int field call age with getter and setter.

List<User> users = Arrays.
//... add user in to list
Stream<User> stream = users.stream();

First without using method reference to calculate the averge age of all users.

Double averageAgeDouble = stream.collect(Collectors.averagingDouble((User u)->u.getAge()));

Using method reference, it looks like below

Double averageAgeDouble = stream.collect(Collectors.averagingDouble(User::getAge));

The method reference is shorter than normal lambda expression. The syntax is className::methodName,  which in our case is User::getAge, means arbitrary object of class User and the method name is getAge().