Meaning of :: in Java syntax [duplicate]

Question

What is the meaning of :: in the following code?

Set<String> set = people.stream()                         .map(Person::getName)                         .collect(Collectors.toCollection(TreeSet::new)); 

Answer 1

This is method reference. Added in Java 8.

TreeSet::new refers to the default constructor of TreeSet.

In general A::B refers to method B in class A.

Answer 2

:: is called Method Reference. It is basically a reference to a single method. i.e. it refers to an existing method by name.

Method reference using :: is a convenience operator.

Method reference is one of the features belonging to Java lambda expressions. Method reference can be expressed using the usual lambda expression syntax format using –> In order to make it more simple :: operator can be used.

Example:

public class MethodReferenceExample {     void close() {         System.out.println("Close.");     }      public static void main(String[] args) throws Exception {         MethodReferenceExample referenceObj = new MethodReferenceExample();         try (AutoCloseable ac = referenceObj::close) {         }     } } 

So, In your example:

Set<String> set = people.stream()                         .map(Person::getName)                         .collect(Collectors.toCollection(TreeSet::new)); 

Is calling/creating a 'new' treeset.

A similar example of a Contstructor Reference is:

class Zoo {     private List animalList;     public Zoo(List animalList) {         this.animalList = animalList;         System.out.println("Zoo created.");     } }  interface ZooFactory {     Zoo getZoo(List animals); }  public class ConstructorReferenceExample {      public static void main(String[] args) {         //following commented line is lambda expression equivalent         //ZooFactory zooFactory = (List animalList)-> {return new Zoo(animalList);};             ZooFactory zooFactory = Zoo::new;         System.out.println("Ok");                Zoo zoo = zooFactory.getZoo(new ArrayList());     } }