Double colon (::) operator in Java

The double colon (::) operator, also known as method reference operator in Java, is used to call a method by referring to it with the help of its class directly. They behave exactly as the lambda expressions. The only difference it has from lambda expressions is that this uses direct reference to the method by name instead of providing a delegate to the method.

Syntax:

<Class name>::<method name>

Example: To print all elements of the stream:

• Using Lambda expression:

  stream.forEach( s-> System.out.println(s));

  Program:

  // Java code to print the elements of Stream
  // without using double colon operator
  
  import java.util.stream.*;
  
  class GFG {
      public static void main(String[] args)
      {
  
          // Get the stream
          Stream<String> stream
              = Stream.of("Geeks", "For",
                          "Geeks", "A",
                          "Computer",
                          "Portal");
  
          // Print the stream
          stream.forEach(s -> System.out.println(s));
      }
  }
  Output:

  Geeks
  For
  Geeks
  A
  Computer
  Portal
  

• Using double colon operator:

  stream.forEach( System.out::println);

  Program: To demonstrate the use of double colon operator

  // Java code to print the elements of Stream
  // using double colon operator
  
  import java.util.stream.*;
  
  class GFG {
      public static void main(String[] args)
      {
  
          // Get the stream
          Stream<String> stream
              = Stream.of("Geeks", "For",
                          "Geeks", "A",
                          "Computer",
                          "Portal");
  
          // Print the stream
          // using double colon operator
          stream.forEach(System.out::println);
      }
  }
  Output:

  Geeks
  For
  Geeks
  A
  Computer
  Portal
  

When and how to use double colon operator?

Method reference or double colon operator can be used to refer:

• a static method,
• an instance method, or
• a constructor.

How to use method reference in Java:

1. Static method

   Syntax:

   (ClassName::methodName)

   Example:

   SomeClass::someStaticMethod

   Program:

   // Java code to show use of double colon operator
   // for static methods
   
   import java.util.*;
   
   class GFG {
   
       // static function to be called
       static void someFunction(String s)
       {
           System.out.println(s);
       }
   
       public static void main(String[] args)
       {
   
           List<String> list = new ArrayList<String>();
           list.add("Geeks");
           list.add("For");
           list.add("GEEKS");
   
           // call the static method
           // using double colon operator
           list.forEach(GFG::someFunction);
       }
   }
   Output:

   Geeks
   For
   GEEKS
   

2. Instance method

   Syntax:

   (objectOfClass::methodName)

   Example:

   System.out::println

   Program:

   // Java code to show use of double colon operator
   // for instance methods
   
   import java.util.*;
   
   class GFG {
   
       // instance function to be called
       void someFunction(String s)
       {
           System.out.println(s);
       }
   
       public static void main(String[] args)
       {
   
           List<String> list = new ArrayList<String>();
           list.add("Geeks");
           list.add("For");
           list.add("GEEKS");
   
           // call the instance method
           // using double colon operator
           list.forEach((new GFG())::someFunction);
       }
   }
   Output:

   Geeks
   For
   GEEKS
   

3. Super method

   Syntax:

   (super::methodName)

   Example:

   super::someSuperClassMethod

   Program:

   // Java code to show use of double colon operator
   // for super methods
   
   import java.util.*;
   import java.util.function.*;
   
   class Test {
   
       // super function to be called
       String print(String str)
       {
           return ("Hello " + str + "\n");
       }
   }
   
   class GFG extends Test {
   
       // instance method to override super method
       @Override
       String print(String s)
       {
   
           // call the super method
           // using double colon operator
           Function<String, String>
               func = super::print;
   
           String newValue = func.apply(s);
           newValue += "Bye " + s + "\n";
           System.out.println(newValue);
   
           return newValue;
       }
   
       // Driver code
       public static void main(String[] args)
       {
   
           List<String> list = new ArrayList<String>();
           list.add("Geeks");
           list.add("For");
           list.add("GEEKS");
   
           // call the instance method
           // using double colon operator
           list.forEach(new GFG()::print);
       }
   }
   Output:

   Hello Geeks
   Bye Geeks
   
   Hello For
   Bye For
   
   Hello GEEKS
   Bye GEEKS
   

4. Instance method of an arbitrary object of a particular type

   Syntax:

   (ClassName::methodName)

   Example:

   SomeClass::someInstanceMethod

   Program:

   // Java code to show use of double colon operator
   // for instance method of arbitrary type
   
   import java.util.*;
   
   class Test {
       String str=null;
         Test(String s)
       {
           this.str=s;
       }
       // instance function to be called
       void someFunction()
       {
           System.out.println(this.str);
       }
   }
   
   class GFG {
   
       public static void main(String[] args)
       {
   
           List<Test> list = new ArrayList<Test>();
           list.add(new Test("Geeks"));
           list.add(new Test("For"));
           list.add(new Test("GEEKS"));
   
           // call the instance method
           // using double colon operator
           list.forEach(Test::someFunction);
       }
   }
   Output:

   Geeks
   For
   GEEKS
   

5. Class Constructor

   Syntax:

   (ClassName::new)

   Example:

   ArrayList::new

   Program:

   // Java code to show use of double colon operator
   // for class constructor
   
   import java.util.*;
   
   class GFG {
   
       // Class constructor
       public GFG(String s)
       {
           System.out.println("Hello " + s);
       }
   
       // Driver code
       public static void main(String[] args)
       {
   
           List<String> list = new ArrayList<String>();
           list.add("Geeks");
           list.add("For");
           list.add("GEEKS");
   
           // call the class constructor
           // using double colon operator
           list.forEach(GFG::new);
       }
   }
   Output:

   Hello Geeks
   Hello For
   Hello GEEKS