The double colon (::) operation, also known as the method reference operator, is used to directly call methods of a specified class. Its behavior is exactly the same as that of a la mbda expression. It and lambda expressions The only difference is that it references the method directly by name, rather than providing a delegate for the method.

Syntax:

<Class name>::<method name>

Example: to print all elements of a Stream:

• Use Lambda expressions:

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

Complete example:

// 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

• Use the double colon operator:

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

Complete example:

// 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 do I use the double colon operator?

Method references or double colon operators are available for the following methods:

• Static method
• Example method
• Constructor

How to use method references in Java:

1. Static method

Syntax:

(ClassName::methodName)

Example:

SomeClass::someStaticMethod

Complete example:

// 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

1. Example method

Syntax:

(objectOfClass::methodName)

Example:

System.out::println

Complete example:

// 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

1. Parent class method

Syntax:

(super :: methodName)

Example:

super::someSuperClassMethod

Complete example:

// 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

1. Instance methods for specific types of objects

Syntax:

(ClassName::methodName)

Example:

SomeClass::someInstanceMethod

Complete example:

// 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

1. class constructor

Syntax:

(ClassName::new)

Example:

ArrayList::new

Complete example:

// 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