Double colon (::) operator in Java

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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:

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

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

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

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

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);
    }
}