Scope Resolution Operator in Java

C++ supports the scope resolution operator (::) that allows us to resolve the ambiguous call or reference to identifiers. Like C++, Java does not support the scope resolution operator. Java uses the same operator (::) but with different names. The scope resolution operator in Java is known as the method reference operator or double colon operator. In this section, we will discuss the scope resolution operator in Java.

The method reference operator is used to call a method by referring to it with the help of its class directly. We can use the method reference operator instead of lambda expressions because it behaves the same as lambda expressions.

The only difference between lambda expression and method reference operator is that instead of providing a delegate to the method it uses a direct reference to the method by name. The target reference is placed before the operator (::) and the method name is written after it.

Syntax:

Both lambda expression and the method reference operator are the ways to call the existing method with method reference that provides better readability. Let's understand it through an example.

First, we will create a Java program and use the lambda expression to access a list. It also clears the differences between lambda expression and the method reference operator.

LambdaExpressionExample.java

import java.util.stream.*; 
public class LambdaExpressionExample 
{ 
public static void main(String[] args) 
{ 
//invoking the of() method of the Stream class that returns a sequential ordered stream
Stream<String> st = Stream.of("San Jose", "Las Vegas", "Austin", "New York", "Denver", "Portland"); 
//print the streams 
st.forEach(city -> System.out.println(city)); 
} 
}

Output:

San Jose
Las Vegas
Austin
New York
Denver
Portland

In the above program, we have used the lambda expression to access the elements.

Let's use the method reference operator in the above program to access the elements. We have made a slight change in the above program.

MethodReferenceOperatorExample.java

import java.util.stream.*; 
public class MethodReferenceOperatorExample 
{ 
public static void main(String[] args) 
{ 
//invoking the of() method of the Stream class that returns a sequential ordered stream
Stream<String> st = Stream.of("San Jose", "Las Vegas", "Austin", "New York", "Denver", "Portland"); 
//using method reference operator to print the stream
st.forEach(System.out::println); 
} 
}

Output:

San Jose
Las Vegas
Austin
New York
Denver
Portland

Using lambda expression and method reference operator, we get the same output. Therefore, it is clear that these are the ways to call the existing method with method reference.

Types of Method References

There are four types of method references in Java:

Types	Description	Syntax	Example
Reference to a static method	It is used to refer a static method from a class.	ContainingClass::staticMethodName	Math::floor is equivalent to Math.floor(x)
Reference to an instance method of a particular object	It refers to an instance method using a reference to a supplied object.	containingObject::instanceMethodName	System.out::println is equivalent to System.out.println(x)
Reference to an instance method of an arbitrary object of a particular type	It invokes the instance method on a reference to an object supplied by the context.	ContainingType::methodName	String::indexOf is equivalent to str.indexOf()
Reference to a constructor	It gives reference to a constructor.	ClassName::new	LinkedList::new is equivalent to new LinkedList()