Java Method References

Java Method References are a shorthand way to refer to an existing method without invoking it. They were introduced in Java 8 to make lambda expressions shorter, cleaner, and more readable. Method references use the double colon (::) operator and are mainly used with functional interfaces.

• Introduced in Java 8 as an alternative to lambda expressions
• Improve code readability and reduce boilerplate
• Used when a lambda expression simply forwards its parameters to a single existing method (pass-through call).

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Explanation: In the above example, we are using method reference to print items. The print method is a static method which is used to print the names. In the main method we created an array of names and printing each one by calling the print method directly.

Syntax

ClassName::methodName
objectReference::methodName
ClassName::new

Types of Method References 

There are four type method references that are as follows:

1. Reference to a Static Method

A static method reference is used to refer to a static method of a class. It replaces a lambda expression that simply calls a static method.

• Uses class name to access the method
• Method must be static
• Commonly used with utility methods

Syntax:

ClassName::staticMethodName

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Explanation:

• square(int n) is a static method that prints the square of a number.
• forEach(MathUtil::square) passes each list element to the square() method.
• Internally, this works like:

list.forEach(n -> MathUtil.square(n));

2. Reference to an Instance Method of a Particular Object

This type of method reference refers to an instance method of a specific object. The object is already created, and its method is referenced directly.

• Uses an existing object
• Method must be non-static
• Suitable for object-specific operations

Syntax:

objectReference::instanceMethod

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Explanation:

• printer is an object of the Printer class.
• printer::print refers to the instance method print().
• Each element of the list is automatically passed to print().
• Equivalent lambda expression:

data.forEach(msg -> printer.print(msg));

3. Reference to an Instance Method of an Arbitrary Object

This method reference refers to an instance method of any object of a particular class. The actual object is determined at runtime.

• Uses class name instead of object
• Mostly used with Stream API
• Method is called on each element

Syntax:

ClassName::instanceMethod

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Explanation:

• String::toUpperCase calls toUpperCase() on each String object.
• map() transforms each element into uppercase.
• System.out::println prints the transformed values.
• Equivalent lambda expression:

map(name -> name.toUpperCase())

4. Reference to a Constructor

A constructor reference is used to create a new object using a functional interface. It replaces a lambda expression that calls a constructor.

• Uses new keyword
• Commonly used with Supplier, Function
• Helps in object creation logic

Syntax:

ClassName::new

Student object created

Explanation:

• Supplier<Student> is a functional interface with get() method.
• Student::new refers to the constructor of Student.
• When supplier.get() is called, a new Student object is created.
• Equivalent lambda expression:

Supplier<Student> supplier = () -> new Student();

Method References and Functional Interfaces

Method references work only with functional interfaces, which contain exactly one abstract method.

• Method signature must match abstract method
• Common functional interfaces include Consumer, Supplier, Function, Predicate
• Used heavily with Streams and Collections

Example:

List<String> list = Arrays.asList("Banana", "Apple", "Mango");

list.sort(String::compareToIgnoreCase);