Method Reference in Java

In Java, method references provide a concise way to refer to methods or constructors without executing them. They are part of Java 8’s lambda expressions feature and are used mainly with functional interfaces. Method references make code shorter and more readable.

Syntax

There are four types of method references:

1. Reference to a static method ClassName::staticMethodName
2. Reference to an instance method of a particular object instance::instanceMethodName
3. Reference to an instance method of an arbitrary object of a particular type ClassName::instanceMethodName
4. Reference to a constructor ClassName::new

Examples

1. Static Method Reference

import java.util.Arrays;

public class Main {
    public static void main(String[] args) {
        String[] names = {"John", "Alice", "Bob"};
        
        // Using method reference
        Arrays.sort(names, String::compareToIgnoreCase);
        
        for(String name : names) {
            System.out.println(name);
        }
    }
}

import java.util.Arrays;

public class Main {
    public static void main(String[] args) {
        String[] names = {"John", "Alice", "Bob"};
        
        // Using method reference
        Arrays.sort(names, String::compareToIgnoreCase);
        
        for(String name : names) {
            System.out.println(name);
        }
    }
}

• Here, String::compareToIgnoreCase refers to the static method for sorting.

2. Instance Method of a Particular Object

import java.util.function.Consumer;

public class Main {
    public static void main(String[] args) {
        Consumer<String> printer = System.out::println;
        printer.accept("Hello, World!");
    }
}

import java.util.function.Consumer;

public class Main {
    public static void main(String[] args) {
        Consumer<String> printer = System.out::println;
        printer.accept("Hello, World!");
    }
}

• System.out::println refers to the println method of the System.out object.

3. Instance Method of an Arbitrary Object of a Particular Type

import java.util.Arrays;

public class Main {
    public static void main(String[] args) {
        String[] names = {"John", "Alice", "Bob"};
        
        Arrays.sort(names, String::compareToIgnoreCase);
        
        for(String name : names) {
            System.out.println(name);
        }
    }
}

import java.util.Arrays;

public class Main {
    public static void main(String[] args) {
        String[] names = {"John", "Alice", "Bob"};
        
        Arrays.sort(names, String::compareToIgnoreCase);
        
        for(String name : names) {
            System.out.println(name);
        }
    }
}

• This is similar to the static example, but it works on any object of a type.

4. Constructor Reference

import java.util.function.Supplier;

class Person {
    String name;
    
    Person() {
        this.name = "Unknown";
    }
}

public class Main {
    public static void main(String[] args) {
        Supplier<Person> personSupplier = Person::new;
        Person person = personSupplier.get();
        System.out.println(person.name); // Outputs "Unknown"
    }
}

import java.util.function.Supplier;

class Person {
    String name;
    
    Person() {
        this.name = "Unknown";
    }
}

public class Main {
    public static void main(String[] args) {
        Supplier<Person> personSupplier = Person::new;
        Person person = personSupplier.get();
        System.out.println(person.name); // Outputs "Unknown"
    }
}

• Person::new refers to the constructor of the Person class.

✅ Key Points

• Method references are shorthand for lambda expressions.
• They improve code readability.
• They can only be used when the method signature matches the functional interface’s abstract method.