Method References

In this lesson, you will learn about method references and explore its types.

We'll cover the following

Method references, as the name suggests are the references to a method. They are similar to object references. As we can have reference to an object, we can have reference to a method as well.

Similar to an object reference, we can now pass behavior as parameters. But, you might be wondering what the difference between a method reference and lambda expressions is. There is no difference. Method references are shortened versions of lambda expressions that call a specific method.

Let’s say you have a Consumer as defined below:

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Consumer<String> consumer = s -> System.out.println(s);

This can be written as:

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Consumer<String> consumer = System.out::println;

Let’s see one more example. Consider we have a Function<T,R> functional interface as defined below:

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Function<Person, Integer>  function = p  -> p.getAge();

This can be written as:

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Function<Person, Integer>  function = Person::getAge;

Four kinds of method references

There are four kinds of method references.

1. Static methods

The syntax to use static methods as method reference is ClassName::MethodName.

In the below example, we have a method getLength() which returns the length of the String. We have written a lambda expression using a method reference to fetch the length of the string.

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import java.util.ArrayList;

import java.util.List;



public class StreamDemo {



    public static int getLength(String str){

        return str.length();

    }



    public static void main(String[] args) {

        List<String> list = new ArrayList<>();

        list.add("done");

        list.add("word");

        list.add("practice");

        list.add("fake");



        // Code without using method reference.

        list.stream()

                .mapToInt(str -> StreamDemo.getLength(str))

                .forEach(System.out::println);



        // Code with method reference.

        list.stream()

                .mapToInt(StreamDemo::getLength)

                .forEach(System.out::println);



    }

}

2. Instance method of a particular object

The syntax to use the instance method as a method reference is ReferenceVariable::MethodName

We will look at the same example as above, but, this time, the getLength() method is not static.

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import java.util.ArrayList;

import java.util.List;



public class StreamDemo {



    public int getLength(String str) {

        return str.length();

    }



    public static void main(String[] args) {

        List<String> list = new ArrayList<>();

        list.add("done");

        list.add("word");

        list.add("practice");

        list.add("fake");



        StreamDemo demo = new StreamDemo();

        // Code without instance method reference.

        list.stream()

                .mapToInt(str -> demo.getLength(str))

                .forEach(System.out::println);



        // Code with instance method reference.

        list.stream()

                .mapToInt(demo::getLength)

                .forEach(System.out::println);



    }

}

3. Instance method of an arbitrary object #

This type of method reference does not require the object of the referenced class. We can directly use the class name in the method reference.

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import java.util.ArrayList;

import java.util.List;



public class StreamDemo {



    public int getLength(String str) {

        return str.length();

    }



    public static void main(String[] args) {

        List<Employee> list = new ArrayList<>();

        list.add(new Employee("Dave", 23, 20000));

        list.add(new Employee("Joe", 18, 40000));

        list.add(new Employee("Ryan", 54, 100000));

        list.add(new Employee("Iyan", 5, 34000));

        list.add(new Employee("Ray", 63, 54000));

        // Code without using method reference

        int totalSalary1 = list.stream()

                .mapToInt(emp -> emp.getSalary())

                .sum();

        

        // Code with method reference

        int totalSalary = list.stream()

                .mapToInt(Employee::getSalary)

                .sum();



        System.out.println("The total salary is " + totalSalary);

    }

}





class Employee {

    String name;

    int age;

    int salary;



    Employee(String name, int age, int salary) {

        this.name = name;

        this.age = age;

        this.salary = salary;

    }



    public String getName() {

        return name;

    }



    public int getAge() {

        return age;

    }



    public int getSalary() {

        return salary;

    }



    @Override

    public String toString() {

        return "Employee{" +

                "name='" + name + '\'' +

                ", age=" + age +

                ", salary=" + salary +

                '}';

    }

}

4. Constructor references

We can refer to a constructor in the same way we reference a static method. The only difference is that we need to use a new keyword.

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import java.util.ArrayList;

import java.util.List;

import java.util.stream.Collectors;



public class StreamDemo {



    public int getLength(String str) {

        return str.length();

    }



    public static void main(String[] args) {

        List<String> list = new ArrayList<>();

        list.add("Dave");

        list.add("Joe");

        list.add("Ryan");

        list.add("Iyan");

        list.add("Ray");



        // Code without constructor reference

        list.stream()

                .map(name -> new Employee(name))

                .forEach(System.out::println);



        // Code with constructor reference

        list.stream()

                .map(Employee::new)

                .forEach(System.out::println);

    }

}





class Employee {

    String name;

    int age;

    int salary;



    Employee(String name) {

        this.name = name;

    }



    Employee(String name, int age, int salary) {

        this.name = name;

        this.age = age;

        this.salary = salary;

    }



    public String getName() {

        return name;

    }



    public int getAge() {

        return age;

    }



    public int getSalary() {

        return salary;

    }



    @Override

    public String toString() {

        return "Employee{" +

                "name='" + name + '\'' +

                ", age=" + age +

                ", salary=" + salary +

                '}';

    }



}

In the next lesson, we will explore the Optional class in Java 8.

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