Prerequisites: Functional Interfaces in Java, Lambda Functions

A method is a collection of statements that perform some specific task and return the result to the caller. A method can perform some specific task without returning anything. Methods allow us to reuse the code without retyping the code. In this article, we will see how to use methods as value.

Introduction:

In Java8 we can use method as if they were objects or primitive values and we can treat them as a variable. The example shows the function as a variable in java:

// This square function is a variable getSquare.
Function<Integer, Integer> getSquare = i -> i * i;
// Pass function as a argument to other function easily
SomeFunction(a, b, getSquare);

Sometimes, a lambda expression only calls an existing method. In those cases, it looks clear to refer to the existing method by name. The method references can do this, they are compact, easy-to-read as compared to lambda expression . A method reference is the shorthand syntax for a lambda expression that contains just one method call. Here’s the general syntax of a method reference:

// To refer a method in a object
Object :: methodName

Following is the example of a lambda expression which just call a single method in its entire execution:

// To print all element in a list
list.forEach(s -> System.out.println(s));

To make the code clear and compact, In above example one can turn lambda expression into a method reference:

// Shorthand to print all element in a list
list.forEach(System.out::println);

The method references can only be used to replace a single method lambda expression . A code is more clear and short if one use lambda expression rather than using an anonymous class and one can use method reference rather than using a single function lambda expression to achieve same. In general, one don’t have to pass arguments to method references.

The following example is about performing some operation on elements in the list and add them. The operation to be performed on elements is a function argument and caller can pass accordingly :

Following are the ways to call above method

There are Four type method reference:

1. Static Method Reference.
2. Instance Method Reference of a particular object.
3. Instance Method Reference of an arbitrary object of a particular type.
4. Constructor Reference.

To look into all this type we will consider a sorting with comparator example.

1. Reference to a static method:

   If a Lambda expression is like:

   // If a lambda expression just call a static method of a class
   (args) -> Class.staticMethod(args)

   then method reference is like:

   // Shorthand if a lambda expression just call a static method of a class
   Class::staticMethod

   Example:
   

   Java

   filter_none

   edit
   close

   play_arrow

   link
   brightness_4
   code

   // This Example shows how one can use // Static method reference to sort // with custom comparator import java.io.*; import java.util.*;    // Object need to be sorted. class Person {     private String name;     private Integer age;        // Constructor     public Person(String name,                   int age)     {         this.name = name;         this.age = age;     }        public Integer getAge()     {         return age;     }     public String getName()     {         return name;     } }    public class GFG {        // A static method to     // compare with name     public static int compareByName(Person a,                                     Person b)     {         return a.getName()             .compareTo(b.getName());     }        // A static method to     // compare with age     public static int compareByAge(Person a,                                    Person b)     {         return a.getAge()             .compareTo(b.getAge());     }        // Main     public static void main(String[] args)     {         // List of person         List<Person> personList             = new ArrayList<>();         // Add Elements         personList             .add(new Person("vicky", 24));         personList             .add(new Person("poonam", 25));         personList             .add(new Person("sachin", 19));            // Use static method reference to         // sort array by name         Collections.sort(personList,                          GFG::compareByName);         System.out.println("Sort by name :");         personList.stream()             .map(x -> x.getName())             .forEach(System.out::println);            // Use static method reference         // to sort array by age         Collections.sort(personList,                          GFG::compareByAge);         System.out.println("Sort by age :");         personList.stream()             .map(x -> x.getName())             .forEach(System.out::println);     } }

   chevron_right

   filter_none

   Output:

   Sort by name :
   poonam
   sachin
   vicky
   Sort by age :
   sachin
   vicky
   poonam
   

2. Reference to an instance method of a particular object:

   If a Lambda expression is like:

   // If a lambda expression just call a default method of a ObjectType
   (obj, args) -> obj.instanceMethod(args)

   then method reference is like:

   // Shorthand if a lambda expression just call a default method of a ObjectType
   ObjectType::instanceMethod

   Example:
   

   Java

   filter_none

   edit
   close

   play_arrow

   link
   brightness_4
   code

   // This Example is same as above but // using object method reference to // sort with custom comparator import java.io.*; import java.util.*;    // Object need to be sorted. class Person {     private String name;     private Integer age;        // Constructor     public Person(String name, int age)     {         this.name = name;         this.age = age;     }        public Integer getAge()     {         return age;     }     public String getName()     {         return name;     } }    // Comparator class class ComparisonProvider {     // A method to compare with name     public int compareByName(Person a,                              Person b)     {         return a.getName()             .compareTo(b.getName());     }        // A method to compare with age     public int compareByAge(Person a,                             Person b)     {         return a.getAge()             .compareTo(b.getAge());     } }    public class GFG {     // Main     public static void main(String[] args)     {         // List of person         List<Person> personList = new ArrayList<>();         // Add Elements         personList.add(new Person("vicky", 24));         personList.add(new Person("poonam", 25));         personList.add(new Person("sachin", 19));            // A comparator class with multiple         // comaparator methods         ComparisonProvider comparator             = new ComparisonProvider();            // Use instance method reference         // to sort array by name         Collections.sort(personList,                          comparator::compareByName);         System.out.println("Sort by name :");         personList.stream()             .map(x -> x.getName())             .forEach(System.out::println);            // Use instance method reference         // to sort array by age         Collections.sort(             personList,             comparator::compareByAge);         System.out.println("Sort by age :");         personList.stream()             .map(x -> x.getName())             .forEach(System.out::println);     } }

   chevron_right

   filter_none

   Output:

   Sort by name :
   poonam
   sachin
   vicky
   Sort by age :
   sachin
   vicky
   poonam
   

3. Reference to an instance method of an arbitrary object of a particular type:

   If a Lambda expression is like:

   // If a lambda expression just call a default method of a object
   (args) -> obj.instanceMethod(args)

   then method reference is like:

   // Shorthand if a lambda expression just call a default method of a object
   obj::instanceMethod

   Example:
   

   Java

   filter_none

   edit
   close

   play_arrow

   link
   brightness_4
   code

   // This Example shows how one can use // Instance type method reference // to sort with custom comparator    import java.io.*; import java.util.*;    public class GFG {     // Main     public static void main(String[] args)     {         // List of person         List<String> personList             = new ArrayList<>();            // Add Elements         personList.add("vicky");         personList.add("poonam");         personList.add("sachin");            // Method reference to String type         Collections.sort(             personList,             String::compareToIgnoreCase);         personList.forEach(             System.out::println);     } }

   chevron_right

   filter_none

   Output:

   poonam
   sachin
   vicky
   

4. Constructor method reference:

   If a Lambda expression is like:

   // If a lambda expression just create a object
   (args) -> new ClassName(args)

   then method reference is like:

   // Shorthand if a lambda expression just create a object
   ClassName::new

   Example:
   

   Java

   filter_none

   edit
   close

   play_arrow

   link
   brightness_4
   code

   // This Example shows how one // can use constructor method reference    import java.io.*; import java.nio.charset.Charset; import java.util.*; import java.util.function.*;    // Object need to be sorted. class Person {     private String name;     private Integer age;        // Constructor     public Person()     {         Random ran = new Random();            // length is bounded by 7         byte[] array = new byte[7];         ran.nextBytes(array);            // Asigning Random values         // to name and age         this.name             = ran                   .ints(97, 122 + 1)                   .limit(7)                   .collect(StringBuilder::new,                            StringBuilder::appendCodePoint,                            StringBuilder::append)                   .toString();         this.age = age;     }        public Integer getAge()     {         return age;     }     public String getName()     {         return name;     } }    public class GFG {        // Get List of objects of given     // length and Supplier     public static <T> List<T>     getObjectList(int length,                   Supplier<T> objectSupply)     {         List<T> list = new ArrayList<>();            for (int i = 0; i < length; i++)             list.add(objectSupply.get());         return list;     }        public static void main(String[] args)     {            // Get 10 person by suppliying         // person supplier, Supplier is         // created by person constructor         // reference         List<Person> personList             = getObjectList(5, Person::new);            // Print names of personList         personList.stream()             .map(x -> x.getName())             .forEach(System.out::println);     } }

   chevron_right

   filter_none

   Output:

   vzskgmu
   iupltfx
   kocsipj
   lyvhxsp
   hbdphyv
   

Conclusion:

As mentioned above, if a lambda expression only calls an existing method then using method reference can make code more readable and clear. There are many more things we can do with Java8 Lambda and Method References while using Java streams.

Attention reader! Don’t stop learning now. Get hold of all the important Java Foundation and Collections concepts with the Fundamentals of Java and Java Collections Course at a student-friendly price and become industry ready.