Function pointers in Java

From Java 8 onwards, the lambda expression is introduced which acts as function pointers.

Lambda expressions are introduced in Java 8 and are touted to be the biggest feature of Java 8. Lambda expression facilitates functional programming and simplifies the development a lot.

Syntax

A lambda expression is characterized by the following syntax.

parameter -> expression body

Following are the important characteristics of a lambda expression.

• Optional type declaration − No need to declare the type of a parameter. The compiler can inference the same from the value of the parameter.
• Optional parenthesis around parameter − No need to declare a single parameter in parenthesis. For multiple parameters, parentheses are required.
• Optional curly braces − No need to use curly braces in expression body if the body contains a single statement.
• Optional return keyword − The compiler automatically returns the value if the body has a single expression to return the value. Curly braces are required to indicate that expression returns a value.

Lambda Expressions Example

Create the following Java program using any editor of your choice in, say, C:\> JAVA.

Java8Tester.java

Live Demo

public class Java8Tester {
   public static void main(String args[]) {
      Java8Tester tester = new Java8Tester();
      //with type declaration MathOperation addition = (int a, int b) -> a + b;
      //with out type declaration MathOperation subtraction = (a, b) -> a - b;
      //with return statement along with curly braces MathOperation multiplication =
         (int a, int b) -> { return a * b; };
      //without return statement and without curly braces MathOperation division =
         (int a, int b) -> a / b;
         System.out.println("10 + 5 = " + tester.operate(10, 5, addition));
         System.out.println("10 - 5 = " + tester.operate(10, 5, subtraction));
         System.out.println("10 x 5 = " + tester.operate(10, 5, multiplication));
         System.out.println("10 / 5 = " + tester.operate(10, 5, division));
      //without parenthesis GreetingService greetService1 = message ->
         System.out.println("Hello " + message);
      //with parenthesis GreetingService greetService2 = (message) ->
         System.out.println("Hello " + message);
         greetService1.sayMessage("Mahesh");
         greetService2.sayMessage("Suresh");
   }

   interface MathOperation {
      int operation(int a, int b);
   }

   interface GreetingService {
      void sayMessage(String message);
   }

   private int operate(int a, int b, MathOperation mathOperation) {
      return mathOperation.operation(a, b);
   }
}

Verify the Result

Compile the class using javac compiler as follows − 

C:\JAVA>javac Java8Tester.java

Now run the Java8Tester as follows − 

C:\JAVA>java Java8Tester

It should produce the following output − 

10 + 5 = 15
10 - 5 = 5
10 x 5 = 50
10 / 5 = 2
Hello Mahesh
Hello Suresh