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This article provides an introduction to lambda expressions introduced in JDK 8 and explains how to run them using functional interfaces.

lambda expressions

What is a Lambda expression in Java ?

 

A Lambda expression represents an Anonymous method.

 

Anonymous method concept is similar to that of an Anonymous class… the difference being it implements a functional interface.

 

Functional interface is a new interface concept in Java 8. A functional interface can only declare one abstract method.

 

Lambda expressions allow the programmer to pass code in a concise manner making the code more clearer and flexible.

 

Lambda syntax

 

A lambda expression contains:

  • A parameter list
  • An arrow symbol(->)
  • Body of the lambda containing statements

 

The syntax for Lambda expressions is:

(parameters) -> {statements;}

 

How to write simple methods as lambda expression

 

Here is a simple method that displays a message “Hello World”.

 

public void hello(){
    System.out.println("Hello World");
  }

To convert this method to a lambda expression, remove the access specifier “public”, return type “void” and method name “hello” and write it as :
() -> {System.out.println("Hello World");}

 

Here is another method example that adds two numbers and returns their sum :

 

int add(int x, int y){
    return x+y;
  }

 

This method can be converted to lambda expression as :

 

(int x, int y) -> {return x+y;}

 

Important points about lambda expressions

 

Here are some notable points regarding lambda expressions :

 

1 ) A lambda expression can have zero or more parameters.

 

Here is an example of lambda expression with zero parameters:

 

() -> {System.out.println("Hello World");}

 

Here is an example of lambda expression with two parameters:

 

(int x, int y) -> {return x+y;}

 

This lambda expression accepts two integer parameters and returns their sum.

 

2) If the type of the parameters can be decided by the compiler, then we can ignore adding them in the lambda expression.

 

(int x, int y) -> {return x+y;} // type mentioned

(x,y) -> {return x+y;}; // type ingored

 

3) If there is only one parameter, the parenthesis of parameter can be omitted.

 

Here are some examples :

x -> {return x+10;}

String s -> s.toUpperCase()

String s -> System.out.println(s)

 

4) If the body has just one expression, the return keyword and curly braces can be omitted as show below:

 

(int x, int y) -> x + y

 

If the return type of parameters is omitted, the compiler determines default parameter types.

 

(x,y) -> x+y

(str, i) -> str.substring(i)

 

5) A lambda can also have empty parameters

 

() -> {return “Hello”;}

() -> System.out.println(“Hello”)

 

6) A lambda can also have empty parameter as well as empty body

 

() -> {}

 

This represents a valid lambda expression that takes no parameters and returns void.

 

How to call a lambda expression ?

 

Once a lambda expression is written, it can be called and executed like a method.

 

For calling a lambda expression, we should create a functional interface.

 

Here is an example that uses functional interface to execute lambda expression :
 

public class FuntionalInterfaceDemo {

  interface MyInter{
    void hello();
  }
  public static void main(String[] args) {
    // TODO Auto-generated method stub
    MyInter infVar = () -> {System.out.println("Hello World");};
    
    infVar.hello();
  }

}

 

Refer more details on functional interface and executing lambda expressions in the following article:

 

Running lambda expression with Functional Interface

 

 

More on Java 8

 

 

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