The lambda expression was introduced first time in Java 8. Its main objective to increase the expressive power of the language.

But, before getting into lambdas, we first need to understand functional interfaces.

What is Functional Interface?

If a Java interface contains one and only one abstract method then it is termed as functional interface. This only one method specifies the intended purpose of the interface.

For example, the Runnable interface from package java.lang ; is a functional interface because it constitutes only one method i.e. run() .

Example 1: Define a Functional Interface in java

import java.lang.FunctionalInterface; @FunctionalInterface public interface MyInterface{ // the single abstract method double getValue(); }

In the above example, the interface MyInterface has only one abstract method getValue(). Hence, it is a functional interface.

Here, we have used the annotation @FunctionalInterface . The annotation forces the Java compiler to indicate that the interface is a functional interface. Hence, does not allow to have more than one abstract method. However, it is not compulsory though.

In Java 7, functional interfaces were considered as Single Abstract Methods or SAM type. SAMs were commonly implemented with Anonymous Classes in Java 7.

Example 2: Implement SAM with anonymous classes in java

public class FunctionInterfaceTest { public static void main(String[] args) { // anonymous class new Thread(new Runnable() { @Override public void run() { System.out.println("I just implemented the Runnable Functional Interface."); } }).start(); } }

Output:

I just implemented the Runnable Functional Interface.

Here, we can pass an anonymous class to a method. This helps to write programs with fewer codes in Java 7. However, the syntax was still difficult and a lot of extra lines of code were required.

Java 8 extended the power of a SAMs by going a step further. Since we know that a functional interface has just one method, there should be no need to define the name of that method when passing it as an argument. Lambda expression allows us to do exactly that.

Introduction to lambda expressions

Lambda expression is, essentially, an anonymous or unnamed method. The lambda expression does not execute on its own. Instead, it is used to implement a method defined by a functional interface.

How to define lambda expression in Java?

Here is how we can define lambda expression in Java.

(parameter list) -> lambda body

The new operator ( -> ) used is known as an arrow operator or a lambda operator. The syntax might not be clear at the moment. Let's explore some examples,

Suppose, we have a method like this:

double getPiValue() { return 3.1415; }

We can write this method using lambda expression as:

() -> 3.1415

Here, the method does not have any parameters. Hence, the left side of the operator includes an empty parameter. The right side is the lambda body that specifies the action of the lambda expression. In this case, it returns the value 3.1415.

Types of Lambda Body

In Java, the lambda body is of two types.

1. A body with a single expression

() -> System.out.println("Lambdas are great");

This type of lambda body is known as the expression body.

2. A body that consists of a block of code.

() -> { double pi = 3.1415; return pi; };

This type of the lambda body is known as a block body. The block body allows the lambda body to include multiple statements. These statements are enclosed inside the braces and you have to add a semi-colon after the braces.

Note: For the block body, you should always have a return statement. However, the expression body does not require a return statement.

Example 3: Lambda Expression

Let's write a Java program that returns the value of Pi using the lambda expression.

As mentioned earlier, a lambda expression is not executed on its own. Rather, it forms the implementation of the abstract method defined by the functional interface.

So, we need to define a functional interface first.

import java.lang.FunctionalInterface; // this is functional interface @FunctionalInterface interface MyInterface{ // abstract method double getPiValue(); } public class Main { public static void main( String[] args ) { // declare a reference to MyInterface MyInterface ref; // lambda expression ref = () -> 3.1415; System.out.println("Value of Pi = " + ref.getPiValue()); } }

Output:

Value of Pi = 3.1415

In the above example,

We have created a functional interface named MyInterface . It contains a single abstract method named getPiValue()

. It contains a single abstract method named Inside the Main class, we have declared a reference to MyInterface . Note that we can declare a reference of an interface but we cannot instantiate an interface. That is,

// it will throw an error MyInterface ref = new myInterface(); // it is valid MyInterface ref;

class, we have declared a reference to . Note that we can declare a reference of an interface but we cannot instantiate an interface. That is, We then assigned a lambda expression to the reference. ref = () -> 3.1415;

Finally, we call the method getPiValue() using the reference interface. When

System.out.println("Value of Pi = " + ref.getPiValue());

Lambda Expressions with parameters

Till now we have created lambda expressions without any parameters. However, similar to methods, lambda expressions can also have parameters. For example,

(n) -> (n%2)==0

Here, the variable n inside the parenthesis is a parameter passed to the lambda expression. The lambda body takes the parameter and checks if it is even or odd.

Example 4: Using lambda expression with parameters

@FunctionalInterface interface MyInterface { // abstract method String reverse(String n); } public class Main { public static void main( String[] args ) { // declare a reference to MyInterface // assign a lambda expression to the reference MyInterface ref = (str) -> { String result = ""; for (int i = str.length()-1; i >= 0 ; i--) result += str.charAt(i); return result; }; // call the method of the interface System.out.println("Lambda reversed = " + ref.reverse("Lambda")); } }

Output:

Lambda reversed = adbmaL

Generic Functional Interface

Till now we have used the functional interface that accepts only one type of value. For example,

@FunctionalInterface interface MyInterface { String reverseString(String n); }

The above functional interface only accepts String and returns String . However, we can make the functional interface generic, so that any data type is accepted. If you are not sure about generics, visit Java Generics.

Example 5: Generic Functional Interface and Lambda Expressions

// GenericInterface.java @FunctionalInterface interface GenericInterface<T> { // generic method T func(T t); } // GenericLambda.java public class Main { public static void main( String[] args ) { // declare a reference to GenericInterface // the GenericInterface operates on String data // assign a lambda expression to it GenericInterface<String> reverse = (str) -> { String result = ""; for (int i = str.length()-1; i >= 0 ; i--) result += str.charAt(i); return result; }; System.out.println("Lambda reversed = " + reverse.func("Lambda")); // declare another reference to GenericInterface // the GenericInterface operates on Integer data // assign a lambda expression to it GenericInterface<Integer> factorial = (n) -> { int result = 1; for (int i = 1; i <= n; i++) result = i * result; return result; }; System.out.println("factorial of 5 = " + factorial.func(5)); } }

Output:

Lambda reversed = adbmaL factorial of 5 = 120

In the above example, we have created a generic functional interface named GenericInterface . It contains a generic method named func() .

Here, inside the Main class,

GenericInterface<String> reverse - creates a reference to the interface. The interface now operates on String type of data.

- creates a reference to the interface. The interface now operates on type of data. GenericInterface<Integer> factorial - creates a reference to the interface. The interface, in this case, operates on the Integer type of data.

Lambda Expression and Stream API

The new java.util.stream package has been added to JDK8 which allows java developers to perform operations like search, filter, map, reduce, or manipulate collections like Lists .

For example, we have a stream of data (in our case a List of String ) where each string is a combination of country name and place of the country. Now, we can process this stream of data and retrieve only the places from Nepal.

For this, we can perform bulk operations in the stream by the combination of Stream API and Lambda expression.

Example 6: Demonstration of using lambdas with the Stream API

import java.util.ArrayList; import java.util.List; public class StreamMain { // create an object of list using ArrayList static List<String> places = new ArrayList<>(); // preparing our data public static List getPlaces(){ // add places and country to the list places.add("Nepal, Kathmandu"); places.add("Nepal, Pokhara"); places.add("India, Delhi"); places.add("USA, New York"); places.add("Africa, Nigeria"); return places; } public static void main( String[] args ) { List<String> myPlaces = getPlaces(); System.out.println("Places from Nepal:"); // Filter places from Nepal myPlaces.stream() .filter((p) -> p.startsWith("Nepal")) .map((p) -> p.toUpperCase()) .sorted() .forEach((p) -> System.out.println(p)); } }

Output:

Places from Nepal: NEPAL, KATHMANDU NEPAL, POKHARA

In the above example, notice the statement,

myPlaces.stream() .filter((p) -> p.startsWith("Nepal")) .map((p) -> p.toUpperCase()) .sorted() .forEach((p) -> System.out.println(p));

Here, we are using the methods like filter() , map() and forEach() of the Stream API. These methods can take a lambda expression as input.

We can also define our own expressions based on the syntax we learned above. This allows us to reduce the lines of code drastically as we saw in the above example.