What is Java Lambda Expression? Explained with Example

Java 8 is one of most popular release in the history of Java and have introduced lots of major features like Lambda Expression, Method Reference, Stream API, new DateTime API, Default and static interface methods etc.. For complete list of new Java 8 feature read What’s New in JDK 8 article by oracle itself.

In this article, we are going to study Java Lambda Expression in details e.g. what is the lambda expression, why it so popular, what are the advantages of the lambda expression, what purpose is solved, how it differs from anonymous classes and why/how should we use it.

Why Java needs Lambda Expression

Java is a multiparadigm programming language (Supports Object Oriented, Imperative, Structured, Declarative style of programming). However, most of its design architecture is influenced by Object Oriented programming style.

And there are two variations of object-oriented programming (OOP) style

• Class-based OOP style
• Prototype-based OOP style → doesn’t support inheritance

Java supports class-based OOP style that’s why there should be a class for every object while JavaScript follows Prototype-based OOP style that’s why we don’t have the concept of class and inheritance in JavaScript. Everything in JavaScript is an object either it is a variable or a function while in Java a function (method) is state of the object.

Apart from this JavaScript also follows the Functional style of programming so we can assign functions to variables, pass functions as argument or return a function from other function etc..

Due to class-based nature of Java programming, it is hard to maintain functions as first class citizen similar as objects. That’s why Java does not support functional style of programming (before Java 8) and we can’t assign functions to variables or pass functions as argument or return them.

Lambda Expression is a way to introduce functional programming to Java. So by using lambda expression we can simply define and use functions wherever we need them (e.g. inside a method) without even creating objects. Due to lambda expression now functions have also become first class citizens in Java language.

What is Lambda Expression

By definition

A Lambda Expression is an anonymous function (a function defined, and possibly called, without being bound to an identifier and a name.

The concept is similar to anonymous classes or anonymous objects, Now we can declare methods wherever we want them without any name. The sole purpose of writing an anonymous class object is to override its method and provide our own functionality but for that single method, we always need to declare the class. With lambda expression, we can eliminate the declaration of the anonymous class and we can simply write the methods.

Since the old days of Swing, we always had written anonymous classes if we wanted to pass some functionality to any method. For example old event listener code and thread creation code look like

Runnable runnable = new Runnable() {
  @Override
  public void run() {
  System.out.println("Running thread using anonymous class");
  }
};
Thread thread = new Thread(runnable);

If you look at above code the actual functionality of the thread is written in the run() method. And we are creating an anonymous class object just to define a single method which is really unnecessary if we can do functional programming (just create the function where it needs). So here the creation of anonymous class Can be replaced by lambda as below which is just a single line of code:

Runnable runnable = () -> System.out.println("Running thread using lambda expression");
Thread thread = new Thread(runnable);

Above we have assigned a lambda to the reference variable but more precisely we create a thread as below if r is not reusable.

Thread t = new Thread(() -> System.out.println("Running thread using lambda expression"));

How to write Lambda Expression

Through lambda expression, we can write functional code but this is still not similar to other functional programming languages like C, C++, JavaScript.

Java is still a class based object oriented language and methods always belong to some class, abstract class or interface. So methods implemented using lambda expression must have to define somewhere. So along with Lambda Expression Oracle engineers have applied some tweaks and introduced the concept of Functional Interfaces.

Functional Interface → An interface with only one abstract method and optionally annotated with @FunctionalInterface annotation.

We can use lambda expressions only with Functional interfaces, in above example the Runnable interface is a functional interface.
Lambda expressions in Java is usual written using syntax (argument) -> (body)

Converting anonymous classes to Lambda Expression

After little bit practice, you will not feel any need for conversion, you will start writing lambdas without any confusion. But whenever you got confused conversion approach is the best way to create lambdas.

We can convert the old anonymous class code to lambda expression as below, suppose we want to create lambda expression for thread generation process:

Runnable runnable = new Runnable() {
  @Override
  public void run() {
  System.out.println("In thread");
  System.out.println("doing something");
  }
};

Remove below things in order
→ new keyword new,
→ class name and it’s parentheses Runnable(),
→ class opening and closing braces { };,
→ @Override annotation (if have any),
→ remove method declaration (leave argument parentheses ()) public void run
→ removing method opening { and closing braces } is not necessary, however, if you have single line of code in it then you can remove them
→ add -> between parentheses () and execution code inside { }

Runnable runnable = new Runnable() {
  @Override
  public void run() -> {
  System.out.println("In thread");
  System.out.println("doing something");
  }
};

So now our lambda expression will become

Runnable runnable = () -> {
  System.out.println("In thread");
  System.out.println("doing something");
}

Single line Zero Argument method: If we have a single line of code inside execution block {} then we can remove them.

Runnable r = () -> System.out.println("In thread");

Multiple Argument Methods: If functional interface’s method definition have arguments then we can write them inside ()

Comparator<Integer> comparator = (Integer i, Integer j) -> i.compareTo(j);

Single Argument Methods: And if there is only one argument then we can eliminate () braces

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

Difference between Lambda Expression and Anonymous class

• An anonymous class object creates a separate class file after compilation which increases the size jar while after compilation Lambda expression becomes invokedynamic which dynamic language implementation.
• We can use this keyword to represent the current class in lambda expression while in the case of anonymous class this keyword represents that particular anonymous class.
• In the case of Lambda expression, we need to provide the function body only while in the case of anonymous class we need to write the redundant class definition.

Advantages of Lambda Expression

Lambda Expressions allows us to code in functional style so it provides all benefits of functional style as well as above we can see Lambda Expressions lets developers

• Simply understand the code.
• Simplify and shorten their code.
• Making it more readable and maintainable.
• Remove more verbose class declarations.

Examples of Lambda Expressions

In below program, I have demonstrated different examples you can also found the source code on Github.

package org.programming.mitra.exercises;

import java.awt.Button;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.util.Comparator;
import java.util.function.Consumer;
import java.util.stream.IntStream;

public class LambdaExpression {

  @SuppressWarnings("unused")
  public static void main(String[] args) {

  // Example : Thread Creation
  // Old Way
  Runnable runnable = new Runnable() {
  @Override
  public void run() {
  System.out.println("Running thread using anonymous class");
  }
  };
  Thread thread = new Thread(runnable);
  thread.start();

  // New Way
  runnable = () -> System.out.println("Running thread using lambda expression");
  thread = new Thread(runnable);
  thread.start();

  // Example : Comparator Creation
  Comparator<Integer> comparator = (Integer i, Integer j) -> i.compareTo(j);

  // Example : Consumer Creation
  Consumer<String> consumer = obj -> System.out.println(obj);

  // Example : Consumer Creation
  Button button = new Button();

  // Old way:
  button.addActionListener(new ActionListener() {
  @Override
  public void actionPerformed(ActionEvent e) {
  System.out.println("button clicked");
  }
  });

  // New way:
  button.addActionListener((e) -> {
  System.out.println("button clicked");
  });

  // Example : Iteration
  System.out.println("####### Old way #######");
  for (int i = 1; i < 10; i++) {
  int j = i * i;
  System.out.println(j);
  }

  System.out.println("####### Lambda (Stream) way #######");
  IntStream.range(1, 10)
  .map(num -> num * num)
  .forEach(i -> System.out.println(i));

  }
}

Lambda expressions are heavily used in Java Stream API, which I am going to cover in later articles.