What are lambda expressions in Java, and how are they used?

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Ashutosh Patel · Answer

Lambda Expressions in Java

Lambda expressions in Java are a shorthand way of representing anonymous functions: functions without a name that can be executed later by passing them around. They are essentially a way of defining a method without formally declaring it in a class.

The syntax for a lambda expression

The simple lambda expressions contain a single parameter and expression,

parameter -> expression

If you need to use more than one parameter, wrap them into a parentheses ().

(parameter1, parameter2) -> expression

Here is a simple example where a lambda expression is used to define a function that calculates the square of a number,

Function without lambda expression

public class Main {
   public static void main(String[] args) {
       
       System.out.println(myFuntion(5, 4));
   }
   public static int myFuntion(int a, int b) {       
       return a * b;
   };
}

Function with lambda expression

import java.util.ArrayList;
class Program {
   public static void main(String args[])
   {
       // Creating an ArrayList with elements {1, 2, 3, 4, 5, 6, 7}
       ArrayList<Integer> arrList = new ArrayList<Integer>();
       arrList.add(1);
       arrList.add(2);
       arrList.add(3);
       arrList.add(4);
       arrList.add(5);
       arrList.add(6);
       arrList.add(7);
       // Using lambda expression to print all elements
       System.out.println("All elements");
       arrList.forEach(n -> System.out.print(n+ "\t"));
       // Using lambda expression to print even elements
       System.out.println("\nList of Even Numbers");
       arrList.forEach(n -> {
           if (n % 2 == 0)
               System.out.print(n+ "\t");
       });
       
       // Using lambda expression to print odd elements
       System.out.println("\nList of Odd Numbers");
       arrList.forEach(n -> {
            if(n%2 != 0)
                System.out.print(n+ "\t"); 
       });
   }
}

Output-

All elements
1	2	3	4	5	6	7	
List of Even Numbers
2	4	6	
List of Odd Numbers
1	3	5	7

Usage

Lambda expressions are mainly used in functional networks, which are exactly one-way abstract interfaces. They provide a concise way to implement a method defined by a user interface.

For example, in Java, the java.util.function package contains many function interfaces such as Predicate, Consumer, and Function, which can be effectively implemented in lambda expressions.

Benefits

Conciseness- Lambda expressions reduce the amount of boilerplate code that comes with anonymous classes.
Readability- The code is made readable by focusing on what needs to be done rather than how.
Functional Programming- Lambda expressions enable a highly efficient functional form in Java, facilitating tasks such as mapping, filtering, and reducing collections.

Limitations

There are some restrictions on lambda expressions:

Can only be used in functional interfaces.
Cannot have return statements without curly braces {} around the body.
Cannot contain void as a return type (use Consumer for the void method).

When to Use

Lambda expressions are especially useful in situations where you need to pass a statement (to some extent) as an argument to a method or do it asynchronously. It is widely used in streams, threads, event handling, and other functional programming constructs.

Also, Read: What are lambda expressions, and how are they used in LINQ queries?

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What are lambda expressions in Java, and how are they used?

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Lambda Expressions in Java

Usage

Benefits

Limitations

When to Use