What are lambda expressions in Java?

A lambda expression is a concise form for an anonymous function that implements methods of a functional interface.

Structure: (parameters) -> { expression body }.

Part	Description	Examples
parameters	List of parameters, can be empty. Types may be omitted (inferred).	()<br>x<br>(x, y)<br>(int x, String y)
->	Arrow operator. Separates parameters and body.	->
expression body	Lambda body. Can be an expression or a block of code.	x * x<br>{ System.out.println(x); }

Examples of usage:

• Implementing functional interfaces:

  import java.util.function.Consumer;
  
  // Using traditional anonymous class
  Consumer<String> consumerAnon = new Consumer<String>() {
      @Override
      public void accept(String s) {
          System.out.println(s);
      }
  };
  consumerAnon.accept("Traditional anonymous class");
  
  // Using lambda expression
  Consumer<String> consumerLambda = s -> System.out.println(s);
  consumerLambda.accept("Lambda expression");
  

• Working with Streams API:

  import java.util.Arrays;
  import java.util.List;
  import java.util.stream.Collectors;
  
  List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5);
  
  // Filtering even numbers with lambda
  List<Integer> evenNumbers = numbers.stream()
                                    .filter(n -> n % 2 == 0)
                                    .collect(Collectors.toList());
  
  System.out.println(evenNumbers); // Output: [2, 4]
  
  // Transforming numbers to their squares
  List<Integer> squares = numbers.stream()
                                .map(n -> n * n)
                                .collect(Collectors.toList());
  
  System.out.println(squares); // Output: [1, 4, 9, 16, 25]
  

Advantages:

• Reduces code compared to anonymous classes.
• Improves readability for simple operations.
• Convenient for use with Streams API and other APIs oriented towards functional programming.