What is a lambda expression in Java?

A lambda expression in Java is an anonymous function that allows you to represent instances of functional interfaces in a more concise form.

Features:

• No name: The lambda has no name; it is defined "on the fly".
• No access modifiers: It does not have public, private, etc.
• No return type: The return type is determined automatically by the compiler.
• Body: Can be an expression (no need for the return keyword) or a block of code.
• Parameters: Passed in parentheses, types can be omitted if the compiler can infer them.

Syntax:

parameters -> body

Examples:

• No parameters: () -> System.out.println("Hello")
• With one parameter: x -> x * x
• With multiple parameters: (a, b) -> a + b
• With explicit types: (int x, int y) -> x + y
• With a code block:

  (a, b) -> {
      int sum = a + b;
      return sum;
  }
  

A functional interface is an interface with one abstract method. The lambda expression implements this abstract method.

// Example using the functional interface Runnable
Runnable myRunnable = () -> System.out.println("Running from lambda");
new Thread(myRunnable).start();

// Example using the functional interface Predicate
import java.util.function.Predicate;

Predicate<Integer> isEven = num -> num % 2 == 0;
boolean result = isEven.test(4); // true

Advantages:

• Conciseness: Reduces boilerplate code.
• Functional programming: Enables functional programming approaches (e.g., when working with Stream API).
• Readability: In some cases, makes the code more understandable, especially when passing simple logic blocks.

Lambda expressions were introduced in Java 8.