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Lessons 10 / 12

Lambdas and Functional Interfaces

Lambdas and Functional Interfaces

Java supports the functional programming paradigm since version 8.

Functional Interfaces

It is an interface that has exactly one abstract method. They are usually marked with @FunctionalInterface.

@FunctionalInterface
public interface Operation {
    int execute(int a, int b);
}

Lambda Expressions

They are a concise way to implement a functional interface without creating a full class.

Syntax: (parameters) -> expression

// Lambda example for addition
Operation sum = (a, b) -> a + b;
System.out.println(sum.execute(5, 3)); // 8

Stream API and Applications

Lambdas are very powerful when used with collections:

  • forEach: Iterates through elements.
  • filter: Filters elements according to a condition.
  • map: Transforms elements.
  • anyMatch / allMatch: Checks conditions.

Example:

List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5, 6);
List<Integer> even = numbers.stream()
    .filter(n -> n % 2 == 0)
    .collect(Collectors.toList());
// Result: [2, 4, 6]