Lambda Expressions & Functional Interfaces

A functional interface is any interface with exactly one abstract method — Java lets you implement one inline with a compact lambda expression instead of a full named class.

interface Greeter {
    String greet(String name);
}

Greeter formal = name -> "Good day, " + name;
System.out.println(formal.greet("Ada")); // Good day, Ada

The standard library ships common functional interfaces ready to use, so you rarely need to declare your own:

import java.util.function.Function;
import java.util.function.Predicate;

Function<Integer, Integer> square = n -> n * n;
System.out.println(square.apply(5)); // 25

Predicate<String> isEmpty = String::isEmpty;
System.out.println(isEmpty.test("")); // true

String::isEmpty is a method reference — shorthand for a lambda that just calls an existing method.

Common mistakes

• Writing s -> s.isEmpty() when String::isEmpty says the same thing more clearly — reach for a method reference whenever the lambda body is just "call this one existing method."