Phase 17 of 20 · Topic 17.2

Lambda Expressions & The 4 Types of Method References

1Concept

Method References (`::`) provide clean shorthand syntax for lambdas that simply call an existing method. There are 4 distinct types: 1. Static (`Class::staticMethod`); 2. Bound Instance (`obj::instanceMethod`); 3. Unbound Instance (`Class::instanceMethod`); 4. Constructor (`Class::new`).

2Architecture Diagram

Lambda:              (user) -> user.getName()
Method Reference:    User::getName (Unbound instance method reference)

3Code Example

Core Java
import java.util.List;
import java.util.function.Function;
import java.util.function.Supplier;

public class MethodReferencesDemo {
    static class User {
        private String name;
        public User() { this.name = "Guest"; }
        public User(String name) { this.name = name; }
        public String getName() { return name; }
        public static boolean isValid(String name) { return name != null && !name.isBlank(); }
    }

    public static void main(String[] args) {
        // 1. Static method reference
        Predicate<String> validator = User::isValid;

        // 2. Unbound instance method reference
        Function<User, String> nameExtractor = User::getName;

        // 3. Constructor reference
        Supplier<User> userFactory = User::new;

        User guest = userFactory.get();
        System.out.println("Created via Constructor Reference: " + nameExtractor.apply(guest));
        System.out.println("Valid name check: " + validator.test("Alice"));
    }
}

4Expected Output

Created via Constructor Reference: Guest
Valid name check: true

5Key Takeaways

  • ✓Method references produce cleaner, more readable code than equivalent verbose lambdas.
  • ✓Constructor references (`ArrayList::new`) are ideal for factory suppliers in stream pipelines.
  • ✓Variables captured by lambdas MUST be final or effectively final.