10.1. Introduction

Consider the following classes.

public class Character {
  public String firstName;
  public String lastName;

  public Character(String firstName, String lastName) {
    this.firstName = firstName;
    this.lastName = lastName;
  }
}

public class Hello {
  void sayHello(Character person) {
    System.out.println("Hello " + person.firstName + " " + person.lastName + "!");
  }
}

public class Main {

  public static void main(String[] args) {
    Character rick = new Character("Rick", "Sanchez");
    Character morty = new Character("Morty", "Smith");
    Character scary = new Character("Scary", "Terry");
    List<Character> characters = List.of(rick, morty, scary);

    Hello hello = new Hello();
    for (Character c : characters)
      hello.sayHello(c);
  }

}

Hello Rick Sanchez!
Hello Morty Smith!
Hello Scary Terry!

If we often want to "say hello", it makes sense to create an interface.

public interface Helloable {
  void sayHello(Character character);
}

We then implement it for each way we say hello.

public class SimpleHello implements Helloable {
  @Override
  public void sayHello(Character character) {
    System.out.println("Hello " + character.firstName + " " + character.lastName + "!");
  }
}

public class FancyHello implements Helloable {
  @Override
  public void sayHello(Character character) {
    System.out.println("Hello Mr. " + character.lastName + ".");
  }
}

public class Main {

  public static void main(String[] args) {
    Character rick = new Character("Rick", "Sanchez");
    Character morty = new Character("Morty", "Smith");
    Character scary = new Character("Scary", "Terry");
    List<Character> characters = List.of(rick, morty, scary);

    SimpleHello simple = new SimpleHello();
    for (Character c : characters)
      simple.sayHello(c);

    FancyHello fancy = new FancyHello();
    for (Character c : characters)
      fancy.sayHello(c);
  }

}

Hello Rick Sanchez!
Hello Morty Smith!
Hello Scary Terry!
Hello Mr. Sanchez.
Hello Mr. Smith.
Hello Mr. Terry.

If we want to define how to say hello "on the fly", we can write an anonymous class.

public class Main {
  public static void main(String[] args) {
    Character rick = new Character("Rick", "Sanchez");
    Character morty = new Character("Morty", "Smith");
    Character scary = new Character("Scary", "Terry");
    List<Character> characters = List.of(rick, morty, scary);

    Helloable custom =
        new Helloable() {
          @Override
          public void sayHello(Character character) {
            System.out.println("Hello " + character.firstName.charAt(0) + ". " + character.lastName);
          }
        };

    for (Character c : characters)
      custom.sayHello(c);
  }
}

Hello R. Sanchez
Hello M. Smith
Hello S. Terry

A little verbose, right?

Lambdas to the rescue!

Since Java 8, we can achieve the same result writing a lot less code by using lambda expressions.

public class Main {
  public static void main(String[] args) {
    Character rick = new Character("Rick", "Sanchez");
    Character morty = new Character("Morty", "Smith");
    Character scary = new Character("Scary", "Terry");
    List<Character> characters = List.of(rick, morty, scary);

    Helloable custom =
        c -> System.out.println("Hello " + c.firstName.charAt(0) + ". " + c.lastName);
    
    for (Character c : characters) {
      custom.sayHello(c);
    }
  }
}

We can even pass these expressions as parameters.

public class Main {
  public static void main(String[] args) {
    Character rick = new Character("Rick", "Sanchez");
    Character morty = new Character("Morty", "Smith");
    Character scary = new Character("Scary", "Terry");
    List<Character> characters = List.of(rick, morty, scary);

    sayHelloEverybody(
        characters, 
        c -> System.out.println("Hello " + c.firstName.charAt(0) + ". " + c.lastName));

    sayHelloEverybody(
        characters,
        c -> System.out.println("Hello Mr. " + c.firstName.charAt(0) + c.lastName.charAt(0)));
  }

  public static void sayHelloEverybody(List<Character> characters, Helloable helloable) {
    for (Character c : characters)
      helloable.sayHello(c);
  }
}

Hello R. Sanchez
Hello M. Smith
Hello S. Terry
Hello Mr. RS
Hello Mr. MS
Hello Mr. ST