• Abstraction: what functionality
• Encapsulation: how to achieve functionality
• Use encapsulation to hide implementation details
• Defensive programming: clearly document and check method contracts

public abstract class Animal {
  // Encapsulation
  private LocalDate birthdate;
  public Animal(LocalDate birthdate) { 
    if (birthdate.after(LocalDate.now())) {
      throw new IllegalArgumentException("...");
    }
    this.birthdate = birthdate; 
  }
  // Access-control: read-only age
  public int getAge() { 
    return Period.between(
        birthdate, 
        LocalDate.now()
      ).getYears(); 
    }

  // Abstraction
  public String sound();
}

public abstract class Pet extends Animal {
  // Encapsulation
  private String name;

  public final String getName() { return name; }
  public final void setName(String name) { 
    if (name.isEmpty()) throw new IllegalArgumentException("...");
    this.name = name; 
  }
}

public class Dog extends Pet {
  @Override
  public String sound() { return "bark"; }
}