Абстрактные классы

Что такое класс abstract?

Код: невозможно создать экземпляр

public class Main {
  // Abstract class with one abstract method and one concrete method
  static abstract class Animal {
    abstract void speak();              // must be implemented by subclasses
    void info() {                       // concrete method allowed
      System.out.println("I am an animal.");
    }
  }

  public static void main(String[] args) {
    // Animal a = new Animal(); // Uncommenting would not compile: cannot instantiate abstract class
    System.out.println("You cannot create an Animal directly if it is abstract.");
  }
}

Зачем нужны классы abstract?

Код: реализация метода abstract

public class Main {
  static abstract class Animal {
    abstract void speak();
    void info() {
      System.out.println("I am an animal.");
    }
  }

  static class Dog extends Animal {
    // Must implement speak()
    void speak() {
      System.out.println("Woof!");
    }
  }

  public static void main(String[] args) {
    Dog d = new Dog();      // allowed: Dog is concrete
    d.info();               // inherited concrete method
    d.speak();              // Dog's implementation
  }
}

Правила и сочетание возможностей

Код: abstract и полиморфизм

public class Main {
  static abstract class Animal {
    abstract void speak();
  }

  static class Dog extends Animal {
    void speak() { System.out.println("Woof!"); }
  }

  static class Cat extends Animal {
    void speak() { System.out.println("Meow!"); }
  }

  public static void main(String[] args) {
    // Polymorphism through abstract base type
    Animal[] pets = { new Dog(), new Cat() };
    for (int i = 0; i < pets.length; i = i + 1) {
      pets[i].speak(); // each subclass provides its own behavior
    }
  }
}

Проверка классов abstract

Итоги