Inheritance: extends

Inheritance allows one class to acquire fields and methods of another. The IS-A relationship is the foundation of code reuse.

What is Inheritance

Inheritance allows a subclass to inherit from a superclass. The IS-A test: Dog IS-A Animal.

public class Animal {
    String name;
    void eat() { System.out.println(name + " eats"); }
}
public class Dog extends Animal {
    void bark() { System.out.println(name + " barks"); }
}
public class Cat extends Animal {
    void meow() { System.out.println(name + " meows"); }
}

Dog d = new Dog(); d.name = "Rex"; d.eat(); d.bark();
Cat c = new Cat(); c.name = "Kitty"; c.eat(); c.meow();

What is Inherited

• public and protected fields/methods
• package-private if same package

NOT inherited: private members, constructors, static methods.

Transitivity

C extends B extends A. C inherits from both B and A.

class A { void a() {} }
class B extends A { void b() {} }
class C extends B { } // has a() and b()

✅ Итоги урока

• extends establishes parent-child relationship.
• IS-A is the key test (Dog IS-A Animal).
• Inherits: public, protected, package-private (same package).
• Does NOT inherit: private, constructors, static.
• Inheritance is transitive (C extends B extends A).
• Use only for genuine IS-A relationships.