Inheritance

The idea

Inheritance creates a subclass that can reuse and specialize a superclass. An extends clause expresses the relationship, and super can call a superclass constructor or implementation. Construction initializes the superclass portion before the subclass constructor body completes. Overriding supplies a new implementation with a compatible method signature. The @Override annotation lets the compiler detect mistakes in your intended override.

A subtype should remain usable where its parent is expected. Inheriting solely to borrow code can produce a misleading model; composition is often clearer when one object contains or uses another. Private superclass fields are not directly accessible in subclass code, so a carefully designed parent exposes the protected or public behavior that extensions actually need.

class Notice {
  String text() { return "Ready"; }
}
class UrgentNotice extends Notice {
  @Override String text() { return super.text() + "!"; }
}
public class Main {
  public static void main(String[] args) {
    Notice n = new UrgentNotice();
    System.out.println(n.text());
  }
}

Ready!

The overridden method is chosen for UrgentNotice. Its super call deliberately uses the parent implementation before adding the exclamation mark.