Inheritance

A class can extend another class, which means it inherits the parent's fields and methods. The subclass starts with everything the parent has, then adds its own features or changes some behavior. In Java:

class CleaningBot extends Robot { }

This is useful because shared behavior lives in one place instead of being copied into every class. A Robot parent can hold common fields like name and batteryLevel, plus a shared method like reportStatus(). Specialized robots can then extend it. A CleaningBot might add a dustBinLevel field and a clean() method. A GuardBot might add an alarmEnabled field and a patrol() method. The common parts stay in Robot; each subclass defines only what makes it different.

A subclass can also override a parent method by defining a method with the same name and signature, replacing the parent's version for that subclass's objects. If CleaningBot overrides reportStatus(), then calling it on a CleaningBot runs the CleaningBot version.

class Robot {
  void reportStatus() { System.out.println("Robot status"); }
}

class CleaningBot extends Robot {
  @Override
  void reportStatus() { System.out.println("CleaningBot status"); }
}

Method lookup is the mechanism behind this. When you call a method on an object, Java first looks in the object's actual class. If it is not there, Java searches the parent, then the grandparent, and keeps climbing until it finds a match. That climb is how inherited methods are found and why overrides win.

Deep inheritance chains can get fragile and hard to reason about, which a later topic on composition addresses.

In the figure below, click classes in a small robot family tree to see own vs inherited vs overridden members, and call a method to watch the lookup climb the chain until it finds an implementation.

Climbing the lookup chain

The robot class tree is hand-authored, but the lookup is not: every walk you watch is a real climb up the parent pointers, and the own, inherited, or overridden tag on each member is resolved by that same logic, not typed in.