3.5.2Hierarchies and types of inheritance

Classes form a tree called a class hierarchy. The most general class is at the top.

Types of inheritance:

Every Java class ultimately inherits from java.lang.Object.

Generalisation and specialisation

Moving up a hierarchy is generalisation: Dog and Cat have things in common, so you collect those into a more general class, Animal. Moving down is specialisation: a Puppy is a more specific kind of Dog. A good design puts each attribute and method at the highest level where it applies to every class below it. name and eat() belong in Animal, because every animal has them, but bark() belongs in Dog.

How to design a hierarchy

1. List the classes you need and the attributes and methods of each.
2. Look for attributes and methods that several classes share. Move them up into a superclass.
3. Check every link with the is-a sentence. "A Manager is an Employee": yes. "A Department is an Employee": no.
4. Keep it shallow. Two or three levels is usually enough; very deep hierarchies are hard to understand and to change.

Example: a hierarchy in code

class Vehicle {
    protected String regNo;
    Vehicle(String regNo) { this.regNo = regNo; }
    void start() { System.out.println(regNo + " started"); }
}

class Car extends Vehicle {                  // hierarchical: Car and Bike
    Car(String regNo) { super(regNo); }      // both extend Vehicle
}

class Bike extends Vehicle {
    Bike(String regNo) { super(regNo); }
}

class SportsCar extends Car {                // multilevel: Vehicle -> Car -> SportsCar
    SportsCar(String regNo) { super(regNo); }
    void turbo() { System.out.println(regNo + " turbo on"); }
}

A SportsCar has regNo and start() from Vehicle and turbo() from itself.

The Object class

If a class does not say extends, Java makes it extend java.lang.Object automatically. So every class inherits a few useful methods, which it can override: