Inheritance

1. Inheritance Chapter 11 in Gaddis

2. Is a relationships in ‘real’ life • Exist when one object is a specialized version of another one • Examples • An english setter is-a dog • A jet is-a motorized plane • A rose is-aflower • A card game is-a game • The specialized object has all of the characteristics of the general object plus additional characteristics that make it special.

3. Is-a relationships in OO Programming • Are created using inheritance • Inheritance is used to create is-a relationships among classes. • Java does not allow what is called multiple inheritance.

4. General Superclass Base class Specific Subclass Derived class Inheritance terminology

5. Super class elements inherited/not inherited by subclass • Inherited • Public fields • Public methods • Accessors • Mutators • Not inherited • Constructors • Private fields • Private methods

6. Important re: constructors • Superclass constructors are not inherited. • Superclass constructors create superclass objects • In an inheritance relationship, the superclass constructor always executes before the subclass constructor. • It happens automatically when the superclass has a no-arg constructor or a default constructor.

7. Important re: constructors • What if superclass has • Programmer-defined 1 argument constructor • Programmer-defined 2 argument constructor • Programmer-defined multi-argument constructor • All of the above • Subclass uses the keyword super to call the desired superclass constructor

8. Constructor Guidelines • The super statement can only appear in the subclass’s constructor • The super statement must be the first statement in the subclass’s constructor • IF the subclass constructor does not explicitly call the superclass’s constructor, Java will automatically call the superclasses no-arg or default constructor just before the subclass constructor executes. • Equivalent to super();

9. Two more important points • If a superclass does not have a no-arg constructor but does have a programmer-defined constructor, the subclass must call it or a subclass compile error will occur. • Subclass header statement must indicate what its superclass is using the reserved word extends

10. Important • When a subclass extends a superclass, the public members of the superclass become public members of the subclass.

11. Important • Inheritance is a one-way relationship. • Subclasses inherit from superclasses. • Superclasses do not inherit from subclasses.

12. Terminology review • Overriding vs Overloading • Overloaded methods have the same name as one or more methods but a different parameter list. • Overridden methods have the same signature

13. Overriding and Overloading • Both can take place in an inheritance relationship. • Overloaded methods can occur: • Within the same class • A subclass can overload a method in a superclass • Overridden methods can only occur in the inheritance relationship • A subclass can not override another method in the same class • A subclass can override a method in its superclass.

14. How to prevent overriding • A method in the superclass with the modifier finalcan not be overridden.

15. More on Inheritance • AFTER the exam