Object Orientation (Part II)

Encapsulation, IS-A, HAS-A

• Encapsulation helps hide implementation behind an interface (or API).
• Encapsulated code has two features: Instance variables are kept protected (usually with the private modifier) and Getter and setter methods provide access to instance variables.
• IS-A refers to inheritance it is expressed with the keyword extends.
• IS-A, "inherits from," and "is a subtype of" are all equivalent expressions.
• HAS-A means an instance of one class "has a" reference to an instance of another class.

Inheritance

• Inheritance is a mechanism that allows a class to be a subclass of a superclass, and thereby inherit variables and methods of the superclass.
• Inheritance is a key concept that underlies IS-A, polymorphism, overriding, overloading, and casting.
• All classes (except class Object), are subclasses of type Object, and therefore
• they inherit Object's methods.

Polymorphism

• Polymorphism means ‘many forms'.
• A reference variable is always of a single, unchangeable type, but it can refer to a subtype object.
• A single object can be referred to by reference variables of many different
• types as long as they are the same type or a supertype of the object.
• The reference variable's type (not the object's type), determines which methods can be called!
• Polymorphic method invocations apply only to overridden instance methods.

Overriding and Overloading

• Methods can be overridden or overloaded; constructors can be overloaded but not overridden.
• Abstract methods must be overridden by the first concrete (nonabstract) subclass.
• final methods cannot be overridden.
• Only inherited methods may be overridden, and remember that private methods are not inherited.
• Overloading means reusing amethod name, but with different arguments.
• Methods from a superclass can be overloaded in a subclass.
• Polymorphism applies to overriding, not to overloading
• Object type (not the reference variable's type), determines which overridden method is used at runtime.
• Reference type determines which overloaded method will be used at compile time.