CS1020

1

• Inheritance and Polymorphism

2
Classes with Family ties

• Sometimes we find that two (or more) instantiable
  classes could be related to each other in the
  sense that
• They are not the exactly the same, but they are
  not different either
• One of the classes is a special case of the
  other, and needs to behave a little differently
• Example Class Beagle is a special case of
  Class Dog
• and maybe Class Rottweiler is also a special
  case of Class Dog

3
The Java mechanism for defining relationships
between related instantiable classes is called
Inheritance

• public abstract class Pet // generic housepet
• public class Dog // may or may not extend Pet
• public class Rottweiler extends Dog //
  specific type of Dog
• public class Beagle extends Dog // specific
  type of Dog
• The extends keyword establishes the inheritance
  relationship
• extends means is a kind of
• Note this is similar to extending an abstract
  class (like Pet), but remember an abstract class
  is itself not instantiable
• Each class gets its own .java file for
  implementation
• unless we want to use inner classes

4
UML and Inheritance
Superclass/Parent Class/Base Class
Subclasses/Child Classes/Derived Classes
5
Implements vs. Extends

• A class or abstract class can implement more than
  one interface
• Even if multiple interfaces declare the same
  behavior
• A class or abstract class can extend only one
  other class or abstract class
• Because attributes or behaviors cannot be
  inherited more than once

6
Rules of inheritance

• All attributes and methods defined in a
  superclass are inherited by all subclasses
• But Only protected and public attributes and
  methods defined in the superclass are accessible
  in the subclasses
• meaning only those superclass attributes are
  visible to the subclasses
• And only those superclass methods are callable by
  the subclasses
• Package attributes and methods are accessible
  only if the subclass is in the same package as
  the superclass
• Private methods and attributes defined in a
  superclass are only accessible within the
  superclass itself

methodA() can onlyaccess these attributes and
methodsof the superclass
7
Visibility from other classes

• doSomething() can only access the public
  attributes and methods of Superclass or
  SubClass
• publicAttr
• publicMethod()
• methodA() (of Subclass)
• Package attributes and methods are accessible if
  SomeOtherClass is in the same package as
  Superclass and Subclass

8
A subclass method can redefine a superclass method

• This is called overriding a method
• Do not confuse with method overloading
• A subclass overrides a superclass method to
  provide a specialized behavior

Beagle provides its own custom implementation of
speak()
9
Polymorphism allows a single variable to refer to
objects from different subclasses in the same
inheritance hierarchy

• For example, if Beagle is a subclass of Dog, then
  the following statements are valid

10
The instanceof operator can help us learn the
class of an object

• The following code counts the number of Beagles
  in a collection

11
Superclasses can keep secrets

• If a superclass declares some data members and
  methods private, they can be inherited but cannot
  be seen by subclasses
• Protected makes a data member or method of a
  superclass accessible to descendant classes
• In Java, protected also means package visible
• Public data members and methods are accessible to
  everyone

12
Inheritance and Member Accessibility
13
The Effect of Three Visibility Modifiers
In this example, Client is not in the same
package as Super and Sub
14
Everything except the private members of the
Super class is visible from a method of the Sub
class
method
15
Data members accessible from an instance are also
accessible from other instances of the same
class.
16
Every Java class has a common ancestor the
Object class

• If a class declaration does not explicitly
  designate the superclass with the extends clause,
  then the classs superclass is the Object class.
• Every Java class ultimately extends Object
• The Object class is the source of the following
  methods which every Java class inherits
• clone() makes a copy of the object
• equals() compares an object to another object
• toString() converts an object to its String
  representation
• and several others (see the Javadoc for the
  Object class)

17
Inheritance and Constructors

• Unlike members of a superclass, constructors of a
  superclass are not inherited by its subclasses.
• You must define a constructor for a class or use
  the default constructor added by the compiler.
• The statement
• super()
• calls the superclasss constructor.