Creating Classes from Other Classes

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Creating Classes from Other Classes

• Chapter 11

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Chapter Contents

• Inheritance
• Invoking constructors from within constructors
• Private fields and methods of the base class
• Overriding, overloading methods
• Protected access
• Multiple inheritance
• Type compatibility and base classes
• The class Object
• Abstract classes and methods
• Polymorphism

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Composition

• When a class has a data field that is an instance
  of another class
• Example an object of type Student.

A "has a" relationship
fig 2-1
Fig. 2-1 A Student object composed of other
objects
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Adapters

• Use composition to write a new class
• Has an instance of an existing class as a data
  field
• Defines new methods needed for the new class
• Example

public class NickName private Name nick
public NickName() nick new Name()
// end default constructor public void
setNickName(String nickName)
nick.setFirst(nickName) // end setNickName
public String getNickName() return
nick.getFirst() // end getNickName // end
NickName
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Inheritance

• A general or base class is first defined
• Then a more specialized class is defined by
• Adding to details of the base class
• Revising details of the more general class
• Advantages
• Saves work
• Common properties and behaviors are define only
  once for all classes involved

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Inheritance

• Inheritance allows a software developer to derive
  a new class from an existing one
• The existing class is called the parent class, or
  superclass, or base class
• The derived class is called the child class or
  subclass.
• As the name implies, the child inherits
  characteristics of the parent
• In programming, the child class inherits the
  methods and data defined for the parent class

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Inheritance

• Inheritance should create an is-a relationship,
  meaning the child is-a more specific version of
  the parent
• Inheritance relationships are often shown
  graphically, with the arrow pointing to the
  parent class

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Inheritance
An "is a" relationship
Fig. 2-2 A hierarchy of classes.
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Inheritance

• A hierarchy of student classes.
• A child class of one parent can be the parent of
  another child, forming class hierarchies

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Class Hierarchies

• Two children of the same parent are called
  siblings
• Good class design puts all common features as
  high in the hierarchy as is reasonable
• Class hierarchies often have to be extended and
  modified to keep up with changing needs
• There is no single class hierarchy that is
  appropriate for all situations

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The Object Class

• All objects are derived from the Object class
• If a class is not explicitly defined to be the
  child of an existing class, it is assumed to be
  the child of the Object class
• The Object class is therefore the ultimate root
  of all class hierarchies
• The Object class contains a few useful methods,
  such as toString(), which are inherited by all
  classes

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References and Inheritance

• An object reference can refer to an object of its
  class, or to an object of any class related to it
  by inheritance
• For example, if the Holiday class is used to
  derive a child class called Christmas, then a
  Holiday reference could actually be used to point
  to a Christmas object
• Holiday day
• day new Christmas()

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Deriving Subclasses

• In Java, the reserved word extends is used to
  establish an inheritance relationship
• class Car extends Vehicle
• // class contents

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Calling the Base Class's Constructor

• Constructors usually initialize data fields
• In a derived class
• The constructor must call the base class
  constructor
• Can use the reserved word super as a name for the
  constructor of the base class
• When super is used, it must be the first action
  in the derived constructor definition
• Must not use the name of the constructor

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Accessing Inherited Data Fields

• Private data field in base class
• Not accessible by name within definition of a
  method from another class including a derived
  class
• Still they are inherited by the derived class
• Derived classes must use public methods of the
  base class
• Note that private methods in a base class are
  also unavailable to derived classes
• But usually not a problem private methods are
  used only for utility duties within their class

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Overriding Methods

• A child class can override the definition of an
  inherited method in favor of its own
• That is, a child can redefine a method it
  inherits from its parent
• The new method must have the same signature as
  the parent's method, but can have different code
  in the body
• The object type determines which method is
  invoked

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Overriding Methods

• When a derived class defines a method with the
  same signature as in base class
• Same name
• Same return type
• Same number, types of parameters
• Objects of the derived class that invoke the
  method will use the definition from the derived
  class
• It is possible to use super in the derived class
  to call an overridden method of the base class

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Overriding Methods
Fig. 2-4 The method toString in CollegeStudent
overrides the method toString in Student
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Overriding Methods

• Multiple use of super
• Consider a class derived from a base that
  itself is derived from a base class
• All three classes have a method with the same
  signature
• The overriding method in the lowest derived class
  cannot invoke the method in the base class's base
  class
• The construct super.super is illegal

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The super Reference

• Constructors are not inherited, even though they
  have public visibility
• Yet we often want to use the parent's constructor
  to set up the "parent's part" of the object
• The super reference can be used to refer to the
  parent class, and is often used to invoke the
  parent's constructor

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The super Reference Revisited

• The super reference can be used to invoke any
  method from the parent class
• This ability is often helpful when using
  overridden methods
• The syntax is
• super.method(parameters)

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Overloading a Method

• When the derived class method has
• The same name
• The same return type but
• Different number or type of parameters
• Then the derived class has available
• The derived class method and
• The base class method with the same name
• Java distinguishes between the two methods due to
  the different parameters

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Overloading a Method

• A programmer may wish to specify that a method
  definition cannot be overridden
• So that the behavior of the constructor will not
  be changed
• This is accomplished by use of the modifier
  final

public final void whatever() . . .
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Overloading vs. Overriding

• Don't confuse the concepts of overloading and
  overriding
• Overloading deals with multiple methods in the
  same class with the same name but different
  signatures
• Overriding deals with two methods, one in a
  parent class and one in a child class, that have
  the same signature
• Overloading lets you define a similar operation
  in different ways for different data
• Overriding lets you define a similar operation in
  different ways for different object types

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Protected Access

• A method or data field modified by protected can
  be accessed by name only within
• Its own class definition
• Any class derived from that base class
• Any class within the same package

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Protected Access

• The visibility modifiers determine which class
  members get inherited and which do not
• Variables and methods declared with public
  visibility are inherited, and those with private
  visibility are not
• But public variables violate our goal of
  encapsulation
• The protected visibility modifier allows a member
  to be inherited, but provides more protection
  than public does

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Protected Access
Fig. 2-5 Public, private, protected, and package
access
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Multiple Inheritance

• Some languages allow programmer to derive class C
  from classes A and B
• Java does not allow this
• A derived class can have only one base class
• Multiple inheritance can be approximated
• A derived class can have multiple interfaces
• Described in Chapter 3

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Object Types of a Derived Class

• Given
• Class CollegeStudent,
• Derived from class Student
• Then a CollegeStudent object is also a Student
  object
• In general An object of a derived class is also
  an object of the base class

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The Class Object

• Every class is a descendant of the class Object
• Object is the class that is the beginning of
  every chain of derived classes
• It is the ancestor of every other class
• Even those defined by the programmer

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Abstract Classes and Methods

• Some base classes are not intended to have
  objects of that type
• The objects will be of the derived classes
• Declare that base class to be abstractpublic
  abstract class Whatever . . .
• The designer often specifies methods of the
  abstract class without a body public abstract
  void doSomething()
• This requires all derived classes to implement
  this method

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Polymorphism

• A polymorphic reference is one which can refer to
  one of several possible methods
• Suppose the Holiday class has a method called
  celebrate, and the Christmas class overrode it
• Now consider the following invocation
• day.celebrate()
• If day refers to a Holiday object, it invokes
  Holiday's version of celebrate if it refers to
  a Christmas object, it invokes that version

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Polymorphism

• In general, it is the type of the object being
  referenced, not the reference type, that
  determines which method is invoked
• Note that, because all classes inherit from the
  Object class, an Object reference can refer to
  any type of object

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Polymorphism

• When one method name in an instruction can cause
  different actions
• Happens according to the kinds of objects that
  invoke the methods
• Example

UndergradStudent ug new UndergradStudent(. .
.)Student s ug // s and ug are
aliasess.displayAt(2)ug.displayAt(4)
The object still remembers it is of type
UndergradStudent
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Polymorphism

• Which displayAt is called
• Depends on the invoking object's place in the
  inheritance chain and is not determined by the
  type of the variable naming the object

Fig. 2-6 The variable s is another name for an
undergraduate object.
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Encapsulation

• Hides the fine detail of the inner workings of
  the class
• The implementation is hidden
• Often called "information hiding"
• Part of the class is visible
• The necessary controls for the class are left
  visible
• The class interface is made visible
• The programmer is given only enough information
  to use the class

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Encapsulation
Fig. 3-1 An automobile's controls are visible to
the driver, but its inner workings are hidden.