ITM 352 Class inheritance, hierarchies Lecture #
ITM 352Class inheritance, hierarchies
Lecture #
Chapter 7 Java: an Introduction to Computer Science & Programming - Walter Savitch 2
Announcements Good job on exam 1
» Exam 2 will be a little different Extra lab tomorrow (Friday) 10-12noon Be sure to get started on HW3
» Several parts!!
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Topics For Today Review of Objects Basic Inheritance UML
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An abstraction that represents both memory and functionality
» memory: resolution of a component’s static qualities such as
attributes and relationships.
» Functionality: set of methods that embody operations
Object Abstractions
Attributes
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Object Relationships Objects interact (share data, invoke operations) through
relationships
kool!
manages
ownsanEnvironment
aWeapon
aRobot
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Object QualitiesYou want to specify the following information for each object:
Identity -
Defining quality -
Name -
Attributes -
Behaviors -
Relationships -
State Groups -
Constraints -
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Classes: Summary Describe objects of the same type (type-of)
» Some qualities– behaviors (methods)– attributes (instance variables)– constraints
» Many possible quality resolutions - maintained in objects Describe how an object of a given type is created (instantiated)
in any given situation (instance)
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Classes and objects (redressed) Every class has objects, or instances, created using the new operation.
Think of the class as an object-producing machine.
new
new
Robot
String
aRobot
anotherRobot
aString
anotherString
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Java Objects Literally an instance of a Class (which shockingly is itself, an
object!)» Class defines and creates (instantiates) an object» always have a reference after instantiation
Behaviors are instance methods» methods contain operations» do not have to accept or return values
Attributes are instance variables» all variables must be typed» defined outside of any method
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Objects and Class (redressed)
Directly describing object structures is inefficient » many objects have the same qualities, just different quality
resolutions Java uses the concept of a Class to group the same “type-of”
objects together» A Class contains the description on how to create objects of
particular type. Sometimes called a “factory object” or “metaobject”
Objects are constructed (instantiated) through their Class definitions
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But there’s more…
Class Inheritanceor
Where there’s a will, there are relatives
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Chapter 7
Inheritance Basics Programming with Inheritance Dynamic Binding and Polymorphism
Inheritance
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Principles of OOP
OOP - Object-Oriented Programming Principles discussed in previous chapters:
» Information Hiding» Encapsulation» Polymorphism
In this chapter» Inheritance
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Why OOP?
To try to deal with the complexity of programs To apply principles of abstraction to simplify the tasks
of writing, testing, maintaining and understanding complex programs
To increase code reuse» to reuse classes developed for one application in
other applications instead of writing new programs from scratch ("Why reinvent the wheel?")
Inheritance is a major technique for realizing these objectives
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Inheritance Overview
Inheritance allows you to define a very general class then later define more specialized classes by adding new detail» the general class is called the base or parent class
The specialized classes inherit all the properties of the general class» specialized classes are derived from the base class» they are called derived or child classes
After the general class is developed you only have to write the "difference" or "specialization" code for each derived class
A class hierarchy: classes can be derived from derived classes (child classes can be parent classes)» any class higher in the hierarchy is an ancestor class» any class lower in the hierarchy is a descendent class
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An Example of Inheritance:a Person Class
The base class: Display 7.1 Constructors:
» a default constructor» one that initializes the name attribute (instance variable)
Accessor methods:» setName to change the value of the name attribute» getName to read the value of the name attribute» writeOutput to display the value of the name attribute
One other class method:» sameName to compare the values of the name attributes for
objects of the class Note: the methods are public and the name attribute private
public class Person { // Instance variable section private String name; // Constructors public Person() { // Default constructor name = "No name yet."; } public Person(String initialName) { // Constructor for just the name name = initialName; } // Set/get methods public void setName(String newName) { name = newName; } public String getName() { return name; } // Other methods public void writeOutput() { System.out.println("Name: " + name); } public boolean sameName(Person otherPerson) { return (this.name.equalsIgnoreCase(otherPerson.name)); } }
A PersonBase ClassDisplay 7.1
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Derived Classes: a Class Hierarchy
The base class can be used to implement specialized classes» For example: student, employee, faculty, and staff
Classes can be derived from the classes derived from the base class, etc., resulting in a class hierarchy
Person
Student Employee
Faculty StaffUndergraduate Graduate
MastersDegree NonDegreePhD
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Example of Adding Constructor in a Derived Class: Student
Two new constructors (one on next slide)» default initializes attribute studentNumber to 0
super must be first action in a constructor definition» Included automatically by Java if it is not there» super()calls the parent default constructor
public class Student extends Person{ private int studentNumber; public Student() { super(); studentNumber = 0; }…
Keyword extends in first line» creates derived
class from base class
» this is inheritanceThe first few lines of Student class(Display 7.3):
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Example of Adding Constructor in a Derived Class: Student
Passes parameter newName to constructor of parent class Uses second parameter to initialize instance variable that is not in parent class.
public class Student extends Person{. . . public Student(String newName, int newStudentNumber) { super(newName); studentNumber = newStudentNumber; }. . .
More lines of Student class(Display 7.3):
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More aboutConstructors in a Derived Class
Constructors can call other constructors Use super to invoke a constructor in parent class
» as shown on the previous slide Use this to invoke a constructor within the class
» shown on the next slide Whichever is used must be the first action taken by the
constructor Only one of them can be first, so if you want to invoke both:
» Use a call with this to call a constructor with super
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Example of a constructor using this
Student class has a constructor with two parameters: String for the name attribute and int for the studentNumber attribute
public Student(String newName, int newStudentNumber) { super(newName); studentNumber = newStudentNumber; }
Another constructor within Student takes just a String argument and initializes the studentNumber attribute to a value of 0:» calls the constructor with two arguments, initialName (String) and 0
(int), within the same class
public Student(String initialName){ this(initialName, 0);}
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Example of Adding an Attribute in a Derived Class: Student
Note that an attribute for the student number has been added» Student has this attribute in addition to name,
which is inherited from Person
A line from the Student class:
private int studentNumber;
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Example of Overriding a Method in a Derived Class: Student Both parent and derived classes have a writeOutput method Both methods have the same parameters (none)
» they have the same signature The method from the derived class overrides (replaces) the
parent's It will not override the parent if the parameters are different (since
they would have different signatures) This is overriding, not overloading
public void writeOutput(){ System.out.println("Name: " + getName()); System.out.println("Student Number : " studentNumber);}
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Call to an Overridden Method
Use super to call a method in the parent class that was overridden (redefined) in the derived class
Example: Student redefined the method writeOutput of its parent class, Person
Could use super.writeOutput() to invoke the overridden (parent) method
public void writeOutput(){ super.writeOutput(); System.out.println("Student Number : " studentNumber);}
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Overriding Verses Overloading
Overriding
Same method name
Same signature One method in
ancestor, one in descendant
Overloading
Same method name
Different signature Both methods can be
in same class
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The final Modifier
Specifies that a method definition cannot be overridden with a new definition in a derived class
Example:
public final void specialMethod()
{
. . . Used in specification of some methods in standard libraries Allows the compiler to generate more efficient code Can also declare an entire class to be final, which means it
cannot be used as a base class to derive another class
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private & publicInstance Variables and Methods
private instance variables from the parent class are not available by name in derived classes» "Information Hiding" says they should not be » use accessor methods to change them, e.g. reset for a Student object to change the name attribute
private methods are not inherited!» use public to allow methods to be inherited» only helper methods should be declared private
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What is the "Type" of a Derived class?
Derived classes have more than one type Of course they have the type of the derived class (the class they
define) They also have the type of every ancestor class
» all the way to the top of the class hierarchy All classes derive from the original, predefined class Object Object is called the Eve class since it is the original class for
all other classes
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Assignment Compatibility
Can assign an object of a derived class to a variable of any ancestor type
Person josephine;
Employee boss = new Employee();
josephine = boss;
Can not assign an object of an ancestor class to a variable of a derived class type
Person josephine = new Person();
Employee boss;
boss = josephine; Not allowed
OK
Person
Employee
Person is the parent class of Employee in this example.
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Character Graphics Example
Figure
Box Triangle
Instance variables:offsetMethods:setOffset getOffsetdrawAt drawHere
Instance variables:offset height widthMethods:setOffset getOffsetdrawAt drawHerereset drawHorizontalLinedrawSides drawOneLineOfSidesspaces
Instance variables:offset baseMethods:setOffset getOffsetdrawAt drawHerereset drawBasedrawTop spaces
InheritedOverridesStatic
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How do Programs KnowWhere to Go Next?
Programs normally execute in sequence Non-sequential execution occurs with:
» selection (if/if-else/switch) and repetition (while/do-while/for)(depending on the test it may not go in sequence)
» method calls, which jump to the location in memory that contains the method's instructions and returns to the calling program when the method is finished executing
One job of the compiler is to try to figure out the memory addresses for these jumps
The compiler cannot always know the address» sometimes it needs to be determined at run time
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Static and Dynamic Binding
Binding: determining the memory addresses for jumps Static: done at compile time
» also called offline Dynamic: done at run time Compilation is done offline
» it is a separate operation done before running a program
Binding done at compile time is, therefor, static, and Binding done at run time is dynamic
» also called late binding
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Example of Dynamic Binding: General Description
Derived classes call a method in their parent class which calls a method that is overridden (defined) in each of the derived classes» the parent class is compiled separately and before
the derived classes are even written» the compiler cannot possibly know which address
to use» therefore the address must be determined (bound)
at run time
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Dynamic Binding: Specific Example
Parent class: Figure» Defines methods: drawAt and drawHere» drawAt calls drawHere
Derived class: Box extends Figure» Inherits drawAt » redefines (overrides) drawHere» Calls drawAt
– uses the parent's drawAt method– which must call this, the derived class's, drawHere
method Figure is compiled before Box is even written, so the address
of drawHere(in the derived class Box) cannot be known then» it must be determined during run time, i.e. dynamically
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Polymorphism
Using the process of dynamic binding to allow different objects to use different method actions for the same method name
Originally overloading was considered to be polymorphism
Now the term usually refers to use of dynamic binding
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Summary
A derived inherits the instance variables & methods of the base class
A derived class can create additional instance variables and methods
The first thing a constructor in a derived class normally does is call a constructor in the base class
If a derived class redefines a method defined in the base class, the version in the derived class overrides that in the base class
Private instance variables and methods of a base class cannot be accessed directly in the derived class
If A is a derived class of class B, than A is both a member of both classes, A and B » the type of A is both A and B
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UML
Universal Modeling Language Used to facilitate OOAD A “standard” choice in many development efforts UML is one approach commonly incorporated into object
oriented modeling software, such as Rational Rose
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UML in ITM 352 (and 353)
We will use only very basic UML» Use-case diagrams» Object relationship diagrams» Basic class (object types) diagrams
– association, part-of, kind-of
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Example Use-case
add new robot starts contest
submits robot to contest gets contest results
1007 Student
Contest Admin.
1007 Instructor
The Robot Warz System
The Robot Warz Actors
monitors contestuses
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Example Use-case
starts contest
Contest Admin.
Contest Admin. Interface
set up contest environment
run contest
ContestDisplay
Behavior
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:RobotPart
:Robot :EnvironmentmyEnvironment
:Weapon
:Radar
:Engine
robotParts
:PlayPen
Example Class diagram
reference
sourceobject
relationship
destinationobject
part-ofrelationship
sub-classrelationship
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More Examples of Inheritance
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Bank Accounts Consider these three classes:
BankAccount
SavingsAccount
CheckingAccount
holds money
earns interest on money held
writes checks on money held
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public class BankAccount {
private double balance;
public BankAccount()
{ balance = 0; }
public BankAccount(double initialBalance)
{ balance = initialBalance; }
public void deposit(double amount)
{ balance = balance + amount; }
public void withdraw(double amount)
{ balance = balance - amount; }
public double getBalance()
{ return balance; }
public void transfer(BankAccount other, double amount)
{ withdraw(amount);
other.deposit(amount);
}
}
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Sub-types SavingsAccount and CheckingAccount
» Use all the behaviors of a BankAccount» Use all the attributes of a BankAccount» Add some new behaviors and attributes
This is because they are both types-of BankAccount » They are a “kind-of” BankAccount » A specialization or sub-type» We are intentionally making a distinction
To implement we could » Create two new classes and simply copy all the instance methods
and instance variables from BankAccount and add the new stuff» Add new behaviors and attributes to BankAccount and use
conditionals to determine when savings vs. checking
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Inheritance Both implementation approaches are somewhat in-elegant Java supports a more elegant approach, inheritance
» Use the extends keyword in the class definition
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Inheritance (cont.)
public class SavingsAccount extends BankAccount
private double interestRate;
{ public SavingsAccount(double rate)
{ interestRate = rate;
}
public void addInterest()
{ double interest = getBalance() * interestRate / 100;
deposit(interest);
}
}
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Terminology BankAccount is called the superclass or base class. SavingsAccount and CheckingAccount are subclasses or
derived classes.
We say SavingsAccount and CheckingAccount inherit from BankAccount , because they obtain the definitions of getBalance and deposit, etc.SavingsAccount
BankAccount
CheckingAccount
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What this meansSavingsAccount has five operations:deposit, withdraw, getBalance, transfer, addInterest
It is as if SavingsAccount were defined as
public class SavingsAccount {
private double balance;
private double interestRate;
public SavingsAccount(){ … }
public SavingsAccount(double initialBalance) { … }
public void deposit(double amount) { … }
public void withdraw(double amount) { … }
public double getBalance() { … }
public void transfer(BankAccount other, double amount) {…}
}
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What this means (cont.)
Similarly for CheckingAccount.
Variables can be declared of type BankAccount. SavingsAccount and CheckingAccount objects can be assigned to them.
BankAccount mySavingsAccount = new SavingsAccount(), myCheckingAccount = new CheckingAccount ();
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Casting and Converting
Both can use any methods and ivars from BankAccount
mySavingsAccount.deposit(100);myCheckingAccount.withdrawal(1000);
myCheckingAccount.transfer(mySavingsAccount, 100);
But notmySavingsAccount.addInterest();
You can re-cast if neededSavingsAccount aSavingsAccount
= (SavingsAccount) mySavingsAccount aSavingsAccount.addInterest();
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new conditional operator
Because you may declare a reference to one of its super classes (including Object), you may need to test if a reference is the type you desire. Java uses the instanceof operator for this purpose
if(myAccount instanceof SavingsAccount) myAccount.addInterest();
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Object Class Actually there is implicitly more inheritance
Everything is a Kind-of Object in Java» Any class that does not specify an extension extends Object
SavingsAccount
BankAccount
CheckingAccount
Object
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:RobotPart
:Robot :EnvironmentmyEnvironment
:Weapon
:Radar
:Engine
robotParts
:PlayPen
UML Inheritance Notation
sub-class
super-class
sub-classrelationship
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A Small Problem… Sometimes you will need to initialize a subclasses instance variables (in
a constructor) but not know (or care) how the superclass initializes the inherited ones.
Java provides a simple solution to this using the super operation.
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super public class SavingsAccount extends BankAccount
private double interestRate;
{ public SavingsAccount(double rate, double startingBal)
{ super(startingBal);
interestRate = rate;
}
public void addInterest()
{ double interest = getBalance() * interestRate / 100;
deposit(interest);
}
}
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Method redefinition in subclasses
A very important aspect of inheritance is that instance methods can be redefined instead of being inherited. Sometimes this is called overriding
You do this by simply using the exact same method name (including parameters and return type)
Real rule is: if B is a subclass of A then it inherits instance variables of A and instance methods of A, except those that it defines itself.
» Instance variables are not overridden – if B re-defines ivars then there would be two independent (local to each class) versions
Why would you do this?
» To modify the behavior of the superclass
» In general you try to limit overriding (elegance again)
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Overriding: Checking Account
A CheckingAccount may want to modify the behavior of the deposit, withdrawal, and transfer methods of BankAccount in order to keep track of the number of transactions (to asses a use fee).
The super keyword can be used to call a method of the superclass if the modifications are additions only.
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public class CheckingAccount extends BankAccount
{ public CheckingAccount(double initialBalance)
{ // construct superclass
super(initialBalance);
// initialize transaction count
transactionCount = 0;
}
public void deposit(double amount)
{ transactionCount++;
// now add amount to balance
super.deposit(amount);
}
public void withdraw(double amount)
{ transactionCount++;
// now subtract amount from balance
super.withdraw(amount);
}
public void deductFees()
{ if (transactionCount > FREE_TRANSACTIONS)
{ double fees = TRANSACTION_FEE *
(transactionCount - FREE_TRANSACTIONS);
super.withdraw(fees);
}
transactionCount = 0;
}
private int transactionCount;
private static final int FREE_TRANSACTIONS = 3;
private static final double TRANSACTION_FEE = 2.0;
}
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Inheritance in the Java API
Inheritance is used extensively in the Java API» The Applet class has definitions of init, paint, repaint,
etc. When you define an applet, you inherit those definitions and redefine the ones you choose to.
» Applet itself inherits from its superclass, Panel, which in turn inherits from Container, which inherits from Component.