Inheritance

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Inheritance

• CSC 171 FALL 2004
• LECTURE 18

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READING

• Read Horstmann, Chapter 11

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Design Methodology

• Problem Definition
• Requirements Analysis
• Architecture
• Construction
• Testing
• Future Improvements

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Classes and Objects

• Object oriented programs
• Define classes of objects
• Make specific object out of class definitions
• Run by having the objects interact
• A class is a type of thing
• Instructor
• An object is a specific thing
• Ted
• An object is an instance of a class

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Hierarchies

• Humans have found that organizing concepts into
  hierarchies a useful method of organizing
  information

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HIERARCHIES

• Hierarchies are nested groupings.
• Examples of hierarchies
• Levels of organization in our bodies
• Organism Organ Systems Organs Tissues Cells
  Organelles
• Ecology
• Biome Community Population Organism
• Political boundaries
• USA New York State Monroe County City of
  Rochester
• Notice how each group is completely subordinate
  to any group on its left.

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Inheritance Hierarchies

• Object oriented languages, such as JAVA allows us
  to group classes into inheritance hierarchies.
• The most general classes are near the root
• superclasses
• The more specific classes are near the leaves
• Subclasses
• Subclasses inherit attributes from superclasses

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Example Banking Systems

• Consider a system that supports Savings and
  Checking accounts
• What are the similarities?
• What are the specifics

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Accounts

• Both savings and Checking accounts support the
  idea of
• Balance
• Deposit
• Withdraw
• Savings accounts pay interest checking accounts
  do not
• Checking accounts have transaction fees, savings
  accounts do not

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Super Sub classes

• More general concepts are in super classes
• More specific concepts are in sub classes
• Sub classes extend (inherit from) superclasses

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Why inheritance?

• The power of inheritance is that sub-classes
  inherit the capabilities of the super-classes
  they extend
• This reduces code redundancy

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Example Banking systems

• Bank accounts are a type of object
• Savings accounts are a type of bank account
• Checking Accounts are bank accounts
• public class BankAccount . . .
• public class SavingsAccount extends BankAccount
  . . .
• public class CheckingAccount extends BankAccount
  . . .

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BankAccount

• Instance variable balance
• Methods deposit and withdraw

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• public class BankAccount public void deposit
  (double amount) balance amount
  public void withdraw(double amount)
• if (amount lt balance)
• balance - amount
• private double balance

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SavingsAccount

• A bank account with an interest rate

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• public class SavingsAccount extends BankAccount
  public SavingsAccount(double rate)
  interestRate rate public void
  addInterest() double interest
  getBalance() interestRate / 100
  deposit(interest) private double
  interestRate

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Inheritance and Methods

• Override method Supply a different
  implementation of a method that exists in the
  superclass
• Inherit method Don't supply a new implementation
  of a method that exists in the superclass
• Add method Supply a new method that doesn't
  exist in the superclass

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Inheritance and Fields

• Inherit field All fields from the superclass are
  automatically inherited
• Add field Supply a new field that doesn't exist
  in the superclass
• Can't override fields

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Checking Account

• A Bank Account with transaction fees
• Need to keep tack of transactions

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Checking Account

• public class CheckingAccount extends BankAccount
  private int transactionCount
• public CheckingAccount()
  transactionCount 0

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• public void deposit(double amount)  
   transactionCount   super.deposit(amount)
• public void withdraw(double amount)  
   transactionCount   super.deposit(amount)

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Excercise

• Define a base class Employee
• Employees have names
• Define constructor

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• public class Employee
• private String name
• public Employee(String name)
• this.name name
• public String name() return name

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Excercise

• Define a sub class HourlyEmployee
• Names, hourly rates, and hours worked
• Define constructor, clockhours and getPay()

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• public class HourlyEmployee extends Employee
• private double wage, hoursworked
• public HourlyEmployee(String name, double wage)
• super(name)
• this.wage wage
• hoursworked 0

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• public void clockHours(double hours)
• hoursworked hours
• public double getPay()
• double pay hoursworked wage
• hoursworked 0
• return pay

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Excercise

• Define a sub-class SalaryEmployee
• Names and annual Salary
• Define constructor, and getPay() (montly)

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• public class SalaryEmployee extends Employee
• private double annualSalary
• public SalaryEmployee(String name, double salary)
  
• super(name)
• annualSalary salary

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• public double getPay()
• return annualSalary / 12

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TERMINOLOGY

• A base class is often called a parent class. A
  sub-class is then called a child class (or
  derived class)
• Parents of parents are called ancestor classes
• Children of children are called descendent classes

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Overriding vs. Overloading

• Overloading refers to having a different
  configuration of parameters for the same method
  name
• Overriding refers to the redefinition of a method
  in a subclass

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ACCESS ISSUES

• private instance variables or methods in a super
  (base,parent) class are not accessible in sub
  (derived, child) classes.
• So, private methods and variable are effectively
  not inherited.
• They are there but you have to use the
  super-class accessor and mutator methods.

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Protected

• protected methods and variables can be accessed
  inside derived classes, or in any class in the
  same package.

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Package Access

• If you don not place any of public, private, or
  protected before an instance variable or method
  definition, then it will have package access
  (also know as default access or friendly access).
  It will be visible to any class in the same
  package, but not outside.