Computer Science (CSCI) Department College of Engineering, Computer Science, & Technology (ECST)

JETT Workshops Object-Oriented Programming Fall 2004

(Example code and figures from Computing Concepts with Java Essentials, 3rd edition, by Cay Horstmann.
Visit www.wiley.com/college/horstmann for information on the current edition of this excellent text.)

• inheritance - extend classes by adding or redefining methods, and adding instance variables
  • Example - a Savings account "is-a" bank account, with the addition of interest
    

    class SavingsAccount extends BankAccount { //new methods //new instance fields }





• the extended class is called the superclass - BankAccount in this example
• the extending class is called the subclass - SavingsAccount in this example
  
• all methods of BankAccount are automatically inherited
  • it is legal to call deposit, getBalance on a SavingsAccount object
• all instance variables of BankAccount are inherited too
  • although they are still private
  • SavingAccount methods must use BankAccount accessor and mutator methods
    
    
• a class diagram -
  • Unified Modeling Language (UML) - used to diagram OO designs
• the solid line and closed arrowhead indicate the "is-a" relationship
  

• the SavingsAccount class
  
  

  public class SavingsAccount extends BankAccount { public SavingsAccount(double rate) { interestRate = rate; } public void addInterest() { double interest = getBalance() * interestRate / 100; deposit(interest); } private double interestRate; }

• a SavingsAccount object

  SavingsAccount retirement = new SavingsAccount(10.0); retirement.deposit(10000.0);

• inheritance hierarchies

  • hierarchies of classes, subclasses, and sub-subclasses are common
  • example - the Swing library for graphical user interface components is an extensive hierarchy
  • the superclass JComponent has methods getWidth, getHeight
  • the AbstractButton class has methods to set/get button text and icon
    
    

• part of the hierarchy of Swing UI components

  

  
  

• inheritance and methods

  • override a method - supply a different implementation of a method that exists in the superclass
  • inherit a method - don't supply a new implementation of a method that exists in the superclass
  • add a method - supply a new method that doesn't exist in the superclass
    
    

• inheritance and instance variables

  • inherit variables - all variables from the superclass are automatically inherited
  • add a variable - supply a new variable that doesn't exist in the superclass
  • can't override variables
    
    

• a bank account hierarchy

  
  
  

• the CheckingAccount class

  • the first three transactions are free
  • there is a $2 charge for every additional transaction
  • must override deposit and withdraw to increment transaction count
    
  • a new method, deductFees, deducts accumulated fees and resets transaction count
    
    

• inherited private instance variables are still private
  

  • consider the deposit method of CheckingAccount

    public void deposit(double amount) { transactionCount++; // now we need to add amount to balance // but how? balance is private! ... }

    
    
  • you can't just add amount to balance
  • balance is a private instance variable of the superclass
  • the subclass must use the public interface of BankAccount
    
    

• invoking a superclass method
  

  • from the AP CS Java Subset
    • The use of super to invoke a superclass method is part of the AP Java subset.
      
      
  • we can't just call deposit(amount) in the deposit method of CheckingAccount
    
  • that is the same as this.deposit(amount)
  • it calls the same method we are currently executing (infinite recursion)
  • instead, invoke the superclass method using the Java reserved word super

    public void deposit(double amount) { transactionCount++; super.deposit(amount); // calls the deposit method in BankAccount }

    
    
    

• superclass construction

  • from the AP CS Java Subset
    • The use of super to invoke a superclass constructor is part of the AP Java subset.
      
      
  • allows you to pass parameters to superclass constructor
  • the call to the superclass constructor must be the first statement in the subclass constructor
    
    
  • example - superclass construction in CheckingAccount

    public class CheckingAccount extends BankAccount { public CheckingAccount(double initialBalance) { // construct superclass super(initialBalance); // initialize transaction count transactionCount =0; } ... }