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Adding more instance variables and methods in a derived class

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• Building on the inherited variables and methods
  • Fact:

    The inherited variables and methods from the base class is insufficient to solve the problem (Otherwise, we can use the program code in the base class to solve the problem and be done !!!)

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  • Building on the inherited variables and methods:

    Using the extends clause, we can define a class that contains All instance variables of the base class All instance methods of the base class We can then build on this class by: Adding (defining) new (additional) instance variables Adding (defining) new (additional) instance methods Note: In this webpage, we will only consider the effect of adding variables and methods that has different names than the ones defined inside the base class. Defining methods that has the same names as the ones defined inside the base class has a shadowing effect that will be discussed in a later web page.

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• Example: building on the BankAccount class by adding a new method
  • Problem description:

    Write a CheckingAccount class that maintain bank account balance of customers. A CheckingAccount can perform the following operations: deposit(amount): deposits the amount into the balance of the checking account withdraw(amount): withdraws the amount from the balance of the checking account transfer( toAccount, amount): transfers the amount from the balance of the checking account into another checking account toAccount.

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  • The BankAccount class that was discussed previously (See: click here)

    public class BankAccount { private int accNum; // Private access private String name; // -- limit to ONLY inside private double balance; // the class "BankAccount" /* ==================================================== Constructor 1: initialize all 3 instance variables ==================================================== */ public BankAccount(int a, String n, double amount) { accNum = a; name = n; balance = amount; } /* ==================================================== Default Constructor ==================================================== */ public BankAccount( ) { accNum = -1; name = "Nobody"; balance = 0.0; } /* ============================================== deposit(amount): Add "amount" to balance ============================================== */ public void deposit( double amount ) { balance += amount; } /* ====================================================== withdraw(amount): Subtract "amount" from balance ====================================================== */ public void withdraw( double amount ) { if ( balance >= amount ) balance -= amount; // Subtract "amount" from balance } /* ============================================== toString(): return a String containing information of BankAccount ============================================== */ public String toString( ) { return( "Account number: " + accNum + ", Name: " + name + ", Balance: " + balance); } }

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  • Building the CheckingAccount class using the BankAccount class:

    Clearly, we can use the variables and methods inside the BankAccount class for the CheckingAccount class However: We lack the transfer() capability required in the CheckingAccount class Solution: We add a transfer() method to the CheckingAccount class

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  • Structure of the CheckingAccount class:

    public class CheckingAccount extends BankAccount { // Inherits: // Variables: accNum, name, balance // Methods: deposit(amount), withdraw(amount) /* --------------------------------------------- Add the transfer method to complete the functionality requirements ---------------------------------------------- */ public void transfer( BankAccount toAccount, double amount ) { deduct amount from this account; add amount to "toAccount"; } }

  • Implementation of the CheckingAccount class:

    public class CheckingAccount extends BankAccount { // Inherits: // Variables: accNum, name, balance // Methods: deposit(amount), withdraw(amount) /* ------------------------------------- transfer(toAccount, amount) ------------------------------------- */ public void transfer( BankAccount toAccount, double amount ) { this.withdraw(amount); // deduct amount from this account; toAccount.deposit(amount); // add amount to "toAccount"; } }

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  • Example: using the CheckingAccount

    public class TestChecking { public static void main( String[] args ) { CheckingAccount acc1 = new CheckingAccount(); CheckingAccount acc2 = new CheckingAccount(); acc1.deposit( 1000.0 ); // uses an inherited method acc2.deposit( 400.0 ); System.out.println( "acc1: " + acc1 ); // Uses toString() inherited from BankAccount System.out.println( "acc2: " + acc2 ); acc1.transfer( acc2, 100.0 ); // Uses added method in CheckingAccount ! System.out.println( "After acc1.transfer( acc2, 100.0 );"); System.out.println( "acc1: " + acc1 ); System.out.println( "acc2: " + acc2 ); } }

    Output:

    acc1: Account number: -1, Name: Nobody, Balance: 1000.0 acc2: Account number: -1, Name: Nobody, Balance: 400.0 After acc1.transfer( acc2, 100.0 ); acc1: Account number: -1, Name: Nobody, Balance: 900.0 acc2: Account number: -1, Name: Nobody, Balance: 500.0

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  • Example Program: (Demo above code)                                                
    • The BankAccount Prog file: click here
    • The CheckingAccount Prog file: click here
    • The TestChecking Prog file: click here

    How to run the program:

    Right click on link and save in a scratch directory To compile:   javac TestChecking.java To run:          java TestChecking

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• Summary
  • The derived class inherits the instance variables and methods from the base class

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  • We can add:

    more instance variables more instance methods

    to the derived class to taylor the program to our needs.

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  • The added instance variables and instance methods (for now) have different names as the ones used in the base class

    Note:

    If an added instance variable or instance method have same name as some in the base class then: The inherited instance variable or instance method with the identical name will be overshadowed (and they cannot be used without using some special syntax (super.Name)).

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• Preview: Overshadowing variables and methods inherited from the base class

  • Advanced note: defining variables and methods with the same name as ones in the base class

    When you define a variable inside the derived class that has the same name as one inside the base class, the variable in the base class is overshadowed It is not advisable to overshadow a variable in the base class. When you define a method inside the derived class that has the same name and parameter types as one inside the base class, the variable in the base class is also overshadowed It is quite common to overshadow an inherited method when the inherited method is unsuitable. The overshadowing of a inherited method is called: polymorphism.

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  • The topic polymorphism will be discuss in a later course (CS171).