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Access specifiers and Inheritance

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• Previous discussed: access specifiers
  • The "scoping" rule (where a variable is accessible) for instance variables is defined using

    Access specifiers

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  • Previously, we have learned about 3 different access specifiers: (See: click here )

    private: the member (variable or method) can only be accessed from within a (any) method inside the same class. (no specifier) (default): the member (variable or method) can be accessed from within any method of a class inside the same package. public: the member (variable or method) can be accessed from within any method inside any class.

    Question: how do these access specifiers apply to access by methods inside a derived class ???

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• Apply this fact: Base class and derived class and different classes !!!
  • Relationship: base class ⇔ derived class

    A derived class and its base class are considered to be different classes

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  • Consequently:

    private members (variables and methods) in the base class cannot be accessed from within any method inside derived class. public members (variables and methods) in the base class can be accessed from within any method inside derived class. (Because public members (variables and methods) can be accessed from within any method inside any class.) As for the default specifier, the access permission to these members depends on whether the base class and the derived class are inside the same package or not

    In other words:

    The access speficiers private, (no specifier) "default" and public have no special meaning to derived classes.

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• Example: accessing private member is forbidded in derived class
  • Problem description:

    Define a constructor method CheckingAccount() to initialize accNum, name and balance.

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  • Failed attempt #1:

    public class CheckingAccount extends BankAccount { // Inherits: // Variables: accNum, name, balance // Methods: deposit(amount), withdraw(amount) /* ---------------------------------------------- Constructor: CheckingAccount(a, n, amount) ---------------------------------------------- */ public CheckingAccount(int a, String n, double amount) { accNum = a; // ERROR !! - Cannot access private variables name = n; // in the base class BankAccount ! balance = 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"; } }

    The Java compiler will report the following errors:

    >> javac CheckingAccount.java CheckingAccount.java:12: accNum has private access in BankAccount accNum = a; // Cannot access private variables ^ CheckingAccount.java:13: name has private access in BankAccount name = n; // in the base class BankAccount ! ^ CheckingAccount.java:14: balance has private access in BankAccount balance = amount; ^ 3 errors

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• The protected access specifier
  • Fact:

    There is a special relationship between a base class and a derived class And there is an access specifier that grant access privilege to methods defined inside the derived class to variables inside the base class (discussed next).

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  • Fact:

    You can grant methods defined inside a derived class special access permission rights to variables and methods inside a base class

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  • The access specifier protected:

    A variable or method that is defined with the access specifier protected is accessible in: Any method inside the class where the protected variable is defined (= it's definition class) Any method inside the same package as the definition class Any method inside a derived class of the definition class

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  • Example: suppose the variables in BankAccount uses the protected access specifier:

    public class BankAccount { protected int accNum; // protected access protected String name; // -- accessible inside "BankAccount" protected double balance; // -- AND: inside any derived class ..... /* ============================================== deposit(amount): Add "amount" to balance ============================================== */ public void deposit( double amount ) { balance += amount; } ..... }

    Then we can access the instance variables inside the derived class CheckingAccount:

    public class CheckingAccount extends BankAccount { // Inherits: // Variables: accNum, name, balance // Methods: deposit(amount), withdraw(amount) /* ---------------------------------------------- Constructor: CheckingAccount(a, n, amount) ---------------------------------------------- */ public CheckingAccount(int a, String n, double amount) { accNum = a; // Cannot access private variables name = n; // in the base class BankAccount ! balance = 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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  • Test program that shows that the constructor CheckingAccount(a, n, amount) works:

    public class TestChecking { public static void main( String[] args ) { CheckingAccount acc1 = new CheckingAccount(123, "John", 1000.0); CheckingAccount acc2 = new CheckingAccount(789, "Mary", 5000.0); System.out.println( "acc1: " + acc1 ); // Uses toString() inherited System.out.println( "acc2: " + acc2 ); // from BankAccount } }

    Output:

    acc1: Account number: 123, Name: John, Balance: 1000.0 acc2: Account number: 789, Name: Mary, Balance: 5000.0

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

    How to run the program:

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