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Classes with private variables --- Abstract Data Types (ADT)

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• Security/correctness considerations in computer programming
  • Consider the BankAccount class:

    public class BankAccount { public int accNum; // These variables are used to public String name; // store information on public double balance; // "BankAccounts" /* ============================================== convToString(): return a String containing information of BankAccount ============================================== */ public String convToString( ) { return( "Account number: " + accNum + ", Name: " + name + ", Balance: " + balance); } /* ============================================== 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 } }

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  • 2 different ways to access instance variables in a BankAccount object:

    Access an instance variable directly: BankAccount stu1 = new BankAccount(); st1.balance += 1000; Access an instance variable indirectly using a method: BankAccount stu1 = new BankAccount(); st1.deposit(1000) ;

    Complete Java program:

    public class Class07 { public static void main(String[] args) { BankAccount stu1; stu1 = new BankAccount(); // Create one BankAccount object with // 3 data fields: // stu1.studID, stu1.name, stu1.major stu1.accNum = 12345; stu1.name = "Mary"; stu1.balance = 1000.0; String x; x = stu1.convToString( ); System.out.println("stu1: " + x); // **** Indirect access to stu1.balance: stu1.deposit( 450 ); x = stu1.convToString( ); System.out.println("After depsoiting $450: " + x); // **** Directly accessing variable stu1.balance: stu1.balance = stu1.balance + 450; x = stu1.convToString( ); System.out.println("After depositing another $450: " + x); } }

    (No demo program available)

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  • Information corruption caveat caused by direct access to instance variables:

    Fact: Different instance variables store related information Example: accNum stores the account number information name stores the name information Relationships between different instance variables: There can be a complex relationship between 2 or more different instance variables If there exist complex relationships between different instance variables, we must maintain the relationship correctly (Or else, the information stored in the instance variables may become inconsistent) Inconstant information = useless lies !!!! Information corruption caveat: Granting unrestricted access to the instance variables can result in incorrect changes and can give result in data inconsistency

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• Access specifiers
  • Access specifiers:

    Access specifier = specifies where a variable/method can be accessed (Each (instance and class) variable and method defined inside a class has an access specifier associated with it.)

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  • Access specifiers avialable in Java:

    public (this is the only access specifier used so far) private protected default (no specifier given)

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  • Meaning of Java's access specifiers:

    public = there are no access restrictions to a variable or method public methods and variables can be accessed from everywhere. private = access is restricted to the same class private methods and variables can only be accessed from within the same class to which the methods and variables belong. protected ..... will be covered later (Chapter 15) (Requires knowledge of subclasses) default ..... will be covered later (Chapter 14) (Requires knowledge of packages)

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• Limiting the accessibility of instance variables using private
  • Suppose we change the access speficiers for the all instance variables in BankAccount to private:

    public class BankAccount { private int accNum; // Private access private String name; // -- limit to ONLY inside private double balance; // the class "BankAccount" /* ============================================== convToString(): return a String containing information of BankAccount ============================================== */ public String convToString( ) { return( "Account number: " + accNum + ", Name: " + name + ", Balance: " + balance); } /* ============================================== 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 } }

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  • Consequence:

    The variables names accNum, name and balance can be accessed by statements written inside the BankAccount class The variables names accNum, name and balance can not be accessed by statements written inside other classes The user is forced to use public access methods provided inside the BankAccount class to access private access variables:

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  • Example:

    public class Class06 { public static void main(String[] args) { BankAccount stu1; stu1 = new BankAccount(); // Create one BankAccount object with // 3 PRIVATE data fields: stu1.accNum = 12345; // Now these statements will be in error. stu1.name = "Mary"; // error... stu1.balance = 1000.0; // error... String x; x = stu1.convToString( ); System.out.println("stu1: " + x); stu1.deposit( 450 ); // ****** NO error ****** // because deposit() is public ! x = stu1.convToString( ); System.out.println("After depsoiting $450: " + x); stu1.balance = stu1.balance + 450; // error.... x = stu1.convToString( ); System.out.println("After withdraw $100: " + x); } }

    Compiler messages:

    > javac Class06.java Class06.java:10: accNum has private access in BankAccount stu1.accNum = 12345; // Now these statements will be in error. ^ Class06.java:11: name has private access in BankAccount stu1.name = "Mary"; // error... ^ Class06.java:12: balance has private access in BankAccount stu1.balance = 1000.0; // error... ^ Class06.java:25: balance has private access in BankAccount stu1.balance = stu1.balance + 450; // error.... ^ Class06.java:25: balance has private access in BankAccount stu1.balance = stu1.balance + 450; // error.... ^ 5 errors

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  • Example Program: (Demo above code)                                                
    • The BankAccount class with private instance variables: click here
    • The Class06.java test prog file: click here

    How to compile the program:

    Right click on links and save in a scratch directory To compile:   javac Class06.java