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Shadowing instance variables (by local and parameter variables)

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• Previously discussed: shadowing a class variable ( click here )
  • We can reference a class variable defined inside the same class without using the class name Example:

    public class ClassVar2 { public static double a; // <----- Class variable public static void main(String[] args) { // Body of method "main" a = 3.1415; // We can omit the classname in this method System.out.println(a); } }

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  • Shadowing a class variable:

    When the method has a local variable or a parameter variable defined in a scope whose name is equal to the name of the class variable, then: The class variable with the same name can no longer accessible with the short hand notation in that scope !!!

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  • Example:

    public class ClassVar4 { public static double a = 3.1415; public static void main(String[] args) { boolean a = true; // Class var a now "shadowed" System.out.println( a ); // prints true System.out.println( ClassVar4.a ); // prints 3.1415 } }

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• Shadowing instance variables
  • Shadowing an instance variable:

    Recall that we can omit the implict parameter this to access an instance variable (This is the short hand notation for instance variables Shadowing can also occur with instance variable when a parameter variable or a local variable is defined inside an instance method that has the same name as an instance variable

    Example:

    public class BankAccount { public int accNum; // public String name; // public double balance; // Shadowed ! /* ============================================== convToString(): return a String containing information of BankAccount ============================================== */ public String convToString( ) { return( "Account number: " + accNum + ", Name: " + name + ", Balance: " + balance); } /* ===================================================== deposit(balance): Add "parameter balance" to balance ===================================================== */ public void deposit( double balance ) { balance += balance; // Add parameter balance to (this.)balance } /* ============================================================ withdraw(balance): Subtract "parameter balance" from balance ============================================================ */ public void withdraw( double balance ) { if ( balance >= balance ) balance -= balance; // Subtract parameter balance from (this.)balance } }

    Problem:

    The same variable name balance is used to refer to 2 different variables: The parameter variable balance The instance variable (this.)balance The Java variable referencing rule (see: click here ) will associate the variable name balance with the parameter variable balance.

    Note:

    The Java compiler will not report any error because the statement is syntactically correct

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• How to overcome the shadowing problem with instance variables
  • Overcome the shadowing problem with instance variables:

    Use the full name to access the instance variable: this.instanceVariableName

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  • Example:

    public class BankAccount { public int accNum; public String name; public double balance; // Shadowed ! /* ============================================== convToString(): return a String containing information of BankAccount ============================================== */ public String convToString( ) { return( "Account number: " + accNum + ", Name: " + name + ", Balance: " + balance); } /* ======================================================= deposit(balance): Add "parameter balance" to balance ======================================================= */ public void deposit( double balance ) { this.balance += balance; // Add parameter balance to this.balance } /* =============================================================== withdraw(balance): Subtract "parameter balance" from balance =============================================================== */ public void withdraw( double balance ) { if ( this.balance >= balance ) this.balance -= balance; // Subtract parameter balance from this.balance } }

    Output: (unchanged)

    stu1: Account number: 12345, Name: Mary, Balance: 1000.0 After depsoiting $450: Account number: 12345, Name: Mary, Balance: 1450.0 After withdraw $100: Account number: 12345, Name: Mary, Balance: 1350.0

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  • Example Program: (Demo above code)                                                
    • The BankAccount class with shadowed instance variabel file: click here
    • The Class06.java Prog file: click here

    How to run the program:

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