Scope of Local Variables

• Scope of variable: Region of program in which the variable can be accessed

• Scope of a local variable extends from its declaration to end of the block that encloses it

• Sometimes the same variable name is used in two methods:

public class RectangleTester
{
   public static double area(Rectangle rect)
   {
      double r = rect.getWidth() * rect.getHeight();
      return r;
   }
   public static void main(String[] args)
   {
      Rectangle r = new Rectangle(5, 10, 20, 30);
      double a = area(r);
      System.out.println(r);
   }
}

• These variables are independent from each other; their scopes are disjoint

• Scope of a local variable cannot contain the definition of another variable with the same name

Rectangle r = new Rectangle(5, 10, 20, 30);
if (x >= 0)
{
   double r = Math.sqrt(x);
   // Error - can't declare another variable called r
      here
   . . .
}

• However, can have local variables with identical names if scopes do not overlap

if (x >= 0)
{
   double r = Math.sqrt(x);
   . . .
   } // Scope of r ends here
else
{
   Rectangle r = new Rectangle(5, 10, 20, 30);
   // OK - it is legal to declare another r here
   . . .
}

Scope of Class Members

• Private members have class scope: You can access all members in any method of the class

• Must qualify public members outside scope
  Math.sqrt
  harrysChecking.getBalance

• Inside a method, no need to qualify fields or methods that belong to the same class

• An unqualified instance field or method name refers to the this parameter
  public class BankAccount
  {
     public void transfer(double amount, BankAccount other)
     {
        withdraw(amount); // i.e., this.withdraw(amount);      
        other.deposit(amount);
     }
     . . .
  }

Overlapping Scope

• A local variable can shadow a field with the same name

• Local scope wins over class scope
  public class Coin
  {
     . . .
     public double getExchangeValue(double exchangeRate)
     {
        double value; // Local variable
        . . .
        return value;
     }
     private String name;
     private double value; // Field with the same name
  }

• Access shadowed fields by qualifying them with the this reference
  value = this.value * exchangeRate;

Consider the deposit method of the BankAccount class. What is the scope of the variables amount and newBalance?

   Answer: The scope of amount is the entire deposit method. The
   scope of newBalance starts at the point at which the variable is
   defined and extends to the end of the method.

What is the scope of the balance field of the BankAccount class?

Answer: It starts at the beginning of the class and ends at the end of the class.