Question

requirement

Assume the availability of an existing class, ICalculator, that models an integer arithmetic calculator and contains:

• an instance variable currentValue that stores the current int value
  of the calculator

• methods add, sub, mul, and div

  Each method in ICalculator receives an int argument and applies its operation to currentValue and returns the new value of currentValue. So, if currentValue has the value 8 and sub(6) is invoked then currentValue ends up with the value 2, and 2 is returned.

So, you are to write the definition of a subclass, ICalculator1, based on ICalculator. The class ICalculator1 has one additional method, sign, that receives no arguments, and doesn’t modify currentValue. Instead, it simply returns 1, 0 or -1 depending on the whether currentValue is positive, zero, or negative respectively.

code:

1 public class ICalculator1 extends ICalculator {
2 public int getCurrentValue() {return currentValue;} //**did not need this line**
3 
4 int sign() {
5 if(currentValue > -1) return 0; //changed to**int val = add(0);**
6 else if(currentValue < 0) return -1; //changed to **if (val > 0) return 1;**
7 else return 1; //changed to **else if (val < 0) return -1;**
8 }}

error message:

ICalculator1.java:2: error: currentValue has private access in ICalculator
public int getCurrentValue() {return currentValue;}
                                     ^
ICalculator1.java:5: error: currentValue has private access in ICalculator
if(currentValue > -1) return 0;
   ^
ICalculator1.java:6: error: currentValue has private access in ICalculator
else if(currentValue < 0) return -1;
        ^
3 errors

Not sure what I’m doing wrong?

Answer 1

So it seams that currentValue is private and thus cannot be accessed directly inside your subclass. Since your methods actually return the value of currentValue, you can exploit this inside your subclass, like this for example :

public int sign() {
   int val = add(0);   // add 0 and return currentValue
   if (val > 0) return 1;
   else if (val < 0) return -1;
   else return 0;
}

As others have suggested, if you can modify the visibility of currentValue, make it protected and you won’t need that add(0) hack. However if you cannot modify ICalculator, this is one possibility.