help please i'm quite new too java. Here's the code what's wrong with these?

the idea is that there is a singleton Calculator object, with a single ThreadLocal object used to hold the total.

Please help.
**
* Simple class that can be used to total numbers.
*
* The class is normally used within the following framework.
* <pre>
* Calculator c = Calculator.getCalculator() ;
* try {
* c.start() ;
*
* c.add( ... ) ; // call the add function as many times as necessary
*
* result = c.getTotal() ;
* } finally {
* c.stop() ;
* }
*</pre>
*
* This class is currently not thread safe since the member field
* total is common to all threads. Use the class ThreadLocal so that
* multiple threads can use the calculator simultaneously.
*
* Do not change any public signatures or interfaces otherwise
* the test application will break.
*/

public class Calculator {
private int total ;

private static ThreadLocal calculator = new ThreadLocal() ;

private Calculator() {
}

public Calculator getCalculator() {
return(Calculator)calculator.get() ;
}

public void start() {
total = 0 ;
}

public void add(int value) {
total += value ;
}

public int getTotal() {
return total;
}

public void stop() {
}
}