Counter.java Identify DBC problem

/**
 *  Counter is a non-negative Integer with restricted operations for counting.
 *
 * @author J. Dalbey
 * @version 2014.2.27
 */
public class Counter
{
    // counter value
    private int count;
    // ending value (optional)
    private int target;

    /**
     * Default Constructor sets counter to zero.
     */
    public Counter()
    {
        count = 0;
    }

    /**
     * Constructor with a target end value.
     * @param end is the ending value
     * @pre end > 0
     */
    public Counter(int end)
    {
        this();
        target = end;
    }

    /**
     * Increment by one.
     */
    public void inc()
    {
        count = count + 1;
    }

    /**
     * Check if counter has not reached the target.
     * @return true if count not equals target, false otherwise
     * @throws InvalidStateException if count > target
     */
    public boolean notDone() throws InvalidStateException
    {
        if (count > target)
        {
            throw new InvalidStateException();
        }
        return count != target;
    }

    class InvalidStateException extends Exception {}
}