/* This class is thread safe */

public class Range {
    private int lower; // guarded by this
    private int upper; // guarded by this
    // INVARIANT: lower <= upper
    private boolean invariantOK(int l, int u) {
	return l <= u;
    }
    public Range(int lower, int upper) {
	if (!invariantOK(lower,upper)) 
	    throw new IllegalArgumentException();
	this.lower = lower;
	this.upper = upper;
    }
    public synchronized boolean inRange(int v) {
	if (v >= lower && v <= upper) return true;
	return false;
    }
    public synchronized void incUpper() throws IllegalArgumentException {
	upper ++;
    }
    public synchronized void incLower() {
	if (!invariantOK(lower+1,upper)) 
	    throw new IllegalArgumentException();
	lower ++;
    }
    public synchronized void decLower() {
	lower-- ;
    }
    public synchronized void decUpper() throws IllegalArgumentException {
	if (!invariantOK(lower,upper-1)) 
	    throw new IllegalArgumentException();
	upper --;
    }
}