Cannot reference external types in Java

Cannot reference external types in Java

I'm trying to use the special type QuickUnionPathCompressionUF (which is a
class in a file called algs4.jar), but the compiler complains. I'm using
DrJava as an IDE.
error: QuickUnionPathCompressionUF cannot be resolved to a type
My source file (part of which is shown below) is here:
~/Percolation/Percolation.java
and the .jar file is here:
~/algs4/algs4.java
I have an environment variable (CLASSPATH) set to
~/algs4/algs4.jar ~/algs4/stdlib.jar
Both of these files are listed under "Extra Classpath" in my IDE.
Here are the first few lines of the source file.
package Percolation;
//import algs4.QuickUnionPathCompressionUF;
public class Percolation {
private QuickUnionPathCompressionUF grid; // Data structure for
quick-union.
private int size; // Size of the grid.
private boolean[] open; // open sites
/* ...*/
}
Any suggestions?