How to solve the error: cannot find symbol class Set
<pre><code>import java.io.*;

public class Sudoku
{

    static Set empty = new Set(9);
    static Set fullSet;
    static int [][] board;
    static Set [][] allowedSets;
    ...
}
</code></pre>

<p>It's just a part of the code.</p>

<p>My problem is:
Sometimes it shows me cannot find symbol class Set and sometimes it shows me that it is an abstract class and can't be instantiated.</p>

<p>I want to create a board [9]x[9] </p>