title: How to solve the error: cannot find symbol class Set
body: <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>