added rook, created bishop file

Piece.java

@@ -1,8 +1,7 @@
 import java.awt.*;
 import java.util.*;
 
-public class Piece {
-  ArrayList<Integer> legalMoves;
+public abstract class Piece {
   Position pos;
   Image sprite;
   String color;
@@ -17,10 +16,7 @@ public class Piece {
     g.drawImage(sprite, pos.x * 40, pos.y * 40, null);
   }
 
-  public ArrayList<Position> getLegalMoves(Board board) {
-    System.out.println("parent method not overriden");
-    return null;
-  }
+  public abstract ArrayList<Position> getLegalMoves(Board board);
 
   public boolean colorMatches(Piece p) {
     return this.color.equals(p.color);