18.1.java

public class Tabby extends Cat {
   public String meow() {
      return "meow meow";
   }
}

public class Siamese extends Cat {
   public String meow() {
      return "if you please";
   }
}

18.2.java

public class Siamese extends Cat {
   public String meow() {
      return "if you please";
   }
}

public class Square extends Shape {
   public Square(int squareSize) {
      super(squareSize, squareSize);
   }

   public int getArea() {
      return width * height;
   }
}

18.3.java

public class Secret implements Encrypt {
   private String[][] secretMessage = new String[4][3];
   private int originalLength;

   public Secret(String code) {
      char[] chars = code.toCharArray();
      String[] letters = new String[12];
      originalLength = chars.length;
      for (int i = 0; i < chars.length; i++) {
         letters[i] = Character.toString(chars[i]);
      }
      for (int filler = chars.length; filler < 12; filler++) {
         letters[filler] = ".";  
      }
      for (int row = 0; row < 4; row++) {
         for (int column = 0; column < 3; column++) {
            secretMessage[row][column] = letters[(row*3)+column];
         }
      }
   }

   public String encode() {
      String ret = "";
      for (int column = 0; column < 3; column++) {
         for (int row = 0; row < 4; row++) {
            ret += secretMessage[row][column];
         }
      }
      return ret;
   }

   public String decode() {
      String ret = "";
      int length = 0;
      for (int row = 0; row < 4; row++) {
         for (int column = 0; column < 3; column++) {
            ret += secretMessage[row][column];
            length++;
            if (length >= originalLength) break;
         }
      }
      return ret;
   }

   public String[][] getSecretMessage() {
      return secretMessage;
   }
}