Issue #34 solution - hashcode for dominoes.

* Add a comment in the Domino class making explicit that
changes in the Domino class are not accepted by the solution. (Believe
me,
I was really surprised during my first submission that my changes in the
Domino class were rejected. As well as my mentee ).

* Change the way equals and hashcode are implemented to use the
  min+max value of each side instead of left+right.

* Added a constraint validation within the Domino class constructor,
that tiles must
  have a maximum number of 9 on each side (this is needed for the hash
implementation).

* In the equals code, a comparison between a Domino and an Object would
generate a
  class cast exception instead of the expected value, false.

exercises/practice/dominoes/.meta/src/reference/java/Domino.java

@@ -1,30 +1,41 @@
 import java.util.Objects;
 
+
+// This class can't be changed by the solution.
+//
 class Domino {
     private int left;
     private int right;
+    private int hash;
     Domino(int left, int right) {
+    if(left < 0 || left > 9 || right < 0 || right > 9 ) {
+        throw new IllegalArgumentException("Domino tiles must have a number between 0 and 9 on each side");
+
+    }
         this.left = left;
         this.right = right;
+    this.hash = Integer.min(left,right)+Integer.max(left,right)*10;
     }
-
+    
     int getLeft() {
         return this.left;
     }
-
+    
     int getRight() {
         return this.right;
     }
-
+    
     @Override
     public boolean equals(Object o) {
+    if (!o instanceof Domino ) {
+        return false;
+    }
         Domino otherDomino = (Domino) o;
-        return (this.getLeft() == otherDomino.getLeft() && this.getRight() == otherDomino.getRight()) ||
-            (this.getLeft() == otherDomino.getRight() && this.getRight() == otherDomino.getLeft());
+    return this.hash == otherDomino.hash;
     }
-
+    
     @Override
     public int hashCode() {
-        return Objects.hash(left, right);
+        return hash;
     }
 }

exercises/practice/dominoes/src/main/java/Domino.java

@@ -1,11 +1,20 @@
 import java.util.Objects;
 
+
+// This class can't be changed by the solution.
+//
 class Domino {
     private int left;
     private int right;
+    private int hash;
     Domino(int left, int right) {
+    if(left < 0 || left > 9 || right < 0 || right > 9 ) {
+        throw new IllegalArgumentException("Domino tiles must have a number between 0 and 9 on each side");
+
+    }
         this.left = left;
         this.right = right;
+    this.hash = Integer.min(left,right)+Integer.max(left,right)*10;
     }
 
     int getLeft() {
@@ -18,13 +27,15 @@ int getRight() {
 
     @Override
     public boolean equals(Object o) {
+    if (!o instanceof Domino ) {
+        return false;
+    }
         Domino otherDomino = (Domino) o;
-        return (this.getLeft() == otherDomino.getLeft() && this.getRight() == otherDomino.getRight()) ||
-            (this.getLeft() == otherDomino.getRight() && this.getRight() == otherDomino.getLeft());
+    return this.hash == otherDomino.hash;
     }
 
     @Override
     public int hashCode() {
-        return Objects.hash(left, right);
+        return hash;
     }
 }