robot-simulator: improve equals for GridPosition

We recently improved the implementation of equals for GridPosition so
that we could use assertEquals in the tests. However, we only changed
the reference implementation, not the starter implementation. This
changes the starter implementation to be the same as the reference
implementation.

exercises/robot-simulator/src/main/java/GridPosition.java

         this.y = y;
     }
 
-    /*
-     * This equals method is of deliberately narrow scope (only allows comparison with another GridPosition) to increase
-     * readability. In general, one should provide a full implementation of Object.equals(Object obj) and a
-     * corresponding implementation of Object.hashCode(). See
-     *
-     * https://docs.oracle.com/javase/7/docs/api/java/lang/Object.html#equals(java.lang.Object)
-     *
-     * and
-     *
-     * https://docs.oracle.com/javase/7/docs/api/java/lang/Object.html#hashCode()
-     *
-     * for more information.
-     */
-    boolean equals(final GridPosition gridPosition) {
-        return this.x == gridPosition.x && this.y == gridPosition.y;
+    @Override
+    public int hashCode() {
+        final int prime = 31;
+        int result = 1;
+        result = prime * result + x;
+        result = prime * result + y;
+        return result;
     }
 
+    @Override
+    public boolean equals(Object obj) {
+        if (this == obj) {
+            return true;
+        } else if (obj == null || getClass() != obj.getClass()) {
+            return false;
+        } else if (x != ((GridPosition) obj).x || y != ((GridPosition)obj).y) {
+            return false;
+        } else {
+            return true;
+        }
+    }
 }