{
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    "# Nullable types\n",
    "\n",
    "Many object-oriented languages follow the *ALGOL W* example and allow *null references*. Tony Hoare invented the concept in 1965 and since then has called it his \"[my] billion dollar mistake\".\n",
    "\n",
    "The follwoing example is destilled from assignment solutions for the lecture *Computergrafik Grundlagen*. \n",
    "\n",
    "The `Shape` interface states that a method `intersect()` exists that takes an object of type `Ray` as an argument and returns an object of type `Hit`."
   ]
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  {
   "cell_type": "code",
   "execution_count": 1,
   "metadata": {},
   "outputs": [],
   "source": [
    "class Hit {double t;}\n",
    "class Ray {}\n",
    "\n",
    "interface Shape {\n",
    "    Hit intersect(Ray r);\n",
    "}"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Concrete instances of `Shape` implement the intersection method differently. For example the `Sphere` class implements it like this:\n",
    "\n",
    "```java\n",
    "class Sphere implements Shape{\n",
    "    public Hit intersect(Ray r) {\n",
    "        if (/* some involved test */)\n",
    "            return new Hit(/* some value for t */);\n",
    "        else\n",
    "            return null;\n",
    "    }\n",
    "}\n",
    "```\n",
    "\n",
    "Client code of the `Shape` interface has to be prepared, that instead of a valid `Hit` object, `null` is returned to signal the absence of a hit. The interface however, does not explicitly state that this might happen.\n",
    "\n",
    "A `NullPointerException` is arguably the most frequently encountered exception in any Java program."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 2,
   "metadata": {},
   "outputs": [],
   "source": [
    "class Sphere implements Shape{\n",
    "    public Hit intersect(Ray r) {\n",
    "        return null;\n",
    "    }\n",
    "}"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 3,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "No hit\n"
     ]
    }
   ],
   "source": [
    "Ray ray = new Ray();\n",
    "Sphere sphere = new Sphere();\n",
    "\n",
    "Hit hit = sphere.intersect(ray);\n",
    "\n",
    "if (hit != null)\n",
    "    System.out.println(hit.t);\n",
    "else\n",
    "    System.out.println(\"No hit\");"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "A better solution is to use the `Optional` type that explicitly says *there might be no value* and forces the programmer to check before using a value."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 4,
   "metadata": {},
   "outputs": [],
   "source": [
    "interface ShapeO {\n",
    "    Optional<Hit> intersect(Ray r);\n",
    "}"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 10,
   "metadata": {},
   "outputs": [],
   "source": [
    "class SphereO implements ShapeO {\n",
    "    public Optional<Hit> intersect(Ray r) {\n",
    "        return Optional.empty();\n",
    "    }\n",
    "}"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 7,
   "metadata": {},
   "outputs": [
    {
     "ename": "UnresolvedReferenceException",
     "evalue": "Attempt to use definition snippet with unresolved references in Snippet:ClassKey(SphereO)#21-class SphereO implements ShapeO {\n    public Optional<Hit> intersect(Ray r) {\n        return new Optional<Hit>();\n    }\n}",
     "output_type": "error",
     "traceback": []
    }
   ],
   "source": [
    "Ray ray = new Ray();\n",
    "SphereO sphere = new SphereO();\n",
    "\n",
    "Optional<Hit> hit = sphere.intersect(ray);\n",
    "\n",
    "if (hit != null)\n",
    "    System.out.println(hit.t);\n",
    "else\n",
    "    System.out.println(\"No hit\");"
   ]
  },
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   "metadata": {},
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