When subclassing a JavaClass, provide helpful exception

rubicon/java/api.py

@@ -797,7 +797,11 @@ class JavaClass(type):
     # This class returns known JavaClass instances where possible.
     _class_cache = {}
 
-    def __new__(cls, descriptor):
+    def __new__(cls, descriptor, bases=None, attrs=None):
+        if bases or attrs:
+            # The user is trying to subclass a JavaClass. This will not work because
+            # rubicon-java relies on the Java reflection APIs, which do not support that.
+            raise NotImplementedError("Unable to subclass a JavaClass.") 
         # print ("Creating Java class", descriptor)
         # Using str descriptor as cache key, to avoid .encode() when checking cache.
         if descriptor in cls._class_cache:

tests/test_rubicon.py

@@ -23,6 +23,14 @@ def test_simple_object(self):
         # with self.assertRaises(Exception):
             # stack.pop()
 
+    def test_subclass_impossible(self):
+        def make_subclass():
+            Stack = JavaClass('java/util/Stack')
+            class ImpossibleStackSubclass(Stack):
+                pass
+
+        self.assertRaises(NotImplementedError, make_subclass)
+
     def test_field(self):
         "A field on an instance can be accessed and mutated"
         Example = JavaClass('org/beeware/rubicon/test/Example')