add cast(classname, obj) method to be able to use an instance from another class def

jnius/__init__.py

@@ -1,4 +1,4 @@
 from .jnius import (JavaObject, JavaMethod, JavaStaticMethod,
         JavaField, JavaStaticField, JavaClass, MetaJavaClass,
-        JavaException)
+        JavaException, cast)
 

jnius/jnius.pyx

@@ -86,7 +86,7 @@ Python::
 '''
 
 __all__ = ('JavaObject', 'JavaClass', 'JavaMethod', 'JavaStaticMethod',
-    'JavaField', 'JavaStaticField', 'MetaJavaClass', 'JavaException')
+    'JavaField', 'JavaStaticField', 'MetaJavaClass', 'JavaException', 'cast')
 
 from libc.stdlib cimport malloc, free
 
@@ -895,3 +895,15 @@ cdef class JavaMethodMultiple(object):
         jm.j_self = self.j_self
         return jm.__call__(*args)
 
+
+def cast(destclass, obj):
+    cdef JavaClass jc
+    cdef JavaClass jobj = obj
+    from reflect import autoclass
+    if isinstance(destclass, basestring):
+        jc = autoclass(destclass)(noinstance=True)
+    else:
+        jc = destclass(noinstance=True)
+    jc.instanciate_from(jobj.j_self)
+    return jc
+

jnius/reflect.py

@@ -1,3 +1,5 @@
+__all__ = ('autoclass', 'ensureclass')
+
 from jnius import JavaClass, MetaJavaClass, JavaMethod, JavaStaticMethod, \
         JavaField, JavaStaticField, JavaMethodMultiple
 
@@ -169,4 +171,3 @@ def autoclass(clsname):
             clsname,#.replace('.', '_'),
             (JavaClass, ),
             classDict)
-

tests/test_cast.py

@@ -0,0 +1,13 @@
+import unittest
+from jnius.reflect import autoclass
+from jnius import cast
+
+class MultipleSignature(unittest.TestCase):
+
+    def test_multiple_constructors(self):
+        String = autoclass('java.lang.String')
+        s = String('hello world')
+        self.assertEquals(s.__javaclass__, 'java/lang/String')
+        o = cast('java.lang.Object', s)
+        self.assertEquals(o.__javaclass__, 'java/lang/Object')
+