Single inheritance has been widely accepted in the current programming practice to avoid the complication that incurred by multiple inheritance. Single inheritance enhances the reusability of codes and eliminates the confusion of identical methods that possibly defined in two super classes. However, the mechanism of inner class in Java potentially reintroduces the problems encountered by multiple inheritances. When the depth of Java inner class is increased, the problem becomes severe. This paper aims at exposing the problems of inheritance at the Java inner class. In addition, a measure is proposed to evaluate the potential problem of inheritance for Java inner class.