We propose method shells, which is a module system for avoiding conflicts on customization by language mechanisms such as aspects in AspectJ and open classes in Ruby. These mechanisms allow programmers to customize a library without rewriting original source code but by only describing differences in a separate file. We call these mechanisms destructive class extensions. A problem with destructive class extensions is conflicts on customization. Different customizations may differently modify the same class. To address this problem, we propose a new module system named method shells. With this system, programmers can avoid conflicts since the module system automatically switches a set of customizations that has to be applied together according to the contexts declared by programmers. We present the idea of this module system and then its formal semantics. We also present an extension of Java that supports method shells.