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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def CreateJarBuilder(env):
 """The Jar builder expects a list of class files which it can package into a jar file. The jar tool provides an interface for passing other types of java files such as .java, directories or swig interfaces and will build them to class files in which it can package into the jar. """

    try:
        java_jar = env['BUILDERS']['JarFile']
    except KeyError:
        fs = SCons.Node.FS.get_default_fs()
        jar_com = SCons.Action.Action('$JARCOM', '$JARCOMSTR')
        java_jar = SCons.Builder.Builder(action = jar_com,
                                         suffix = '$JARSUFFIX',
                                         src_suffix = '$JAVACLASSSUFFIX',
                                         src_builder = 'JavaClassFile',
                                         source_factory = fs.Entry)
        env['BUILDERS']['JarFile'] = java_jar
    return java_jar