#vulnerable code 
                                public void setSuperclass(String superclass)
        throws CannotCompileException
    {
        if (constPool.getClassInfo(superClass).equals("java.lang.Object")) {
            if (superclass != null)
                try {
                    superClass = constPool.addClassInfo(superclass);
                    setSuperclass2(superclass);
                }
                catch (BadBytecode e) {
                    throw new CannotCompileException(e);
                }
        }
        else {
            if (superclass == null)
                superclass = "java.lang.Object";

            renameClass(constPool.getClassInfo(superClass), superclass);
        }
    } 
                            #location 4 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                                public void setSuperclass(String superclass)
        throws CannotCompileException
    {
        if (superclass == null)
            superclass = "java.lang.Object";

        try {
            superClass = constPool.addClassInfo(superclass);
            LinkedList list = methods;
            int n = list.size();
            for (int i = 0; i < n; ++i) {
                MethodInfo minfo = (MethodInfo)list.get(i);
                minfo.setSuperclass(superclass);
            }
        }
        catch (BadBytecode e) {
            throw new CannotCompileException(e);
        }
    }