Caching contents to avoid fetching information through slow I/O process is a popular method to improve the system efficiency. However, we have to consider how to refresh our cache data. For example, if we cache some data in the client side, when we send our http request to the web server, we would like to know if we need retrieve the data from the backend database. This requires us to understand if the cached content in the client side is the same as in the backend database. Usually, we use hashcode to do the comparison. Do some research of what is the hashcode and how the hashcode is generated from the content and answer the following questions: Can I use hash key as an id? Specify the reason. How to find the difference between two list of strings How to handle the duplication of the hash code? How to deal with a shared hash table being used on the web server