We'll tackle this on a component level. POI refers to the whole project.

So why should you use POIFS or HSSF?

You'd use POIFS if you had a document written in OLE 2 Compound Document Format, probably written using MFC, that you needed to read in Java. Alternatively, you'd use POI to write OLE 2 Compound Document Format if you needed to inter-operate with software running on the Windows platform. We are not just bragging when we say that POIFS is the most complete and correct port of this file format to date!

You'd use HSSF if you needed to read or write an XLS (Excel) file using Java. You can also read and modify spreadsheets using this API, although right now writing is more mature.

POI stands for Poor Obfuscation Implementation. Why would we name our project such a derogatory name? Well, Microsoft's OLE 2 Compound Document Format is a poorly conceived thing. It is essentially an archive structured much like the old DOS FAT filesystem. Redmond chose, instead of using tar, gzip, zip or arc, to invent their own archive format that does not provide any standard encryption or compression, is not very appendable and is prone to fragmentation.

Poi is also a Hawaiian delicacy that Merriam Webster's dictionary defines as: "A Hawaiian food of taro root cooked, pounded, and kneaded to a paste and often allowed to ferment." This seemed strangely descriptive of the file format.

So if you like acronyms, then POI is an acronym. If you hate them, then we just used the name of the food for our project. If you wish to signify your love or hate for acronyms, use POI or Poi to refer to the project respectively.

A common misconception is that POI writes Excel files. POI is the name of the project. POI contains several components, one of which, HSSF, writes Excel files. The following are components of the entire POI project and a brief summary of their purpose.

POIFS is the oldest and most stable part of the project. It is our port of the OLE 2 Compound Document Format to pure Java. It supports both read and write functionality. All of our components ultimately rely on it by definition. Please see the POIFS project page for more information.

HSSF is our port of the Microsoft Excel 97(-2002) file format (BIFF8) to pure Java. It supports read and write capability. Please see the HSSF project page for more information.

HDF is our port of the Microsoft Word 97 file format to pure Java. It supports read and write capability. Please see the HDF project page for more information. This component is in the early stages of design. Jump in!

HPSF is our port of the OLE 2 property set format to pure Java. Property sets are mostly use to store a document's properties (title, author, date of last modification etc.), but they can be used for application-specific purposes as well. Currently HPSF supports read functionality only. Please see the HPSF project page for more information.