POI supports the older OLE2 format Excel (XLS) files using HSSF, and using XSSF can also handle the newer OOXML format (XLSX) used by Microsoft Office 2007, 2008 and onwards.
If you want to open an Excel file in either format then you can use something like the following:
FileInputStream in = new FileInputStream(file);
Workbook workbook = WorkbookFactory.create(in);
The Apache POI project helps you read and write Microsoft OLE2 and Office Open XML (OOXML) documents.
OLE2 includes Excel (XLS), Word (DOC) and Powerpoint (PPT) files.
Office Open XML Format is the new standards based XML file format found in Microsoft Office 2007 and 2008. This includes XLSX, DOCX and PPTX files.
Use POIFS to read OLE2 files
HSSF is used to read and write Excel (XLS) files.
XSSF is used to access OOXML Excel files (XLSX).
The combined SS interface (SS = HSSF + XSSF) is designed to allow you to access all Excel files (XLS and XLSX).
We shall be posting some POI examples in the next few weeks, let us know if there is anything specific you need.