how to count no.of sheets in excel through POI jar as well as through Java?

I have an excel file, it contains 'n' no.of sheets(the no.of sheets will be dynamic ) .I want to read all sheets one by one in java by using POI jar. I am able to read one sheet at a time by using POI jar(workbook.getSheetAt(0)). But I am not able to read multiple sheets one by one. Could you please let me know the method to count the no.of sheets in excel through POI jar or else java.