Ivor Horton's Beginning Java™ 2, JDK™ 5th Edition

10.1. File I/O Basics

If you are new to programming file operations, there are a couple of things about how they work that may not be apparent to you and can be a source of confusion so I'll clarify these before I go any further. If you already know how input and output for disk files work, you can skip this section.

First, let's consider the nature of a file. Once you have written data to a file, what you have is just a linear sequence of bytes. The bytes in a file are referenced by their offset from the beginning, so the first byte is byte 0, the next byte is byte 1, the third byte is byte 2, and so on through to the end of the file. If there are n bytes in a file, the last byte will be at offset n−1. There is no specific information in the file about how the data originated or what it represents unless you explicitly put it there. Even if there is, you need to know that it's there and read and interpret the data accordingly.

For example, if you write a series of 25 binary values of type int to a file, it will contain 100 bytes. Nothing in the file will indicate that the data consists of 4-byte integers so there is nothing to prevent you from reading the data back as 50 Unicode characters or 10 long values followed by a string, or any other arbitrary collection of data items that corresponds to 100 bytes. Of course, the result is unlikely to be very meaningful unless you interpret the data in the form in which it was originally written. This implies that to read data from a ...