J.10. Introduction to Files and Streams

Data stored in variables and arrays is temporary—it’s lost when a local variable goes out of scope or when the program terminates. For long-term retention of data, even after the programs that create the data terminate, computers use files. You use files every day for tasks such as writing a document or creating a spreadsheet. Data maintained in files is persistent data—it exists beyond the duration of program execution.

Files as Streams of Bytes

Java views each file as a sequential stream of bytes (Fig. J.10). Every operating system provides a mechanism to determine the end of a file, such as an end-of-file marker or a count of the total bytes in the file that’s recorded in a system-maintained administrative data structure. A Java program processing a stream of bytes simply receives an indication from the operating system when it reaches the end of the stream—the program does not need to know how the underlying platform represents files or streams. In some cases, the end-of-file indication occurs as an exception. In other cases, the indication is a return value from a method invoked on a stream-processing object.