Introduction

Storage of data in variables and arrays is temporarythe data is lost when a local variable goes out of scope or when the program terminates. Computers use files for long-term retention of large amounts of data, even after the programs that created the data terminate. You use files every day for tasks such as writing an essay or creating a spreadsheet. We refer to data maintained in files as persistent data because it exists beyond the duration of program execution. Computers store files on secondary storage devices such as hard disks, optical disks and magnetic tapes. In this chapter, we explain how Java programs create, update and process data files.

File processing is one of the most important capabilities a language must have to support commercial applications, which typically store and process massive amounts of persistent data. In this chapter, we discuss Java's powerful file-processing and stream input/ output features. The term "stream" refers to ordered data that is read from or written to a file. We discuss streams in more detail in Section 14.3. File processing is a subset of Java's stream-processing capabilities, which enable a program to read and write data in memory, in files and over network connections. We have two goals in this chapterto introduce file-processing concepts (making the reader more comfortable with using files programmatically) and to provide the reader with sufficient stream-processing capabilities to support the networking features introduced in Chapter 24, Networking. Java provides substantial stream-processing capabilitiesfar more than we can cover in one chapter. We discuss three forms of file processing heretext-file processing, object serialization and random-access file processing.

We begin by discussing the hierarchy of data contained in files. We then cover Java's architecture for handling files programmatically by discussing several classes in package java.io. Next we explain that data can be stored in two different types of filestext files and binary filesand cover the differences between them. We demonstrate retrieving information about a file or directory using class File and then devote several sections to the different mechanisms for writing data to and reading data from files. First we demonstrate creating and manipulating sequential-access text files. Working with text files allows the reader to quickly and easily start manipulating files. As you will learn, however, it is difficult to read data from text files back into object form. Fortunately, many object-oriented languages (including Java) provide ways to write objects to and read objects from files (known as object serialization and deserialization). To demonstrate this, we recreate some of the sequential-access programs that used text files, this time by storing objects in binary files. We then consider random-access files, which enable us to directly and quickly access specific portions of a file. Understanding random-access files provides a foundation for understanding databases, discussed in Chapter 25, Accessing Databases with JDBC.