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Trail: Essential Java Classes

Lesson: I/O

Often a program needs to bring in information from an external source or to send out information to an external destination. The information can be anywhere: in a file, on disk, somewhere on the network, in memory, or in another program. Also, the information can be of any type: objects, characters, images, or sounds. This chapter covers the Java platform classes that your programs can use to read and to write data.

Overview of I/O Streams

This section describes each type of stream and shows the classes in that implement them according to the division in the class hierarchy.

Using the Streams

This section has several "how-to" pages to show you how to use selected streams based on their purpose:

Scanning and Formatting

Thi section describes reading formatted input and writing formatted output.

Object Serialization

Two of the byte streams, ObjectInputStream and ObjectOutputStream, are specialized streams that let you read and write objects. Reading and writing objects is a process known as object serialization. Object serialization has many uses, including remote method invocation (RMI). In addition to the object streams, has other classes and interfaces that define the API to help classes perform serialization for its instances.

Working with Random Access Files

The character and byte streams are all sequential access streams. In contrast, RandomAccessFile lets you randomly access the contents of a file. This section talks about how to use random access files. It also provides a special section that shows you how to write filters for objects that implement the DataInput and DataOutput interfaces. Filters implemented in this fashion are more flexible than regular filter streams because they can be used on random access files and on some sequential files.

The New I/O Packages

An overview of the New I/O packages, java.nio.*.


A summary of the key points covered in this trail.

Questions and Exercises

Test what you've learned in this trail by trying these questions and exercises.

The I/O Classes in Action

Many of the examples in the next trail, Custom Networking (in the Essential Java Classes trail) use the I/O streams described in this lesson to read from and write to URLs, URLConnections, and Sockets.

Security consideration:  Some I/O operations are subject to approval by the current security manager. The example programs contained in these lessons are standalone applications, which by default have no security manager. This code might not work in an applet depending on the in which browser or viewer it is running. See Security Restrictions (in the Essential Java Classes trail) for information about the security restrictions placed on applets.

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