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Java IO Tutorial

Jakob Jenkov
Last update: 2014-10-04

Java IO is an API that comes with Java which is targeted at reading and writing data (input and output). Most applications need to process some input and produce some output based on that input. For instance, read data from a file or over network, and write to a file or write a response back over the network.

The Java IO API is located in the Java IO package ( If you look at the Java IO classes in the package the vast amount of choices can be rather confusing. What is the purpose of all these classes? Which one should you choose for a given task? How do you create your own classes to plugin? etc. The purpose of this tutorial is to try to give you an overview of how all these classes are grouped, and the purpose behind them, so you don't have to wonder whether you chose the right class, or whether a class already exists for your purpose.

The Scope of the Java IO ( Package

The package doesn't actually address all types of input and output. For instance, input from and output to a GUI or web page is not covered in the Java IO package. Those types of input are covered elsewhere, for instance by the JFC classes in the Swing project, or the Servlet and HTTP packages in the Java Enterprise Edition.

The Java IO package is primarily focused on input and output to files, network streams, internal memory buffers etc. However, the Java IO package does not contain classes to open network sockets which are necessary for network communication. For that purpose you need to use the Java Networking API. Once you have opened a socket (network connection) though, you read and write data to and from it via Java IO's InputStream and OutputStream classes.

Java NIO - The Alternative IO API

Java also contains another IO API called Java NIO. It contains classes that does much of the same as the Java IO and Java Networking APIs, but Java NIO can work in non-blocking mode. Non-blocking IO can in some situations give a big performance boost over blocking IO.

More Java IO Tools, Tips etc.

The tutorial trail called Java How To's and Utilities also contain a few Java IO utilities - e.g. replacing strings in streams, iterating streams using buffers etc.

The Scope of this Java IO Tutorial

The tutorial starts by giving you a solid overview of how the Java IO APIs work, and how you are supposed to use them. After that the tutorial switches to covering the core classes in the Java IO API.

The coverage of the classes in this tutorial is not just an API listing. It's more than just a class listing (you can get that from Sun's official Java Doc's). Rather each text is a short introduction to the class, its purpose, and a few examples of how to use it. In other words, some of the stuff you don't find in Sun's official Java Doc's.

Java 5 to Java 8

The first version of this Java IO tutorial was written based on Java 5, but the classes work pretty much the same all the way up to Java 8 which is the latest version of Java at the time of writing.

Jakob Jenkov

Copyright  Jenkov Aps
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