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Java, A Beginner’s Guide, 5th Edition by Herbert Schildt

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The Predefined Streams

As you know, all Java programs automatically import the java.lang package. This package defines a class called System, which encapsulates several aspects of the run-time environment. Among other things, it contains three predefined stream variables, called in, out, and err. These fields are declared as public, final, and static within System. This means that they can be used by any other part of your program and without reference to a specific System object.

System.out refers to the standard output stream. By default, this is the console. System.in refers to standard input, which is by default the keyboard. System.err refers to the standard error stream, which is also the console by default. However, these streams can be ...

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