Basic IO

This section covers the Java platform classes used for basic input and output. But before we begin we need to have a concrete understanding of what input and output means in programming. To grasp this concept, think of the Java platform as a system.

The Java platform is an isolated entity, a space on your OS in a way, where everything outside this system is its environment. The interaction between the system and its environment is a two-way dialog of sorts. Either the system receives messages from its environment, or it conveys its messages to the same. When a message is received by the system, it is called an input , its opposite is an output . On a whole, this communication is termed input/output abbreviated as I/O.

The following chapters are designed to introduce basic input and output in Java, including reading text input from the keyboard, outputting text to the monitor, and reading/writing files from the file system. More advanced user interaction using Graphics and Graphical User Interface (GUI) programs is taken up in the later section on Swing.

There are two packages for I/O: the older java.io package (does not support symbolic links) and the newer java.nio ("new io") package that has improved exception handling at java.nio.file.

Writing to the screen is very easy, and can be accomplished using one of two methods:

System.out.print("Hello world");

Hello world

System.out.println("Hello world");

Hello world

As of version 5, Java provides a class in the java.util package called Scanner that simplifies keyboard input.

ScannerkbdIn=newScanner(System.in);// Instantiating a new Scanner object
System.out.print("Enter your name: ");// Printing out the prompt
Stringname=kbdIn.nextLine();// Reading a line of input (until the user hits enter) from the keyboard
// and putting it in a String variable called name
System.out.println("Welcome, "+name);// Printing out welcome, followed by the user's name

Enter your name: John Doe
Welcome, John Doe

Alternatively, one could write a method to handle keyboard input:

publicStringreadLine(){
// Creates a new BufferedReader object
BufferedReaderx=newBufferedReader(newInputStreamReader(System.in));
// Reads a line of input and returns it directly
returnx.readLine();
}

Note that the code above shouldn't be used in most applications, as it creates new Objects every time the method is run. A better alternative would be to create a separate class file to handle keyboard input.

• Book:Java Programming

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