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.

Understanding input and output

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.

Simple Java Output: Writing to the Screen

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

Code section 9.1: Print "Hello world" without advancing to a new line System.out.print("Hello world");	Output on the screen Hello world
Code section 9.2: Print "Hello world" and advance to a new line System.out.println("Hello world");	Output on the screen Hello world

Simple Java Input: Inputting from the keyboard

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

Code section 9.3: Inputting with Scanner Scanner kbdIn = new Scanner(System.in); // Instantiating a new Scanner object System.out.print("Enter your name: "); // Printing out the prompt String name = 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

On the screen Enter your name: John Doe Welcome, John Doe

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

Code section 9.4: Line reader public String readLine() { // Creates a new BufferedReader object BufferedReader x = new BufferedReader(new InputStreamReader(System.in)); // Reads a line of input and returns it directly return x.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.