Java Programming/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 dialogue of sorts. Either the system receives messages from its environment, or it conveys its messages to the same. When a message is received to 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:

• System.out.print("Hello world") will print "Hello world" (no quotes) to the screen without advancing to a new line.
• System.out.println("Hello world") will print "Hello world" (no quotes) to the screen and advance to a new line.

Simple Java Input

Inputting from the keyboard

As of version 1.5.0, Java provides a class in the java.util package called Scanner that simplifies keyboard input.

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

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

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

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.