The Way of the Java/Java IO

Input and Output in Java

System objects

System is the name of the built-in class that contains methods and objects used to get input from the keyboard, print text on the screen, and do file input/output (I/0).

System.out is the name of the object we have used to print text. When you invoke print and println, you invoke them on the object named System.out.

Interestingly, you can print System.out:

verbatim System.out.println (System.out); verbatim

The output is:

verbatim java.io.PrintStream@80cc0e5 verbatim

As usual, when Java prints an object, it prints the type of the object, which is PrintStream, the package in which that type is defined, java.io, and a unique identifier for the object. On my machine the identifier is 80cc0e5, but if you run the same code, you will probably get something different.

There is also an object named System.in that has type BufferedInputStream. System.in makes it possible to get input from the keyboard. Unfortunately, it does not make it easy to get input from the keyboard.

Keyboard input

First, you have to use System.in to create a new InputStreamReader.

verbatim

   InputStreamReader in = new InputStreamReader ("System.in");

verbatim

Then you use in to create a new BufferedReader:

verbatim

   BufferedReader keyboard = new BufferedReader (in);

verbatim

The point of all this manipulation is that there is a method you can invoke on a BufferedReader, called readLine, that gets input from the keyboard and converts it into a String. For example:

verbatim

   String s = keyboard.readLine ();
   System.out.println (s);

verbatim

reads a line from the keyboard and prints the result.

There is only one problem. There are things that can go wrong when you invoke readLine, and they might cause an IOException. There is a rule in Java that if a method might cause an exception, it should say so. The syntax looks like this:

verbatim
 public static void main (String[] args) throws IOException 

verbatim

This indicates that main might ``throw an IOException. You can think of throwing an exception as similar to throwing a tantrum.

File input

Reading input from a file is equally stupid. Here is an example:

verbatim

   public static void main (String[] args)
       throws FileNotFoundException, IOException 

       processFile ("/usr/dict/words");
   

   public static void processFile (String filename)
       throws FileNotFoundException, IOException 

       FileReader fileReader = new FileReader (filename);
       BufferedReader in = new BufferedReader (fileReader);

       while (true) 
           String s = in.readLine();
           if (s == null) break;
           System.out.println (s);
       
   

verbatim

This program reads each line of the named file (/usr/dict/words) into a String and then prints the line. Again, the declaration throws FileNotFoundException, IOException is required by the compiler. The object types FileReader and BufferedReader are part of the insanely complicated class hierarchy Java uses to do incredibly common, simple things. Other than that, you couldn't possibly care how this program works.