(c) Copyright 2000, 2001, 2002, 2004 by Daniel Berleant 

   Review: a First Java Program

import java.lang.*;
class HelloWorld
   {
   public static void main

     (String args[])
     {
     System.out.println

        ("Hello World!");
     }
   }

(Note the I/O statement in it)

New Topic: I/O 

I/O can involve console, file, or URL

    ("console" - screen and keyboard)

I/O with screen and keyboard

    Similar to I/O from a file, but simpler

Data "comes from" a keyboard or file

Data "goes to" a screen or file

Keyboard and screen are automatically opened for I/O

...so, no need for open statement

(Other files must be explicitly opened)

Keyboard, screen, files (and internet http connections for I/O with URLs) are all streams

             Stream: an ordered sequence of bytes, characters, or other data

             (Analogy: a stream of data flowing like the water in a stream of water)

The keyboard stream is automatically declared as stream System.in

The screen stream is automatically declared as stream System.out

System is a class, 

    out is a "member" of the class,

    out itself contains stuff

    ...it has a method ("function") called println(...)

    Hence: System.out.println("Hello!")

    To get to a member of a class, use the dot operator "."

className.classMember gets you to the member

         objectName.objectMember gets you to the member

       (C++ uses "::" for one case and "." for the other)

System.out contains various methods 

        print(), println(), etc.

C: a "bunch of statements" is called a function

Java: a "bunch of statements" is called a method

To get to the println() method associated with stream System.out

... use the dot (".") operator!

Conclusion: to print, call System.out.println()

Printing to the Screen: More Details

System.out.println

    ("Learn Java for fun and profit");

takes a string argument

But System.out.println(5)works too!

Is this a good quality of Java?

How can it do this?

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Answer: There are several different

   println() methods

The computer can determine which to use

        How do you suppose the computer does this?