Lecture 1. Introduction to Programming and Java MIT- AITI 2003

What is a Computer Program? For a computer to be able to do anything (start up, play a song, run a word processor), it must first be given the instructions to do so. The instructions are written by humans in different computer languages like C, C++, Java, etc. The computer does not understand these languages. They must first be converted (compiled) to code that the machine can run ( )

How Are Programs Designed? Programs are written in a step by step instruction method which gets run every time you “run the program”.

For example, a program can be compared to a script for a play –A script is a set of instructions that calls for specific: Dialogue Acting directions Scenery –A program is also a set of instructions that might call for specific: Displays Commands –Additionally, a script may also produce a different looking play each time it is performed because of: Different actors Different theaters Different directors –Similarly, the same computer program can behave differently because of: Different user input Different software/hardware Different platforms

How Do Computers Interpret Code? Some programming languages are converted to machine code by a compiler and then can run anytime. However, because different platforms (PCs, Macs, Suns) interpret the same machine code differently, you must compile separately for each platform Programming Language compiler PC MAC Sun

Other programming languages only need to be compiled in one way and then an interpreter (software) on the machine converts it to the appropriate machine code. This is how Java works. Java Program compiler Java Bytecode PC MAC Sun Interpreter

Advantages: –Only need to compile once and then it works on every platform Disadvantages: –Software or drivers must be installed on the computers in order to run the programs

Welcome to Java The programming language you will learn in this course is called Java. A little background about Java: –It was developed in 1990 by Sun Microsystems engineer James Gosling for use in small appliances –Unexpectedly, its popularity grew because of its compatibility with the emerging World Wide Web.

Advantages of Java It is effective for programming on the web It is small and simple therefore making for faster programs It is secure for added protection against hacking It is portable; it can run on any machine without re-compiling

Programs in Java The most common Java programs are applications and applets. Applications are standalone programs. Applets are similar to applications, but they don't run standalone. Instead, applets adhere to a set of conventions that lets them run within a Java-compatible browser.

Writing Your First Application Open your text-editor and type the following piece of Java code exactly: class HelloWorld1 { public static void main(String[] args) { System.out.println("Hello World!"); } } Save this file as HelloWorld1.java (watch capitalization) in the following directory: C:\Java

Compiling and Running Your First Application Open the command prompt in Windows To run the program that you just wrote, type at the command prompt: cd C:\Java Your command prompt should now look like this: C:\Java> To compile the program that you wrote, you need to run the Java Development Tool Kit Compiler as follows: At the command prompt type: C:\Java> javac HelloWorld1.java You have now created your first compiled Java program named HelloWorld.class To run your first program, type the following at the command prompt: C:\Java>java HelloWorld1 Although the file name includes the.class extension, this part of the name must be left off when running the program with the Java interpreter.

You’ve created your first Java application!!

Writing Your First Applet Open your text-editor and type the following piece of Java code exactly: import java.applet.Applet; import java.awt.Graphics; public class HelloWorld2 extends Applet { public void paint(Graphics g) { g.drawString("Hello world!", 50, 25); } } Save this file as HelloWorld2.java (watch capitalization) in the following directory: C:\Java

Compiling Your First Applet Open the command prompt in Windows To run the program that you just wrote, type at the command prompt: cd C:\Java Your command prompt should now look like this: C:\Java> To compile the program that you wrote, you need to run the Java Development Tool Kit Compiler as follows: At the command prompt type: C:\Java> javac HelloWorld2.java

Running Your First Applet In order to run your first Java applet, you must first write an HTML page that the applet will run in Open your text-editor and type the following piece of HTML code exactly: Applet HTML Page Applet HTML Page

Save this file as HelloWorld2.html (watch capitalization) in the following directory: C:\Java Finally, to run your first applet, type the following at the command prompt: C:\Java> appletviewer HelloWorld2.html You will learn more about HTML in following lectures

You’ve created your first Java applet!!