Chapter 2: Your First Program!

Chapter 2: Your First Program!

Hello World:Let’s Program • All programs must have the extension .java • Our first program will be named: HelloWorld.java • Let’s get started…

The name of the class must be EXACTLY the same as the name of the file! The class starts here A Comment:the “//” tells the computer to ignore this line. The only line that DOES anything!Prints “Hello World” and goes to the next line.This is a statement and must end with a semicolon. The main function:the code in here is what happens when you “run” the program. And ends here.ALL code will be inside the class. Hello World: a closer look All Java programs are inside a “class”. public class HelloWorld{ public static void main(String [ ] args){//Prints Hello World System.out.println("Hello World"); }}

That’s nice, but what did we just do? • HelloWorld.java is written in a high-level language : • Easy for you to read • Not so easy for the computer to read • We used a program called a compiler to break the program down into something more computer-friendly. • javac HelloWorld.java • Now we are able to run the program • java HelloWorld

Java:what’s the big deal? • In a normal programming language, the compiler translates your program into machine language (1’s and 0’s). • Unfortunately, different machines speak different languages. • With these languages, code will need to be recompiled to run on different machines.

Java:what’s the big deal? • The Java compiler changes your code into byte code. • byte code is like machine language for a hypothetical machine. • HelloWorld.class: byte code output from compiler. • The Java Virtual Machine (JVM) is our hypothetical machine. • acts as an interpreter between byte code and the computer • There is a JVM for each computer architecture • The JVM must be installed before your computer can run a java program.

HelloWorld.java HelloWorld.class Byte Code Source Code Compiler You JVM JVM Mr. PC Mr. Mac Let’s take a look

C++ vs. Java • C++: I just wrote this beautiful code, but… • You wanna run it on your machine, you better find your own compiler and compile it yourself. • Java: • Write once, Compile once, run anywhere • (Anywhere that has the JVM installed)

Why Does Java Hate Me? • Java is case sensitive! • public class HelloWorld NOTpublic Class helloworld • System.out.println(“Hello World”);NOTsystem.out.Println(“Hello World”); Capitalization errors and other typos (missing semicolons) will result in compile-time errors (aka syntax errors)

Unfinished quote SEMICOLON!!! Why, that’s a large “P” you have there. Hello World Gone Bad • Can you spot the 3 syntax errors? public class HelloWorld{ public static void main(String [ ] args){//Prints Hello World System.out.Println(“Hello World) }}

A Note on Readability… publicclassVariableStuff{ publicstaticvoidmain(String[]args){//Haiku oneSystem.out.println("Programmer's Hiaku"); System.out.println("an expression of the soul"); System.out.println("code flows from the heart"); System.out.println(); //Haiku twoSystem.out.println("Words liberated"); System.out.println("escaping through the keyboard"); System.out.println("fleeing to the screen"); }} • Look at this beautiful program:

…and the lack thereof • This actually compiles and provides the same output. • VERY bad style and indentation publicclassVariableStuff{publicstaticvoidmain(String[]args){//Haiku oneSystem.out.println("Programmer's Hiaku");System.out.println("an expression of the soul");System.out.println("code flows from the heart");System. out. println();//Haiku twoSystem.out.println("Words liberated");System.out. println("escaping through the keyboard");System.out.println("fleeing to the screen");}}

HelloWorld: the sequel • Now write a file called HelloWorld2.java. • This program should print out the following: • A general greeting to the world • An introduction (“my name is…”) • Some general advice (e.g. “trust no one”) • A farewell • Don’t forget to compile and execute the program.

Chapter 2: Your First Program!