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…

Hello World: a closer look public class HelloWorld{ public static void main(String [ ] args){ //Prints Hello World System.out.println("Hello World"); } } All Java programs are inside a “class”. The name of the class must be EXACTLY the same as the name of the file! The class starts here And ends here. ALL code will be inside the class. The main function: the code in here is what happens when you “run” the program. 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.

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.

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

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 NOT public Class helloworld  System.out.println(“Hello World”); NOT system.out.Println(“Hello World”); Capitalization errors and other typos (missing semicolons) will result in compile-time errors (aka syntax errors )

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) } } Why, that’s a large “P” you have there. Unfinished quote SEMICOLON!!!

A Note on Readability…  Look at this beautiful program: public class VariableStuff { public static void main(String[] args) { //Haiku one System.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 two System.out.println("Words liberated"); System.out.println("escaping through the keyboard"); System.out.println("fleeing to the screen"); } }

…and the lack thereof  This actually compiles and provides the same output.  VERY bad style and indentation public class VariableStuff{public static void main(String[] args) {//Haiku one System.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 two System.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.