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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.

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hello world let s 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

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
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
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 deal1
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.
let s take a look

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++ 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
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)

hello world gone bad

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
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
…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
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.
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