Chapter 2 your first program
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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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Chapter 2: Your First Program!

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Chapter 2 your first program

Chapter 2: Your First Program!


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 WorldSystem.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 WorldSystem.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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