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Introduction to Java
IB Computer Science II
Paul Bui
What does a computer look like?
Central
Processing
Unit
(CPU)
Memory
What is a computer program?
1010111101010101001100100000111
0101101011110101010100110010000
0111010101111010101010011001000
0011101010111101010101001100100
0001110101011110101010100110010
0000111010101111010101010011001
0000011101010111101010101001100
1000001110101011110101010100110
0100010010000011101010111101010
Assembly Language



Also known as machine code/language or
low-level language
Example:
ADD R1, R2, R3
SUB R1, R1, 1
MULT R3, R1, R2
Why can’t I run my PC programs on a
Mac?!?!?

Different from machine to machine
I Hate Tedious Stuff


Old school programs
Invention of the high-level language
ADD R1, R2, R3
SUB R1, R1, 1
MULT R4, R1, R2


D = B*(B+C-1)
Waaaay easier to understand
Compilers


Translates or converts a high-level
language (e.g., Java, C++) to a lowlevel language
Does other stuff too!




Checks syntax
Optimizations
and more!
One would say, “Yeah, so I compiled
my code to an executable or binary”
Before Java…
Applications or Programs
Operating System (Windows XP, Mac OSX, etc.)
Hardware (CPU + Memory + etc.)
How Java Works
Java Application or Java Program
Java Runtime Environment or Java Virtual Machine
Operating System (Windows XP, Mac OSX, etc.)
Hardware (CPU + Memory + etc.)
What does this mean?

Java bytecode (Java’s low-level
language)
Java sourcecode (high-level)  Java bytecode (low-level) 
assembly

Two different operating systems with
the JRE or JVM can run the same
Java program
Ok, so I want to program now!
//My first Java program!
class HelloWorld{
public static void main(String[] args) {
System.out.println("Hello World!");
}
}
How to Compile and Execute
1)
2)
3)
Open a text editor (gedit, tea, vi, etc.)
Code Code Code
Save file as CLASSNAME.java
Example: HelloWorld.java
4)
5)
6)
Open a terminal
javac HelloWorld.java
java HelloWorld