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Java
• Java is a programming language developed by
Sun Microsystems in 1995.
• Java is one of the first languages to be platform
independent.
– Java is compiled to byte-code that runs on a Java
interpreter
Java VM
• A Java program never executes directly
(i.e., natively) on a machine; instead the
Java interpreter reads the byte code and
executes the corresponding native
machine instructions.
Java API
• To run Java programs on a computer, all
that is needed is the interpreter (java
virtual machine) and some library routines
(the API)
The Java API
• The Java API is the code that comes with
the java interpreter (a.k.a. the java virtual
machine) when you download java.
• The java API is a collection of Java
classes---take a look at the API for Java
SE 6
• See if you can find the String class.
A Simple Java Program
public class HelloWorld {
public static void main(String[] args) {
System.out.println("Hello World!");
}
}
• This program prints Hello World! to the monitor
• Things to notice:
– All programs are classes
– All programs have a method called main() that has
one array of Strings parameter.
Running Java outside of
Processing
Follow these steps to run HelloWorld
1. Download HelloWorld.java and save it to your
Desktop.
2. Open a terminal window.
3. In the terminal window, use the command cd to
change to the Desktop directory where
HelloWorld.java is stored.
4. In the terminal window, type javac HelloWorld.java
to compile your program to byte code
5. In the terminal window, type java HelloWorld to run
your compiled program on the java virtual machine.
In-class exercise
• Run the following program and explain
what happens; how is the output
generated?
• The program requires 3 class: Pets.java,
Cat.java, and Dog.java
• These classes are shown on the following
slides.
Cat Class
public class Cat
{
public void hiss ()
{
System.out.println ("Hiss!");
}
public void scratch (Dog victum)
{
System.out.println ("I'm scratching the dog");
victum.growl ();
}
public void bite (Dog sillyDog)
{
System.out.println ("I'm bitting the dog");
sillyDog.yelp ();
scratch (sillyDog);
}
}
Dog Class
public class Dog
{
public void bark ()
{
System.out.println ("Arf!");
System.out.println ("Arf!");
}
public void growl ()
{
System.out.println ("Grrrr!");
}
public void yelp ()
{
System.out.println ("Awooo!");
}
public void beenBittenBy (Cat sillyCat)
{
System.out.println
("I've been bitten by a cat with a mean hiss:");
sillyCat.hiss ();
}
}
Pets Class
public class Pets
{
/* Creates a Cat and a Dog
*/
public static void main (String [] args)
{
Cat tom = new Cat ();
// create Cat object
Dog spike = new Dog ();
// create Dog object
// demonstrate Cat behavior
tom.bite (spike);
System.out.println ();
// Skip a line of output
// demonstrate Dog behavior
spike.beenBittenBy (tom);
}
}