Java start

Use in DCU (Win and Linux)

Java is already installed on your DCU machine.
To see version:
```
$ java -version
```
You can compile and run programs on the command-line.

Use in DCU (Win)

Command Prompt (Windows)
List of DOS commands
To run Java on the command-line, this must be in the PATH:
```
 C:\Program Files\Java\VERSION\bin 
```
In DCU this is done for you if you open the command-line through this batch file:

Start - Programs - Programming - JDK - Java shell

Use on your own machine

If you want to install on your own machine:

If you are on Windows:

Just run the downloaded EXE to install.
Add this to PATH:
```
 C:\Program Files\Java\VERSION\bin 
```

To change PATH on Win

PATH is a list of directories separated by ";"
To change PATH on Win:
1. Change system-level environment variable:
  My Computer - Properties - Advanced - Environment Variables
```
PATH    (directories);C:\Program Files\Java\VERSION\bin 
```
2. If this is restricted (as at DCU), change user-level environment variable:
```
PATH    C:\Program Files\Java\VERSION\bin 
```
  and this gets appended to system-level PATH.
3. If all else fails, change PATH at command-line:
```
path=%path%;C:\Program Files\Java\VERSION\bin 
```
4. You can put the previous line in a .BAT file called SETPATH.BAT. Then to set the path you just type SETPATH.

To write a Java program

To write a Java program:
1. edit prog.java (text file)
2. $ javac prog.java (makes prog.class, a platform independent class file)
3. $ java prog (runs prog.class)
Notes on distributing a Java program
- class file - each class is compiled into a separate .class file
- jar file - complete program with multiple classes, double-click to execute (if Java installed)
- exe file

What to write code in

Any text editor will do, but you may prefer an IDE.
Some of these installed at DCU. Some not.

Text editors (Win):
- Notepad
- Wordpad
- Notepad++
- EditPlus
- NoteTab
Text editors (Linux):
- Gedit
- Emacs
Java development environments (Win):
- Eclipse
- JCreator
Java development environments (Linux):
- Eclipse

"Hello World" program

 
public class Hello 
{
  public static void main ( String[] args ) 
  {
    System.out.println("Hello World");		// comment	

    for ( int i=0; i < args.length; i++ ) 
      { System.out.println ( args[i] ); }
  }
}

Get it working:
1. Save as Hello.java
2. file name = class name
3. case matters
4. the main() method accepts command-line arguments
5. run it as:
```
$ java Hello arg1 arg2
```
Notes:
1. case matters
2. System.out
3. println
4. command-line arguments come in as a single array of elements of type String
5. array is indexed args[0] .. args[n-1] where n = args.length
For meaning of "public" read here:
- Classes and Objects
  - Controlling Access to Members of a Class
- Object-Oriented Programming
  - Data Access Levels
For meaning of "static" read here:
- Understanding Instance and Class Members
Getting started
- "Hello World"

Java programs

Books

Java in a Nutshell, David Flanagan, 2005.
Library section - 005.13 - Programming languages