Java start

• Java
• OO (Object Oriented) programming

• Java Reference

---

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:

1. Download Java Development Kit (JDK)
2. JDK install instructions

If you are on Windows:

1. Just run the downloaded EXE to install.
2. 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

---

Installed software at DCU

• Text editors (Win):
  • Programs - Accessories - Notepad
  • Programs - Accessories - Wordpad
  • EditPlus
  • NoteTab (not installed in DCU)

• Text editors (Linux):
  • gedit
  • Emacs
  • Bluefish

• Java development environments (Win):
  • Eclipse
  • JCreator (not installed in DCU)

• Java development environments (Linux):
  • Eclipse
  • BluJ

• CA Win software

---

"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