Java Packages

Jussi Pohjolainen

Tampere Polytechnic University of Applied Sciences » ICT

Java Packages: intro

• Why Java packages?
  • Types easier to find
  • Avoid naming conflicts
  • Control access
  • Bundle groups of related types into packages
• When project gets bigger, it is better to put classes in to different packages
• Packages: organize files into directories

Creating packages

• Put package - keyword with the package name in top of your source file:

• package mygraphics;
  
  public class Circle {
  }
  

• You have to save your source - file in corresponding directory. In this example, Circle.java is saved into directory called mygraphics.

Compiling and running

• Compile your source-code as usual
  • mygraphics> javac Circle.java
• When you try to run your .class the usual way, you get an error message:

  • 	
    mygraphics> java Circle
    Exception in thread "main" java.lang.NoClassDefFoundError: 
    Circle (wrong name: mygraphics/Circle) at 
    java.lang.ClassLoader.defineClass1(Native Method)
    

• The reason for this is that your file is in mygraphics package now.
• JVM needs full name of your class and now it is mygraphics.Circle

  • mygraphics> java mygraphics.Circle
    mygraphics> cd ..
    > java mygraphics.Circle
    

Naming packages

• Problem: programmer (from company A) defines package and class mygraphics.circle and another programmer (from company B) defines exactly the same name mygraphics.circle (implementation is different)
• Preventing problem, use convention.
• Companies use Internet domain names in packages.
• For example: com.companyname.package, fi.tamk.mygraphics, fi.uta.mygraphics
• Name collisions occuring inside a company can be handled within that company

Package members

• Import your package and use classes as usual
• Do not import classes and use long names:
  • mygraphics.Circle a = new mygraphics.Circle()
• Importing all the classes: import mygraphics.*;
• Importing one class: import mygraphics.Circle