Lecture Set 17 Packages 1 Packages CMSC 131 Fall 2007 Jan Plane adapted from Bonnie Dorr Java Program Organization Program Organization Java program is composed of 1 or more Java source files Source file can have 1 or more class and or interface declarations In our projects we have implemented one class interface per file Public Class Interface If a class interface is declared public the source file must use the same name Only one public class interface is allowed per source file Can you have non public classes We will discuss this later Packages When a program is very large its classes can be further organized hierarchically into packages CMSC 131 Fall 2007 Jan Plane adapted by Bonnie Dorr 1 Packages Package a collection of related classes and or interfaces Examples The Java API javax swing classes dealing with the development of GUIs java lang essential classes required by the Java language java text facilities for formatting text output java util classes for storing accessing collections of objects java net for network communication Hierarchical Packages can be divided into subpackages java awt classes for basic GUI elements and graphics java awt font classes and interface relating to fonts java awt geom classes for defining 2 dimensional objects There is no limit to the nesting depth CMSC 131 Fall 2007 Jan Plane adapted by Bonnie Dorr 2 1 Access to Package Members Review of Package Basics Accessing Package Members Fully qualified name E g javax swing JOptionPane Importing a single class import javax swing JOptionPane JOptionPane showMessageDialog Importing all the classes import javax swing JOptionPane showMessageDialog Import semantics import does not insert the Java files as C C do with include files Instead it tells the compiler where to look to find classes that the program refers to Multiple import statements You can have as many as you like They go at the top of your java file before any classes or interfaces java lang is automatically imported into every program CMSC 131 Fall 2007 Jan Plane adapted by Bonnie Dorr 3 Defining your own package Why packages Packages enable a programmer organize the code into smaller logically related units A large program may consists of hundreds of classes Although we may not need to use them for the little projects in CMSC 131 but it is important to know how to create packages for when you will need them Every class is part of some package Default package If you do not specify a package a class becomes part of the default package Doesn t require the package statement at the top of the java file What special privileges do packages provide Classes defined within the same package can access one another more easily without the need for importing or fully qualified names CMSC 131 Fall 2007 Jan Plane adapted by Bonnie Dorr 4 Defining your own package Defining a package Add a package statement to specify the package containing the classes of this file package mypackage public class myClass myClass is part of mypackage This must be the first statement of your file Other than comments Subpackages Packages organized into subpackages This is specified using the notation main subpackage Example package mypackage mysubpackage public class myClass2 myClass2 is part of mysubpackage which is within mypackage Packages in Eclipse File New Package Enter the full name of the package e g mypackage or mypackage mysubpackage Without Eclipse Just insert this yourself package mypackage CMSC 131 Fall 2007 Jan Plane adapted by Bonnie Dorr 5 2 Class Access and Packages Class access within a package Classes within a package can refer to each other without full qualification If a class is not given an access specifier public or private it is assumed to have package access and it can only be accessed by other classes within the package Class access across packages A public class can be accessed from other packages When this is done either its name is fully qualified or is imported We can view public classes of a package as the interface of the package with the outside world This is analogous to public methods of a class forming its interface Subpackages are not automatically imported When you import a package e g import java awt it does not import the subpackages e g java awt font must be explicitly imported CMSC 131 Fall 2007 6 Jan Plane adapted by Bonnie Dorr Package Visibility Package visibility is half way between public and private Package Visibility of classes If a class is not declared public or private it can be accessed by all and only the classes within the package Package visibility of class members If instance variables or instance methods are not declared public or private they can be accessed by all and only the classes within the package Package visibility is used in industry A team working on a certain part of a project is likely to have its own package within which elements are shared among classes within the package Used for cases where shared elements are not intended to be publicly visible e g to other teams CMSC 131 Fall 2007 Jan Plane adapted by Bonnie Dorr 7 Example To illustrate these points let s consider an example of a simple hierarchical package which we will create Packages graphics graphics shapes Files Classes Files Driver java Driver Files Circle java Rectangle java OtherShape java Circle Rectangle OtherShape graphics otherstuff Files PublicClass1 java PublicClass2 java CMSC 131 Fall 2007 Jan Plane adapted by Bonnie Dorr PublicClass1 NonPublicClass1 PublicClass2 8 3 Example graphics shapes package File Circle java package graphics shapes public class Circle private double radius public String toString return I m a circle File Rectangle java package graphics shapes public class Rectangle private double height width public String toString return I m a rectangle package graphics shapes public class OtherShape private Circle c private Rectangle r File OtherShape java Note Classes of this package can be accessed without the need for import or using graphics shapes Circle CMSC 131 Fall 2007 Jan Plane adapted by Bonnie Dorr 9 Example graphics otherstuff package File PublicClass1 java package graphics otherstuff Public class public class PublicClass1 Accessible everywhere public String toString return This is a PublicClass NonPublicClass1 message Nonpublic class Only accessible in this package class NonPublicClass1 static public String message return I m a nonpublic class package graphics otherstuff File PublicClass2 java public class
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