DEPARTMENT OF COMPUTER SCIENCE & TECHNOLOGY FACULTY OF SCIENCE & TECHNOLOGY UNIVERSITY OF UWA WELLASSA ‏ Packages

Definition: A package is a grouping of related classes and interfaces providing access protection and name space management. One of the main features of Object Oriented Programming is its ability to reuse the code that is already created. If it is necessary to use the classes from other programs without physically copying them into a new program, packages can be used. In Java packages are the way of grouping a variety of classes and/or interfaces together. The grouping is usually done according to the functionality of the class. In fact, packages act as “containers” for classes. 2 Introduction

By organizing the classes into packages the following benefits can be achieved. 1.The classes contained in the packages of other programs can be easily reused. 2. In packages, classes can be unique compared with classes in other packages. That is, two classes in two different packages can have the same name. They may be referred by their fully qualified name (package name and class name). 3. Packages provide a way to hide classes thus preventing other programs or packages from accessing classes that are meant for internal use only 3

Classification of packages In Java packages are classified into two types, 1. Java API packages. 2. user defined packages. Java API packages. Java API (Application Programming Interface) provides a large number of classes grouped into different packages according to functionality. Following figure shows some of the frequently used packages 4

API packages Using System Packages. The classes in the Java API are organized in to a hierarchical structure. Two ways to access the classes stored in a package. The first approach is to use the fully qualified class name of the class that need to be used. This is done by using the package name containing the class and then appending the class name to it using the dot operator. example Vector in the util package, java.util.Vector v = new java.util.Vector(); note that the util is a package within the package java and the hierarchy is represented by separating the levels with dots. 5

API packages… In many situations it is necessary to use a class in a number of places in the program or may need to use many of the classes contained in the same package. This can be easily done by using the import statement. import packagename.classname; or import packagename.*; (* represents all the classes of the package) These are known as import statements and must appear at the top of the file, before any class declaration. 6

API packages… Example import java.awt.Color; imports the class Color and therefore the class name can now be directly used in the program. There is no need to use the package name to qualify the class. The following statement imports all the classes contained in the java.awt package. import java.awt.*; 7

API packages… Naming conventions Packages can be named using the standard Java naming rules. By convention, however, packages begin with lowercase letters. This makes it easy for users to distinguish package names from class names when looking at an explicit reference to a class. 8

User Defined Packages Creating the package. Java provides the facility to create user defined packages. Need to perform the following steps. 1. Declare the package at the beginning of a file using the statement, package packagename; 2. Define the class that is to be put in the package and declare it public. 3. Create a subdirectory under the directory where the main source files are stored. 4. Store the classname.java files in the subdirectory created. Note that the subdirectory name must match the package name exactly. 9

User Defined Packages… Accessing a package Similar way as accessing the API packages, the import command is used; import package1 [.package2] [.package3].classname; Or import packagename.* ; 10

User Defined Packages… Example: This is a Java example of using a package for a application to handle employee details of a company. Step 1: Code the Java file giving a package name. package company; public class Employee {private String name; public void setName(String name) { this.name=name; } public String getName() { return this.name; } 11

User Defined Packages… Step 2: Save the file with the name “Employee.java” in a folder “company” (Eg: D:\ Java \ company) Step 3: import the Employee class in the company package into the driver class. Save the driver class in the directory D: \ Java. import company.Employee; class Test { public static void main(String args[]) { Employee e1=new Employee(); e1.setName("Nimal Perera"); System.out.println(e1.getName()); } 12

User Defined Packages… Note that in this example the driver class is saved in the directory D: \ Java. Therefore it is possible to give the relative path into the Employee class in the import statement as company.Employee (or company.*). But if the driver class is stored in a directory which can not resolve the relative path to the company package it will produce a compile time error. 13

User Defined Packages… To solve this problem, need to set the absolute path to the place where the package is stored. Then from that place the relative path company.Employee will be considered. This is done by setting the classpath environmental variable. In the command prompt type; set classpath =.;D:\Java Note that the symbol “.” before the absolute path (D:\Java) is to represent the current working directory of Java. When this part is omitted the interpreter may not be able to find the class file of the driver class 14