Unit 051 Packages What is a Package? Why use Packages? Creating a Package Naming a Package Using Package Members Managing Source and Class Files Visibility Modifiers

Unit 052 What is a Package? A package is a collection of related classes and interfaces that provides access protection and namespace management. The classes in Java API are organized into packages. For example: Applet classes are in java.applet package I/O classes are in java.io package GUI classes are in java.awt. You can also organize the classes you create into packages and you should. Why?

Unit 053 Why use Packages? Easy to know where to find classes and interfaces that provide a particular set of functions. –For I/O functions go to java.io package. Avoid conflict with class names in other packages. Classes in different packages can have same name. –A package provides a new namespace. You can allow classes within a package to have access to each other.

Unit 054 Creating a Package To create a package, put a package statement at the top of each source file for the class or interface that belongs to the package. For example, to create a package, graphics, to store the classes: Shape, Circle, Rectangle, and Square, we should add the statement, package graphics; as the first statement in each of the source files. 1.package graphics; 2.public abstract class Shape{ 3.public String name(){ 4.return getClass().getName(); 5.} 6.public abstract double area(); 7.public abstract perimter(); 8.public String toString(){ 9.return name()+"\nPerimeter: "+perimeter()+"\nArea: "+area(); 10.} 11.}

Unit 055 Creating a Package-Cont’d The scope of the package statement is the entire source file, so if source file contains more than one class or interface, they all belong to the package specified by the package statement. If package is not specified for a class or interface, then the class or interface belongs to the default package, which is a package that has no name.

Unit 056 Naming a Package By convention, packages are named using lower case letters. Dots are used to group related packages together. For example, the packages: java.awt, java.io, java.lang, are named as such to indicate that they all belong to Java API. Similarly, java.awt.color, java.awt.geom, java.awt.event, are named as such to indicate that they all belong to a group of classes for programming GUI applications. Notice that this grouping is for the human reader only. As far as the system is concerned, the packages java.awt.color is completely independent of java.awt.geom.

Unit 057 Naming a Package-Cont’d Within one organization, it is clear that packages can be used to avoid conflict in class names. What about between different organizations? It is likely that two organizations working on similar projects may come up with the same package names and probably common class names. Thus, a user dealing with these two organizations may face conflict. To avoid this, by convention, organizations use their reversed Internet domain name in their package names, like: com.company.package We shall name our packages in this course starting with the prefix, ics201. Example: ics201.graphics, ics201.lab01, etc.

Unit 058 Using Package Members To use a public package member (a class or interface) from outside its package, you must do one of the following: oRefer to the member by its long name oImport the package member, or oImport the member's entire package. Each of these methods is appropriate for different situations, as explained in the following sections.

Unit 059 Referring to a Package Member by Full Name So far, our examples have referred to classes and interfaces by their simple names, such as Rectangle. You can use a package member’s simple name if: –The code you are writing is in the same package as the member –If the member’s package has been imported. However, if a member is from a different package and that package has not been imported, you must use the member’s fully qualified name: ics201.graphics.Rectangle To create an instance of the member we use: ics201.graphics.Rectangle myRect = new ics201.graphics.Rectangle();

Unit 0510 Importing a Package Member To import a specific package member into the current file, put an import statement at the beginning of the file before any class or interface definitions but after the package statement ( if there is one). 1.package ics201.lab02; 2.import ics201.graphics.Shape; 3.import ics201.graphics.Rectangle; 4.import ics201.graphics.Square; 5.import ics201.graphics.Circle; 6.public class TestShapes{ 7.public static void main(String[] args){ 8.Shape[] shapes = new Shape[2]; 9.shape[0] = new Rectangle(5, 10); 10.shape[0] = new Square(10); for(int i = 0; i<shape.length; i++) 13.System.out.println(shape[i]); 14.} 15.} This approach is used if few members are needed from a package.

Unit 0511 Importing an Entire Package To import all of the classes and interfaces contained in a particular package, use the asterisk (*). 1.package ics201.lab02; 2.import ics201.graphics.*; 3.public class TestShapes{ 4.public static void main(String[] args){ 5.Shape[] shape = new Shape[2]; 6.shape[0] = new Rectangle(5, 10); 7.shape[0] = new Square(10); 8. 9.for(int i=0; i< shape.length; i++) 10.System.out.println(shape[i]); 11.} 12.} As noted earlier, ics201 and ics201.graphics are independent packages, so the statement, import ics201.*; only imports ics201 package but not ics201.graphics.

Unit 0512 Importing an Entire Package- Cont’d Also note that the asterisk cannot be used to match a subset of a package. For example, the following does not match all the classes in the graphics package that begin with a: For convenience, the Java runtime system automatically imports three entire packages: oThe default package (the package with no name) oThe java.lang package o The current package What if two packages have classes with the same name and both packages are imported? -- You must use fully qualified names.

Unit 0513 Managing Source and Class Files Declaring package names at the beginning of source files is what is required to create a package. However, for the Java system to locate your source and class files, you must store them in a directory whose name matches the name of their package. For example, the source code for the Rectangle class should be in a file, Rectangle.java, and must be in a directory, ics201\graphics. The ics201\graphics directory can be anywhere on the file system.

Unit 0514 Managing Source and Class Files- Cont’d Note however, that the.class and.java files do not have to be in the same directory as shown below: Since the directory structure can be anywhere in the file system, it is obvious that the package name alone does not provide a java tool, such as the Compiler, with all the information need to locate a class. The remaining information is provided by CLASSPATH.

Unit 0515 Managing Source and Class Files- Cont’d A class path is an ordered list of directories or jar files in which to search for class files. Each directory listed in the class path is a top-level directory in which package directories appear. For example, suppose that both.java and.class files for the ics201.graphics package are stored in the same directory. Suppose also that the directory structure for the package is in a directory z:\cs, as shown below, then we must add z:\cs to the class path:

Unit 0516 Visibility Modifiers Now that we have learnt about packages, this is a good time to summarize what we learnt about visibility modifiers. Visibility modifiers are used to allow/deny access to other classes or interfaces. There are four of them, namely, public, private, and protected. The fourth, the default, does not have a keyword, but is assumed if none of the other three is specified.

Unit 0517 Example 1.package package1; 2.public class A{ 3. public A(){ 4. System.out.println("Inside Class A"); 5. } 6. int method1(int x){ 7. return x*x; 8. } 9. public int method2(int x){ 10. return method1(x) + method1(x); 11. } 12.}

Unit 0518 Example (cont’d) 1.package package2; 2.import package1.*; 3.public class B extends A{ 4. public B ( ){ System.out.println ("Inside Class B"); } 5. public int method1 ( int x ) { 6. A a = new B(); 7. return a.method2 (x); 8. } 9. public int method2 ( int x ){return x; } 10. public static void main ( String [] x ) { 11. B b = new B ( ); 12. System.out.println ( b.method1 ( 3 ) ); 13. } 14.}

Unit 0519 Exercises 1.Add the statement: package ics202.graphics; to each of our shape classes, except TestShape. Store all the files in some root folder. Try compiling and running the TestShape class. What happens? 2.Create a folder, ics201\graphics, and store the shape files in it, except the TestShape class. Do not put any import statement in the TestShape. Try compiling and running TestShape. What happens? 3.Add the import statement, import ics201.graphics.*; to TestShape Try compiling and running TestShape without giving the class path for the ics201.graphics package. What happens? 4.Try compiling and running TestShape, giving the class path for the ics201.graphics package. It should compile and run successfully. 5.Try each of the methods of importing classes discussed in this session and the TestShape class. 6.Add the import statement; java.awt.*; What happens? How can this problem be resolved?