Java Program Organization: Understanding Packages and Class Access, Study notes of Computer Science

Download Java Program Organization: Understanding Packages and Class Access and more Study notes Computer Science in PDF only on Docsity! CMSC 131: Chapter 20 (Supplement) Packages 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. 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. 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. 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: Really? Why haven’t we seen more of them? Default package: If you do not specify a package a class becomes part of the "default package". 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). Example: graphics.otherstuff package File: PublicClass1.java package graphics.otherstuff; public class PublicClass1 { // A public class public String toString( ) { return "This is a PublicClass: " + NonPublicClass1.message( ); } } class NonPublicClass1 { // A nonpublic class: Only accessible in the package static public String message( ) { return "I'm a nonpublic class"; } } File: PublicClass2.java package graphics.otherstuff; public class PublicClass2 { private Driver d; // NO! We have no direct access to parent package private Circle c1; // NO! We have no direct access to sister package private graphics.shapes.Circle c2; // Okay. Can access public classes elsewhere public String toString( ) { return "This is a PublicClass2: " + NonPublicClass1.message( ); } } Example: graphics.package File: Driver.java package graphics; import graphics.shapes.Circle; public class Driver { public static void main(String[] args) { testShapes( ); testOtherStuff( ); } public static void testShapes( ) { Circle c = new Circle( ); System.out.println( c.toString( ) ); Rectangle r = new Rectangle( ); // NO! Cannot access without import } public static void testOtherStuff( ) { PublicClass1 x = new PublicClass1( ); // NO! Cannot access without import graphics.otherstuff.PublicClass1 y = new graphics.otherstuff.PublicClass1( ); System.out.println( y ); graphics.otherstuff.NonPublicClass1 z; // NO! not visible here } } File Structure Java organizes the package files using your system’s directory structure. graphics: Driver.class Driver.java otherstuff/ shapes/ graphics/otherstuff: NonPublicClass1.class PublicClass1.java PublicClass2.java PublicClass1.class PublicClass2.class graphics/shapes: Circle.class OtherShape.class Rectangle.class Circle.java OtherShape.java Rectangle.java Packages and .jar Files Packages are convenient, but large directories are not. Jar File: Java allows you to bundle up an entire directory of files into a single file, called a jar file. Creating a jar file: – In Eclipse: In the Package Explorer window, right click on the project and select “Export  JAR file”. – On Unix: jar –cvf myJarFile.jar … (list the file names and/or directories) (c = create; t = list names; x = extract; v = verbose; f = jar file) Examples: cmsc131PictureLib.jar: A jar file we created for the picture library. cmsc131PuzzleLib.jar: A jar file we created for HW#5. C:\...\Java\j2re1.4.2_05\lib\rt.jar: This is a large (20+Mbyte) file with all the class files from the Java runtime library. Packages and Classpath You may have many different packages on your system (e.g. the Java runtime library, cmsc131PictureLib, cmsc131PuzzleLib) in many different locations. How does Java know where to look for them? ClassPath: is a system environment variable that gives a list of directories and jar files where Java should look up its classes. Windows Example: Suppose we want to use – graphics package: stored in directory C:\MyJavaPackages\graphics. – cmsc131PictureLib.jar: stored in C:\MyJars\cmsc131PictureLib.jar – classes compiled in the current working directory: The current directory is denoted by “.” (period) on most systems. C:\>set CLASSPATH=.;C:\MyJavaPackages;C:\MyJars\cmsc131PictureLib.jar Packages and Classpath In Eclipse: The ClassPath is already set with important default directories (e.g. the Java runtime library). To modify the ClassPath: – In the Package Explorer, right click on the project name – Select “Properties  Java Build Path  Libraries” – To add Jars: Select: “Add External JARs…” and browse for the file name. – To add a directory to the ClassPath: Select: Add Variable…  Configure Variables  New and add the name of the new directory.