Docsity
Docsity
Prepare for your exams
Prepare for your exams

Study with the several resources on Docsity

Earn points to download
Earn points to download

Earn points by helping other students or get them with a premium plan

Guidelines and tips
Guidelines and tips
Sell on Docsity
Sell on Docsity
Docsity AI
Docsity AI
Log inSign up

Prepare for your exams

Study with the several resources on Docsity

Find documents

Prepare for your exams with the study notes shared by other students like you on Docsity

Search for your university

Find the specific documents for your university's exams

Docsity AINEW

Summarize your documents, ask them questions, convert them into quizzes and concept maps

Explore questions

Clear up your doubts by reading the answers to questions asked by your fellow students

Earn points to download

Earn points by helping other students or get them with a premium plan

Share documents
download point icon20 Points

For each uploaded document

Answer questions
download point icon5 Points

For each given answer (max 1 per day)

All the ways to get free points
Get points immediately

Choose a premium plan with all the points you need

Study Opportunities

Choose your next study program

Get in touch with the best universities in the world. Search through thousands of universities and official partners

Community

Ask the community

Ask the community for help and clear up your study doubts

Free resources

Our save-the-student-ebooks!

Download our free guides on studying techniques, anxiety management strategies, and thesis advice from Docsity tutors

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

University of MarylandComputer Science

An overview of java program organization, focusing on the use of packages and class access. It covers the concept of packages as collections of related classes and interfaces, their hierarchical organization, and access to package members. The document also includes examples of package usage and file structure.

Typology: Study notes

Pre 2010

Uploaded on 07/30/2009

koofers-user-bzj
koofers-user-bzj 🇺🇸

9 documents

1 / 7

Toggle sidebar

This page cannot be seen from the preview

Don't miss anything!

bg1
CMSC 131: Chapter 20 (Supplement)
CMSC 131: Chapter 20 (Supplement)
Packages
Packages
Java Program Organization
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
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.
pf3
pf4
pf5

Partial preview of the text

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)CMSC 131: Chapter 20 (Supplement)

Packages Packages

Java Program Organization 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 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 MembersAccess 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 packageDefining 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).

ExampleExample

To illustrate these points, let’s consider a (contrived) example of a hierarchical

package that we will create.

graphics

graphics.shapes

graphics.otherstuff

Example: graphics.shapes package 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"; } }

File: Othershape.java

package graphics.shapes; public class OtherShape { private Circle c; // Can access other classes in this package directly private Rectangle r; }

Example: graphics.otherstuff packageExample: 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 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 } }

Packages and ClasspathPackages 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 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.