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Exam Objective 5.1 Describe the purpose of packages in the Java language, and recognize the proper use of import and package statements. Packaging is a common approach used to organize related classes and interfaces. Most reusable code is packaged. Unpackaged classes are commonly found in books and online tutorials, as well as software applications with a narrow focus. This section will show you how and when to package your Java classes and how to import external classes from your Java packages. The following topics will be covered:
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n Package design n Package and import statements
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Packages are thought of as containers for classes, but actually they define where classes will be located in the hierarchical directory structure. Packaging is encouraged by Java coding standards to decrease the likelihood of classes colliding. Packaging your classes also promotes code reuse, maintainability, and the objectoriented principle of encapsulation and modularity. When you design Java packages, such as the grouping of classes, the following key areas (shown in Table 1-1) should be considered. Let s take a look at a real-world example. As program manager you need two sets of classes with unique functionality that will be used by the same end product. You task Developer A to build the first set and Developer B to build the second. You do not define the names of the classes, but you do define the purpose of the package and what it must contain. Developer A is to create several geometry-based classes including a point class, a polygon class, and a plane class. Developer B is to build classes that will be included for simulation purposes, including objects such as hot air balloons, helicopters, and airplanes. You send them off to build their classes (without having them package their classes). Come delivery time, they both give you a class named Plane.java that is, one for the geometry plane class and one for the airplane class. Now you have a problem because both of these source files (class files, too) cannot coexist in the same directory since they have the same name. The solution is packaging. If you had designated package names to the developers, this conflict never would have happened (as shown in Figure 1-1). The lessoned learned is: Always package your code, unless your coding project is trivial in nature.
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Benefits of applying the package attribute
Package dependencies are reduced with class coupling. Package dependencies are reduced with system coupling. Typically, larger packages support reusability, whereas smaller packages support maintainability. Often, software changes can be limited to a single package when the package houses focused functionality. Consider conventions when naming your packages. Use reverse domain name for the package structure. Use lowercase characters delimited with underscores to separate words in package names.
Package Attributes Considerations
1:
Packaging, Compiling, and Interpreting Java Code
FIgUre 1-1
Separate packaging of classes with the same names
com.scjaexam.simulator Plane
com.scjaexam.geometry Plane
Point
Polygon
HotAirBalloon
Helicopter
package and import statements
You should now have a general idea of when and why to package your source files. Now you need to know exactly how. To place a source file into a package, use the package statement at the beginning of that file. You may use zero or one package statements per source file. To import classes from other packages into your source file, use the import statement. The java.lang package that houses the core language classes is imported by default. The following code listing shows usage of the package and import statements. You can continue to come back to this listing as we discuss the package and import statements in further detail throughout the chapter.
package com.scjaexam.tutorial; // Package statement /* Imports class ArrayList from the java.util package */ import java.util.ArrayList; /* Imports all classes from the java.io package */ import java.io.*; public class MainClass { public static void main(String[] args) { /* Creates console from java.io package */ Console console = System.console(); String planet = console.readLine("\nEnter your favorite planet: "); /* Creates list for planets */ ArrayList planetList = new ArrayList(); planetList.add(planet); // Adds users input to the list planetList.add("Gliese 581 c"); // Adds a string to the list System.out.println("\nTwo cool planets: " + planetList); } } $ Enter your favorite planet: Jupiter $ Two cool planets: [Jupiter, Gliese 581 c]
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