Java 程序设计

1. Java程序设计 Java Programming Fall, 2010 成都信息工程学院 计算机系

2. Contents • Package Chapter 8 Nested Classes & Interfaces, & Exceptions

3. Java Program Organization • Java program • 1 or more Java source files • Source file • 1 or more class and/or interface declarations. • If a class/interface is public, the source file must use the same (base) name • So, only one public class/interface per source file • Can have non-public classes! We will discuss later • Packages • When a program is large, its classes can be organized hierarchically into packages. Chapter 8 Nested Classes & Interfaces, & Exceptions

4. Package(包) • Why? • A large program may consists of hundreds of classes (Example: 800 in one current project with NASA). • Packages enable a programmer organize the code into smaller logically related units. • Every class is part of some package. • If you do not specify a package a class becomes part of the default package Chapter 8 Nested Classes & Interfaces, & Exceptions

5. Java Packages • Package: A collection of related classes and/or interfaces. • The classes stored in each directory form a package. • System-defined packages - JRE System Library • User-defined packages Chapter 8 Nested Classes & Interfaces, & Exceptions

6. Chapter 8 Nested Classes & Interfaces, & Exceptions

7. Chapter 8 Nested Classes & Interfaces, & Exceptions

8. Some Predefined Java Packages Chapter 8 Nested Classes & Interfaces, & Exceptions

9. java “java” Package containing “lang”, “awt”,.. packages; Can also contain classes. lang awt Graphics awt Package containing classes Font Classes containing methods Image Java Packages • The packages are organised in a hierarchical(分层) structure. For example, a package named “java” contains the package “awt”, which in turn contains various classes required for implementing GUI (graphical user interface). …

10. Examples of Java Packages • java.langpackage: • Object • Math • The wrapper classes • String • StringBuffer Chapter 8 Nested Classes & Interfaces, & Exceptions

11. Using System Packages • Using a fully qualified component name: x = java.lang.Math.sqrt(3); • Using an importstatement: import packagename.*; // to allow unqualified references to // all package classes import packagename.class_name; // to allow unqualified references to // a particular package class

12. This code java.util.Date d = new java.util.Date(); java.awt.Point p = new java.awt.Point(1,2); java.awt.Button b = new java.awt.Button(); Can be abbreviated import java.util.date; import java.awt.*; … Date d = new Date(); Point p = new Point(1,2); Button b = new Button(); Import Examples Chapter 8 Nested Classes & Interfaces, & Exceptions

13. Defining Packages • To define: add package statement to specify package containing the classes of a file • Must be first non-comment statement in file package mypackage; //must be first line … //myClass is part of mypackage public class myClass { … }

14. Creating Sub-Packages • As packages in Java are organised hierarchically, sub-packages can be created as follows: package myPackage.secondPakage.thirdPackage; • Store “thirdPackage” in a subdirectory named “myPackage\secondPackage”. package mypackage.mysubpackage; … //myClass2 is part of mysubpackage, which is //within mypackage public class myClass2 { … } Chapter 8 Nested Classes & Interfaces, & Exceptions

15. User-Defined Packages • Access（包的访问） • Classes defined within the same package can access one another more easily (no need for imports, fully qualified names); • Some classes, object fields only accessible to classes in same package. Chapter 8 Nested Classes & Interfaces, & Exceptions

16. Visibility Rules and Packages • Instance variables without explicitly declared visibility have package visibility; • Instance variables, static variables, instance methods, and static methods with package visibility are only visible to methods defined in classes belonging to the same package; • Classes not explicitly declared public are not visible outside the package. Chapter 8 Nested Classes & Interfaces, & Exceptions

17. Visibility Modifiers Chapter 8 Nested Classes & Interfaces, & Exceptions

18. Using a Package • Let us store the code listing below in a file named “ClassA.java” within subdirectory named “myPackage” within the current directory (say “abc”). package myPackage; public class ClassA { // class body public void display() { System.out.println("Hello, I am ClassA"); } } Chapter 8 Nested Classes & Interfaces, & Exceptions

19. Using a Package • Within the current directory (“abc”) store the following code in a file named “ClassX.java” import myPackage.ClassA; public class ClassX{ public static void main(String args[]){ ClassA objA = new ClassA(); objA.display(); } } Chapter 8 Nested Classes & Interfaces, & Exceptions

20. package myPackage; public class ClassA { public void display(){ System.out.println("Hello, I am ClassA"); } } import myPackage.ClassA; public class ClassX{ public static void main(String args[]){ ClassA objA = new ClassA(); objA.display(); } }

21. 创建包示例 X1.java package card; //创建包 public class X1{ int x, y; public X1(int i, int j){ this.x=i; this.y=j; System.out.println("x="+x+" "+"y="+y); } public void show(){ System.out.println("this class is a X1!"); } }

22. 创建包示例 X2.java package card; //创建包 public class X2 { int m,n; public X2(int i,int j) { this.m=i; this.n=j; System.out.println("m="+m+" "+"n="+n); } public void show(){ System.out.println("this class is a X2!"); } }

23. 包的引用示例：Pack.java不在card包内，但和card包在同一个目录中。包的引用示例：Pack.java不在card包内，但和card包在同一个目录中。 import card.X1; //导入包card中的X1类 import card.X2; //导入包card中的X2类 //import card.*； public class Pack { public static void main(String args[]){ X1 aa=new X1(4,5); aa.show(); X2 bb=new X2(10,20); bb.show(); } }