THIS chapter explains how to bundle classes and interfaces into packages, how to use classes that are in packages, and how to arrange your file system so that the compiler can find your source files.
To make types easier to find and use, to avoid naming conflicts, and to control access, programmers bundle groups of related types into packages.
A package is a grouping of related types providing access protection and name space management. Note that types refers to classes, interfaces, enumerations, and annotation types. Enumerations and annotation types are special kinds of classes and interfaces, respectively, so types are often referred to in this chapter simply as classes and interfaces.
The types that are part of the Java platform are members of various packages that bundle classes by function: Fundamental classes are in java.lang
, classes for reading and writing (input and output) are in java.io
, and so on. You can put your types in packages too.
Suppose you write a group of classes that represent graphic objects, such as circles, rectangles, lines, and points. You also write an interface, Draggable
, that classes implement if they can be dragged with the mouse:
//in the Draggable.java file public interface Draggable { ... } //in the Graphic.java file public abstract class Graphic { ... } //in the Circle.java file public class Circle extends Graphic implements Draggable { ... } //in the Rectangle.java file public class Rectangle extends Graphic implements Draggable { ... } //in the Point.java file public class Point extends Graphic implements Draggable { ... } //in the Line.java file public class Line extends Graphic implements Draggable { ... }
You should bundle these classes and the interface in a package for several reasons, including the following:
You and other programmers can easily determine that these types are related.
You and other programmers know where to find types that can provide graphics related functions.
The names of your types won’t conflict with the type names in other packages because the package creates a new namespace.
You can allow types within the package to have unrestricted access to one another yet still restrict access for types outside the package.
To create a package, you choose a name for the package (naming conventions are discussed in the next section) and put a package
statement with that name at the top of every source file that contains the types (classes, interfaces, enumerations, and annotation types) that you want to include in the package.
The package
statement (for example, package graphics;
) must be the first line in the source file. There can be only one package
statement in each source file, and it applies to all types in the file.
If you put multiple types in a single source file, only one can be public
, and it must have the same name as the source file. For example, you can define public class Circle
in the file Circle.java
, define public interface Draggable
in the file Draggable.java
, define public enum Day
in the file Day.java
, and so forth.
You can include non-public types in the same file as a public type (this is strongly discouraged, unless the non-public types are small and closely related to the public type), but only the public type will be accessible from outside of the package. All the top-level, non-public types will be package-private.
If you put the graphics interface and classes listed in the preceding section in a package called graphics
, you would need six source files, like this:
// in the Draggable.java file package graphics; public interface Draggable { ... } // in the Graphic.java file package graphics; public abstract class Graphic { ... } // in the Circle.java file package graphics; public class Circle extends Graphic implements Draggable { ... } // in the Rectangle.java file package graphics; public class Rectangle extends Graphic implements Draggable { ... } // in the Point.java file package graphics; public class Point extends Graphic implements Draggable { ... } // in the Line.java file package graphics; public class Line extends Graphic implements Draggable { ... }
If you do not use a package
statement, your type ends up in an unnamed package. Generally speaking, an unnamed package is only for small or temporary applications or when you are just beginning the development process. Otherwise, classes and interfaces belong in named packages.
With programmers worldwide writing classes and interfaces using the Java programming language, it is likely that many programmers will use the same name for different types. In fact, the previous example does just that: It defines a Rectangle
class when there is already a Rectangle
class in the java.awt
package. Still, the compiler allows both classes to have the same name if they are in different packages. The fully qualified name of each Rectangle
class includes the package name. That is, the fully qualified name of the Rectangle
class in the graphics
package is graphics.Rectangle
, and the fully qualified name of the Rectangle
class in the java.awt
package is java.awt.Rectangle
.
This works well unless two independent programmers use the same name for their packages. What prevents this problem? Convention.
Package names are written in all lowercase to avoid conflict with the names of classes or interfaces.
Companies use their reversed Internet domain name to begin their package names—for example, com.example.orion
for a package named orion
created by a programmer at example.com
.
Name collisions that occur within a single company need to be handled by convention within that company, perhaps by including the region or the project name after the company name (for example, com.company.region.package
).
Packages in the Java language itself begin with java.
or javax
.
In some cases, the Internet domain name may not be a valid package name. This can occur if the domain name contains a hyphen or other special character, if the package name begins with a digit or other character that is illegal to use as the beginning of a Java name, or if the package name contains a reserved Java keyword, such as “int”. In this event, the suggested convention is to add an underscore (see Table 7.1).
The types that comprise a package are known as the package members.
To use a public
package member from outside its package, you must do one of the following:
Refer to the member by its fully qualified name
Import the package member
Import the member’s entire package
Each is appropriate for different situations, as explained in the sections that follow.
So far, most of the examples in this tutorial have referred to types by their simple names, such as Rectangle
and StackOfInts
. You can use a package member’s simple name if the code you are writing is in the same package as that member or if that member has been imported.
However, if you are trying to use a member from a different package and that package has not been imported, you must use the member’s fully qualified name, which includes the package name. Here is the fully qualified name for the Rectangle
class declared in the graphics
package in the previous example:
graphics.Rectangle
You could use this qualified name to create an instance of graphics.Rectangle
:
graphics.Rectangle myRect = new graphics.Rectangle();
Qualified names are all right for infrequent use. When a name is used repetitively, however, typing the name repeatedly becomes tedious and the code becomes difficult to read. As an alternative, you can import the member or its package and then use its simple name.
To import a specific member into the current file, put an import
statement at the beginning of the file before any type definitions but after the package
statement, if there is one. Here’s how you would import the Rectangle
class from the graphics
package created in the previous section:
import graphics.Rectangle;
Now you can refer to the Rectangle
class by its simple name:
Rectangle myRectangle = new Rectangle();
This approach works well if you use just a few members from the graphics
package. But if you use many types from a package, you should import the entire package.
To import all the types contained in a particular package, use the import
statement with the asterisk (*)
wildcard character:
import graphics.*;
Now you can refer to any class or interface in the graphics
package by its simple name:
Circle myCircle = new Circle(); Rectangle myRectangle = new Rectangle();
The asterisk in the import
statement can be used only to specify all the classes within a package, as shown here. It cannot be used to match a subset of the classes in a package. For example, the following does not match all the classes in the graphics
package that begin with A
:
import graphics.A*; // does not work
Instead, it generates a compiler error. With the import
statement, you generally import only a single package member or an entire package.
Another, less common form of import
allows you to import the public nested classes of an enclosing class. For example, if the graphics.Rectangle
class contained useful nested classes, such as Rectangle.DoubleWide
and Rectangle.Square
, you could import Rectangle
and its nested classes by using the following two statements:
import graphics.Rectangle; import graphics.Rectangle.*;
Be aware that the second import statement will not import Rectangle
.
Another less common form of import
, the static import statement, will be discussed at the end of this section.
For convenience, the Java compiler automatically imports three entire packages for each source file: (1) the package with no name, (2) the java.lang
package, and (3) the current package (the package for the current file).
At first, packages appear to be hierarchical, but they are not. For example, the Java API includes a java.awt
package, a java.awt.color
package, a java.awt.font
package, and many others that begin with java.awt
. However, the java.awt.color
package, the java.awt.font
package, and other java.awt.xxxx
packages are not included in the java.awt
package. The prefix java.awt
(the Java Abstract Window Toolkit) is used for a number of related packages to make the relationship evident, but not to show inclusion.
Importing java.awt.*
imports all of the types in the java.awt
package, but it does not import java.awt.color
, java.awt.font
, or any other java.awt.xxxx
packages. If you plan to use the classes and other types in java.awt.color
as well as those in java.awt
, you must import both packages with all their files:
import java.awt.*; import java.awt.color.*;
If a member in one package shares its name with a member in another package and both packages are imported, you must refer to each member by its qualified name. For example, the graphics
package defined a class named Rectangle
. The java.awt
package also contains a Rectangle
class. If both graphics
and java.awt
have been imported, the following is ambiguous:
Rectangle rect;
In such a situation, you have to use the member’s fully qualified name to indicate exactly which Rectangle
class you want. For example,
graphics.Rectangle rect;
There are situations where you need frequent access to static final fields (constants) and static methods from one or two classes. Prefixing the name of these classes over and over can result in cluttered code. The static import statement gives you a way to import the constants and static methods that you want to use so that you do not need to prefix the name of their class.
The java.lang.Math
class defines the PI
constant and many static methods, including methods for calculating sines, cosines, tangents, square roots, maxima, minima, exponents, and many more. For example,
public static final double PI 3.141592653589793 public static double cos(double a)
Ordinarily, to use these objects from another class, you prefix the class name, as follows:
double r = Math.cos(Math.PI * theta);
You can use the static import statement to import the static members of java.lang.Math
so that you don’t need to prefix the class name, Math
. The static members of Math
can be imported either individually:
import static java.lang.Math.PI;
or as a group:
import static java.lang.Math.*;
Once they have been imported, the static members can be used without qualification. For example, the previous code snippet would become:
double r = cos(PI * theta);
Obviously, you can write your own classes that contain constants and static methods that you use frequently, and then use the static import statement. For example,
import static mypackage.MyConstants.*;
Many implementations of the Java platform rely on hierarchical file systems to manage source and class files, although The Java Language Specification does not require this. The strategy is as follows.
Put the source code for a class, interface, enumeration, or annotation type in a text file whose name is the simple name of the type and whose extension is .java
. For example:
// in the Rectangle.java file package graphics; public class Rectangle() { ... }
Then, put the source file in a directory whose name reflects the name of the package to which the type belongs:
...graphicsRectangle.java
The qualified name of the package member and the path name to the file are parallel, assuming the Microsoft Windows file name separator backslash (for UNIX, use the forward slash):
class name |
|
pathname to file |
|
As you should recall, by convention a company uses its reversed Internet domain name for its package names. The Example company, whose Internet domain name is example.com
, would precede all its package names with com.example
. Each component of the package name corresponds to a subdirectory. So, if the Example company had a com.example.graphics
package that contained a Rectangle.java
source file, it would be contained in a series of subdirectories like this:
...comexamplegraphicsRectangle.java
When you compile a source file, the compiler creates a different output file for each type defined in it. The base name of the output file is the name of the type, and its extension is .class
. For example, if the source file is like this:
// in the Rectangle.java file package com.example.graphics; public class Rectangle{ ... } class Helper{ ... }
then the compiled files will be located at:
<path to the parent directory of the output files>com examplegraphicsRectangle.class <path to the parent directory of the output files>com examplegraphicsHelper.class
Like the .java
source files, the compiled .class
files should be in a series of directories that reflect the package name. However, the path to the .class
files does not have to be the same as the path to the .java
source files. You can arrange your source and class directories separately, as:
<path_one>sourcescomexamplegraphicsRectangle.java <path_two>classescomexamplegraphicsRectangle.class
By doing this, you can give the classes
directory to other programmers without revealing your sources. You also need to manage source and class files in this manner so that the compiler and the Java Virtual Machine (JVM) can find all the types your program uses.
The full path to the classes
directory, <path_two>classes
, is called the class path, and is set with the CLASSPATH
system variable. Both the compiler and the JVM construct the path to your .class
files by adding the package name to the class path. For example, if
<path_two>classes
is your class path, and the package name is
com.example.graphics
then the compiler and JVM look for .class files
in
<path_two>classescomexamplegraphics.
A class path may include several paths, separated by a semicolon (Windows) or colon (UNIX). By default, the compiler and the JVM search the current directory and the JAR file containing the Java platform classes so that these directories are automatically in your class path.
To display the current CLASSPATH
variable, use these commands in Windows and UNIX (Bourne shell):
In Windows: C:> set CLASSPATH In UNIX: % echo $CLASSPATH
To delete the current contents of the CLASSPATH
variable, use these commands:
In Windows: C:> set CLASSPATH= In UNIX: % unset CLASSPATH; export CLASSPATH
To set the CLASSPATH
variable, use these commands (for example):
In Windows: C:> set CLASSPATH=C:usersgeorgejavaclasses In UNIX: % CLASSPATH=/home/george/java/classes; export CLASSPATH
To create a package for a type, put a package
statement as the first statement in the source file that contains the type (class, interface, enumeration, or annotation type).
To use a public type that’s in a different package, you have three choices: (1) use the fully qualified name of the type, (2) import the type, or (3) import the entire package of which the type is a member.
The path names for a package’s source and class files mirror the name of the package.
You might have to set your CLASSPATH
so that the compiler and the JVM can find the .class
files for your types.
Assume you have written some classes. Belatedly, you decide they should be split into three packages, as listed in Table 7.2. Furthermore, assume the classes are currently in the default package (they have no package
statements).
Download the source files as listed here:
Client: tutorial/java/package/QandE/question/Client.java
Server: tutorial/java/package/QandE/question/Server.java
Utilities: tutorial/java/package/QandE/question/Utilities.java
13.59.141.75