Importing Packages

Importing Packages

Given that packages exist and are a good mechanism for compartmentalizing diverse classes from each other, it is easy to see why all of the built-in Java classes are stored in packages. There are no core Java classes in the unnamed default package; all of the standard classes are stored in some named package. Since classes within packages must be fully qualified with their package name or names, it could become tedious to type in the long dot-separated package path name for every class you want to use. For this reason, Java includes the import statement to bring certain classes, or entire packages, into visibility. Once imported, a class can be referred to directly, using only its name. The import statement is a convenience to the programmer and is not technically needed to write a complete Java program. If you are going to refer to a few dozen classes in your application, however, the import statement will save a lot of typing.

In a Java source file, import statements occur immediately following the package statement (if it exists) and before any class definitions. This is the general form of the import statement:

import pkg1 [.pkg2].(classname | *);

Here, pkg1 is the name of a top-level package, and pkg2 is the name of a subordinate package inside the outer package separated by a dot (.). There is no practical limit on the depth of a package hierarchy, except that imposed by the file system. Finally, you specify either an explicit classname or a star (*), which indicates that the Java compiler should import the entire package. This code fragment shows both forms in use:

import java.util.Date; 

import java.io.*;

All of the standard Java classes included with Java are stored in a package called java. The basic language functions are stored in a package inside of the java package called java.lang. Normally, you have to import every package or class that you want to use, but since Java is useless without much of the functionality in java.lang, it is implicitly imported by the compiler for all programs. This is equivalent to the following line being at the top of all of your programs:

import java.lang.*;

If a class with the same name exists in two different packages that you import using the star form, the compiler will remain silent, unless you try to use one of the classes. In that case, you will get a compile-time error and have to explicitly name the class specifying its package.

It must be emphasized that the import statement is optional. Any place you use a class name, you can use its fully qualified name, which includes its full package hierarchy. For example, this fragment uses an import statement:

import java.util.*;

class MyDate extends Date {

}

The same example without the import statement looks like this:

class MyDate extends java.util.Date {

}

In this version, Date is fully-qualified.

As shown in Table 9-1, when a package is imported, only those items within the package declared as public will be available to non-subclasses in the importing code. For example, if you want the Balance class of the package MyPack shown earlier to be available as a stand-alone class for general use outside of MyPack, then you will need to declare it as public and put it into its own file, as shown here:

package MyPack;

/*     Now, the Balance class, its constructor, and its show() method are public. This means that they can be used by non-subclass code outside their package.

*/

public class Balance { String name;

double bal;

public Balance(String n, double b) { name = n;

bal = b;

}

public void show() { if(bal<0)

System.out.print("--> "); 

System.out.println(name + ": $" + bal);

}

}

As you can see, the Balance class is now public. Also, its constructor and its show( ) method are public, too. This means that they can be accessed by any type of code outside the MyPack package. For example, here TestBalance imports MyPack and is then able to make use of the Balance class:

import MyPack.*;

class TestBalance {

public static void main(String args[]) {

/* Because Balance is public, you may use Balance class and call its constructor. */

Balance test = new Balance("J. J. Jaspers", 99.88);

test.show(); // you may also call show()

}

}

As an experiment, remove the public specifier from the Balance class and then try compiling TestBalance. As explained, errors will result.