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.*;
CAUTION The star form may increase compilation time--especially if you import several large
AUTION
packages. For this reason it is a good idea to explicitly name the classes that you want to use
rather than importing whole packages. However, the star form has absolutely no effect on the
run-time performance or size of your classes.
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 {
Search WWH :
