Packages
and imports
In Java, it is usual for classes to be grouped
into a package, i.e., a set of related classes. A `packaged' class must be
identified by its package name and class name, or the 'import' instruction must be
used to tell the compiler to look for classes in a particular package.
You may have noticed at the start of many of
the example programs a line like this:
import
java.applet.Applet;
This is an indication that the program will
make use of the class Applet which is in the package java.applet.
The Java developers have designed the compiler
to automatically import the standard Java package java.lang. This defines important classes such as String. You will need to refer to a Java reference, such as Deitel & Deitel or
the on-line documentation to identify what to import to use those classes that are not
'built-in' through automatic importing.
You will also have seen the line
import
java.awt.*;
This means 'search every class in the
package java.awt'. When the
compiler comes across a class it does not recognise in the current program, it will see if
it exists inside the package java.awt.
Because an applet imports java.applet.Applet, then we can write the following statement (secure in the knowledge
the Java compiler knows where to find the Applet class
being extended):
class
MyApplet extends Applet
If we did not use the import line,
the program would still compile as long as I wrote
class
MyApplet extends java.applet.Applet
If you are creating a large, sophisticated
program, you may well want to group your classes up into packages of related classes.
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