The "Hello World" Applet |
The first two lines of the following listing import two classes used in the applet:Applet
andGraphics
.import java.applet.Applet; import java.awt.Graphics; public class HelloWorld extends Applet { public void paint(Graphics g) { g.drawString("Hello world!", 50, 25); } }If you removed the first two lines, the applet could still compile and run, but only if you changed the rest of the code like this:
As you can see, importing thepublic class HelloWorld extends java.applet.Applet { public void paint(java.awt.Graphics g) { g.drawString("Hello world!", 50, 25); } }Applet
andGraphics
classes lets the program refer to them later without any prefixes. Thejava.applet.
andjava.awt.
prefixes tell the compiler which packages it should search for theApplet
andGraphics
classes. Both thejava.applet
andjava.awt
packages are part of the core Java API -- API that every Java program can count on being in the Java environment. Thejava.applet
package contains classes that are essential to Java applets. Thejava.awt
package contains the most frequently used classes in the Abstract Window Toolkit (AWT), which provides the Java graphical user interface (GUI).You might have noticed that the "Hello World" application example from the previous lesson uses the
System
class without any prefix, and yet does not import theSystem
class. The reason is that theSystem
class is part of thejava.lang
package, and everything in thejava.lang
package is automatically imported into every Java program.Besides importing individual classes, you can also import entire packages. Here's an example:
In the Java language, every class is in a package. If the source code for a class doesn't have a package statement at the top, declaring the package the class is in, then the class is in the default package. Almost all of the example classes in this tutorial are in the default package. See Creating and Using Packages for information on using theimport java.applet.*; import java.awt.*; public class HelloWorld extends Applet { public void paint(Graphics g) { g.drawString("Hello world!", 50, 25); } }package
statement.Within a package, all classes can refer to each other without prefixes. For example, the
java.awt Component
class refers to thejava.awt Graphics
class without any prefixes, without importing theGraphics
class.
The "Hello World" Applet |