import java.applet.Applet;
import java.awt.Graphics;

public class EchoApplet extends Applet {
  String s;

  public void init () {
    s = getParameter("text");
    if (s == null) s = "What, no parameter to echo?";
  }

  public void paint(Graphics g) {
    g.drawString(s, 40, 50);
    g.drawString(s, 40, 70);
  }
}

Importing classes

We begin with two import statements: by importing the java.applet.Applet and java.awt.Graphics classes we can later refer to these classes by the short names Applet and Graphics and we have access to the classes of the Java development environment for creating applets and for drawing graphics on the screen.

Defining the applet subclass

Every applet is a subclass of the java.applet.Applet class. So it has access to all methods for creating and using applets. For example, we can use the method getParameter in this class to pass parameters from the Web page to our applet. The extends keyword indicates that EchoApplet is a subclass of the Applet class. The EchoApplet class is declared public so that it can be accessed by the parent class.

Types

Every object belongs to a class or is of basic type (integer, boolean). In the line String s; we only declare s to be a String class. In the initialization of the applet, when we assign a value to it, an object or instance of this class is created with the name s.

The init method

The initialization init of the applet consists of reading the parameter text from the Web page that includes the applet and of creating an object or instance of the String class with the name s having the value of text.
The notation = is used for assignment.
In the if-clause we check whether the parameter text is indeed present in the Web page, and if not, we initialize the string s to What, no parameter to echo?.
The notation == is used for equality testing and null is a special value that indicates that a variable does not refer to any object. In the case of no parameter passed to the applet, i.e., in case the PARAM tag is not present, you should see on the screen something like the following

The init method is invoked only once when the applet is loaded in the Web browser or appletviewer.

The paint method

The "painting" of the screen paint takes care of the two echoes. The Graphics object g passed into the paint method refers to the applet's on-screen drawing context. The first argument to the Graphics drawString method is the string to draw onscreen. The second and third arguments are the (x,y) position of the lower left corner of the text onscreen. The applet's coordinate system starts at (0,0), which is at the upper left corner of the applet's display area. The positive x direction is to the right and the positive y direction is downward.

Let us look once more on the way the drawString method is called. It is an example of the following more general dot-notation: object.method(arguments). First you mention the receiving object, then you select some method from the class to which the object belongs, and finally you apply the chosen method on the arguments.