Source

import java.applet.*;
import java.awt.*;

public class AnAppletWithParms extends Applet {
	public void init() {
		String parmString =  getParameter("number");
		System.err.println("The single parameter: " + parmString);
		n = Integer.parseInt(parmString);
		setBackground(Color.blue);
	}

	public void paint(Graphics g) {
		System.out.println("In paint: n = " + n);
		n++;
	}

	int n;
} 

Description and Objective

• Shows how to place parameters in the HTML file
• Shows how to retrieve the parameter within the applet code
• Shows how to convert String values to int
• Illustrates when the paint method is invoked

Notes

• A Java application receives its parameters the array of String objects passed to main

  public static void main(String [] args) {...}
  				

• Since applets don't use use a main method, an alternative method for passing parameters to the applet is employed

• Parameters are specified in the HTML file using the <param> tag using a name/value representation

  <param name="number" value="1">
  				

  • In the above case, the name of the parameter is number and its value is 1
• The param tag appears betwen the <applet> and </applet> tags
• An arbitrary number of parameters may be specified

  <applet code="AppletWithParms.class" width=50 height=50>
  	<param name="number" value="1">
  </applet>
  				

• One can retrieve the value of the parameter via its name using the getParameter method
  • This method accepts a parameter's name as its input and returns the value of the parameter (as a String)
• Once retrieved, the value of the parameter can be used. If the parameter is to be used in a fashion other than as a String, a conversion may be required.

This Applet's Behavior

• A parameter named number is passed to the applet with a value of 1.

• The init Method
  • The parameter is retrieved in the init method and its value is converted using the parseInt method of the Integer wrapper class, and then stored in the instance variable n.
    • parseInt is a static method-- it has no receiver (and requires none, since the only thing it needs is the String is is converting -- and that is passed as a parameter.
    • A consequence of parseInt being static (and thus not having a receiver) is that the method is invoked using the name of the class (in this case Integer) rather than a receiver object.
    • One of the reasons the Integer class is called a wrapper is because it contains (or wraps itself around) several methods that provide support for working for integers (parseInt, for exmaple).
  • A diagnostic message is also printed in init
  • The background is set to blue
    • Since nothing is being done to the applet's surface, and thus the applet would remain blank, setting the background to blue indicates that the applet loaeded OK.
• The paint Method
  • paint prints out a diagnostic message with the value of the instance variable n and then increments n.
    • Successive invocations of paint will print out increasing values of n.
    • How can you make this happen?

Things to Do

• The Applet class
  • Find the getParameter method
    • What does the related getParameterInfo method do?
• The Integer wrapper class
  • What package is it in?
  • Find the parseInt method
  • What are the other wrapper classes analogous to Integer for other types

Place the following three applets on a single page:

1. Go back to the AppletSubClass example and pass in three integers corresponding to an RGB value and use the appropriate Color constructor to set the background color.
2. Modify AnAppletWithParms so that it accepts three parameters: operand1, operand2, and operator. If operator is a +, the sum of the two numbers is sent to System.out, similarly for -, *, and /
3. Same as the first applet, but this time, the parameter should be the name of a color (you can use names corresponding to several of the color constants in the Color class-- or use your own color names).
   • There is no Color constructor that accepts the name of a color (as a String). Instead, you must have logic to test the parameter (e.g., a series of conditionals) and based on the value of the parameter, you would create the appropriate color using the RGB constructor for String.
   • To test two String objects for equality, you must use the equals method (the == operator when used on reference variables for objects merely tests if they reference (i.e., point to) the same object. Thus:

     String x = "Hello",
            y = "Hello",
            z = x;
     							 	 	 	 			 	

     would result in:

     x == y	false	not same object two different String objects containing the value Hello
     x.equals(y)	true	the values are equal (by whatever semantics were used in the equals method for the Stringclass)
     x == z	true	z was initialized with a reference to the same object x is referecning
     x.equals(z)	true	the values are equal (by whatever semantics were used in the equals method for the Stringclass)