Java 天地·Core Java·Lecture Notes·Unitt 3 - Core Java Programming Concepts

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Content Page # 51

Writing Interactive Programs - Activities

A note on displaying applications using Windows rather than a text screen

Java has a large number of classes organised into packages which are called the Java class library or the Java Applications Programming Interface (API). We have already usedio, applet in the first examples. The recommended text Deitel and Deitel uses thejavax.swing package in the introductory chapters. The classJOptionPane from thejavax.swing package is useful for its Graphical User Interface (GUI) components that make the task of data entry into a program or formatting (organising the display) of the output easier.The Java GUI and event handling will be examined in greater detail in Unit 8.

The following isGreetings.javaexample written using thejavax.swing.JOptionPane

line

Source code statements of the application GreetingsVersion2.java

import javax.swing.JOptionPane;

public class GreetingsVersion2{

public static void main(String[] args)

{

JOptionPane.showMessageDialog(null, "Good afternoon ");

System.exit(0);

}

Line 1 is animport statement. This statement tells the compiler to load the classes required for compiling the program. In this case the compiler will load theJOptionPane class from thejavax.swing package

Line 2 begins the class definition forclass GreetingsVersion2.It is important to remember that the file in which the program is saved has to have the same name i.e.GreetingsVersion2.java

Line 3 is the method main part of every Java application. Every Java application must have one method calledmain. The body of the method is enclosed within the braces{}. This includes the lines 4 and 5.

Line 4 sends a message showMessageDialog toJOptionPane. TheshowMessageDialog is a method with two arguments, the first of which is alwaysnull, and the second a String to be displayed. The method is static, which means that it is called using class name ??method name, rather than object name ??method name.

Line 5 is another static method of theSystem class used to terminate the application. TheSystem class is part of thejava.lang package which is always imported by default.

Writing Interactive Programs

The first examples only had output, and data was either 慼ardcoded?(fixed) or generated by the programs. We are more accustomed to interactive work, which means input of data to a program usually from the keyboard and ? output from the program usually on the screen.

For example if we wish to change the TicketDiscount program so that instead of the age being randomly generated, the user is expected to enter the age after which the relevant message is displayed.

We will first examine the TicketDiscount.java listed below and decide on the changes needed to produce the new version TicketDiscountVersion2.java.

line

Source code statements of the application TicketDiscount.java

import java.io.*;

class TicketDiscount {

public static void main( String args[] )

{

int age;

age = (int) ( 100 * Math.random() );

System.out.println("age is: " + age);

if( age >= 60 )

{

System.out.println( age + "is 60 or over" );

System.out.println("qualifies for discount");

}

Line 5 would need to be replaced with code what will obtain input from the user. A prompt should be displayed on the screen telling the user that input is expected e.g. 揈nter age: ?

Line 6 will no longer be necessary because the user input will be shown on the screen.

Another change would be to display a message even if there is no discount "Sorry no discount" because with interactive programs users expect a response from the program. For this reason the else branch of the selection will be added.

A note on Java input and output

In Java all data displayed on the screen or input from the keyboard is in the form of Strings. Strings are a series of characters treated as a single entity and as text.

If we need to use data in a different form, other than text e.g. numbers the String data needs to be converted. Java has a range of parse methods, which convert strings into a variety of types. parse methods that convert Strings to simple/scalar/built-in data belong to wrapper classes such as:Integer, Float, Double, Long, Boolean, Character. To convert a String sto anintwe use the methodparseInt, a metod of wrapper classInteger.

Line 8 reads a line of input.

Line 9 converts the input String to an integer.

line

Source code statements of the application TicketDiscountVersion2.java

import java.io.*;

class TicketDiscountVersion2 {

public static void main( String args[] ) throws IOException

{

int age;

String s;

BufferedReader in =new BufferedReader(new

InputStreamReader(System.in));

System.out.print("Enter age: ");

s = in.readLine();

age = Integer.parseInt(s);

if( age >= 60 )

{

System.out.println( age + " is 60 or over" );

System.out.println("qualifies for discount");

}

else

System.out.println( "Sorry no discount");

}

The changes made to the program lines 3, 5, 6, 7, 8, 9, 13, 14 are shown in boldface font.

If the user enter 54 the output will be:

Enter age: 54
Sorry no discount

Using the JOptionPane dialog box

Source code statements of the application TicketDiscountVersion3.java

import javax.swing.JOptionPane;

public class TicketDiscountVersion3{

public static void main(String[] args)

{

String s;

s = JOptionPane.showInputDialog("Enter age: ");

int age = Integer.parseInt(s);

if( age >= 60 )

JOptionPane.showMessageDialog( null, age + " is 60

or over\n qualifies for discount");

else

JOptionPane.showMessageDialog(null, "Sorry no

discount");

System.exit(0);

}

The output would be as follows:

When user inputs the number 60 and presses OK,

the message is displayed is :

Activity 5Interactive AdditionApplet

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