Java 天地·Core Java·Lecture Notes·Unit 10 - Applications, File Handling and Exceptions

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Activity 4 — Catching an exception

A common problem when processing numeric input from users is that they enter real values, or values with comma’s etc. when, say, an integer is required. So for example a program might request an age from the user (expecting an integer), and a small child might enter ‘3.5’ or ‘3 and a half’.

The application below uses a Swing dialogue box to request and get input from the user. This showInputDialogue() method returns the value entered by the user as a String , whether we wanted a String , or char , or int or double or whatever. One convenient way to convert from a String to an int is to use the class method parseInt() provided by the String class. This method converts from a String . , such as ‘21’ (i.e. character ‘2’ followed by character ‘1’) to the int 21.

When the application is run it first displays a message in the console window (via System.out.println() ):

Age parsing application

Then the application presents to the user an input dialogue, requesting their age:

four.gif (2880 bytes)

Then the application displays the current age (having parsed it to an int), and the age of the person next birthday. Then the application terminates:

The problem occurs if what the user enters is not a valid integer (say 4.5). Then the application crashes with the following messages:

The listing of the application is as follows:

import javax.swing.*;
public class Exception1
{
public static void main( String args[] )
    {
    String ageString;
    int age;
    System.out.println("Age parsing application");
    ageString = JOptionPane.showInputDialog(null, "Enter age
    (1-100)", "Entry of age", JOptionPane.QUESTION_MESSAGE);
    age = Integer.parseInt( ageString );
    int ageNextBirthday = (age + 1);
    System.out.println("Age now : " + age);
    System.out.println("Age next birthday: " + ageNextBirthday);
    System.exit(0);
    }
} // class

Find out about parseInt() and the exception it throws when its String is not a valid, and amend the application to display the following message before terminating if the age entered is invalid:

sorry - your age entry of ‘<age>‘ is not a valid integer

where <age> is whatever the user entered. So if the user entered ‘4.5’ the message would be:

sorry - your age entry of ‘4.5’ is not a valid integer

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