Learning Java Through Games

Chapter 13

Exception Handling and Files

13.1 Handling Exceptions .................................................................... 293

13.2 Text Files ............................................................................... 299

13.2.1 The File Chooser Dialog ........................................................ 299

13.2.2 Reading from Text Files ........................................................ 300

13.2.3 Writing to Text Files ........................................................... 303

13.3 Data Files ............................................................................... 306

13.4 Summary ................................................................................ 313

13.5 Syntax .................................................................................. 313

13.6 Important Points ....................................................................... 315

13.7 Exercises ................................................................................ 316

13.8 Lab ...................................................................................... 317

13.9 Project .................................................................................. 317

This chapter covers two important topics: files and exception handling. The topics are

covered together because every time we are working with a file, an exception can occur.

This exception can be that the file does not exist, we do not have permission to open it,

the hard disk is full and we are trying to save the file, and so on. We will cover two types

of files: text and data. For text files, we will read and write regular text to a file. When

it comes to data files, we will read and write objects to a file. We will cover checked and

unchecked exceptions. While Java forces us to handle checked exceptions, such as opening a

file, Java does not require that one handles unchecked exceptions, such as division by zero.

In the spirit of the textbook, we will describe all new Java primitives in the context of

applications. We will first extend the Notepad software and add features to read and write

to a file. Next, we will examine how the data from a bank application can be stored on the

hard disk. The latter is an interesting example because it demonstrates how to store on the

hard disk interlinked objects that belong to a multiple classes.

13.1 Handling Exceptions

Consider an application that wants to read a positive integer from the keyboard. How-

ever, the application should be bullet-proofed. In particular, the application should not

crash if the user enters a string, for example. Below is one possible way to implement such

an application.

import java . uti l .∗ ;

public c la ss Test {

public s ta tic void main( String args [] ) {

int i=−1;

while (i <=0) {

i = getNumber () ;

}

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294 Learning Java through Games

System.out.println(i);

}

public s ta tic int getNumber() {

try {

Scanner console = new Scanner ( System . in ) ;

System . out . p ri n t ("Enter a positive integer : " );

return console . nextInt () ;

} catch ( InputMismatchException exception ) {

return −1;

}

}

}

The getNumber method asks the user to enter an integer and reads the integer. How-

ever, the nextInt method will raise an exception if the user does not enter an integer. An

exception is an object that is automatically generated by Java. Every exception belongs to

an exception class. For example, InputMismatchException is an exception class. An ex-

ception of type InputMismatchException is generated when Java expects an input of one

type (e.g., an integer), but receives an input of a different type (e.g., a string). All exception

classes inherit from the Exception class. In the above code, the exception object is passed

as a parameter in the catch statement, but not used.

To handle an exception, create a try block. The try block must be followed by

either one or more catch blocks, a finally block, or both. The finally block, when

present, must be the last block in the statement.

An InputMismatchException is generated by the call to the nextInt method when the

user does not enter an integer. This exception is handled in the catch part of the statement.

As a result, the getNumber method returns −1 when the user does not enter an integer.

This signals to the main method that something went wrong and the user is required to

enter the integer again. In order for this approach to work, we assume that the user must

enter a positive integer.

Next, suppose that we require that the user enters an arbitrary integer, not just a

positive integer. In this case, the getNumber method should return two pieces of data: the

integer that is entered (assuming it is an integer) and a Boolean value that tells us if the

user entered an integer. Of course, as we already know, a method cannot return two pieces

of data. The only way to circumvent this restriction is to give the method an address where

to write the result. Here is the new implementation.

import java . uti l .∗ ;

public c la ss Test {

public s ta tic void main( String args [] ) {

int a[] = new i n t [1];

int i;

while( ! getNumber(a ) ) ;

i=a[0];

System.out.println(i);

}

p ub l ic static boolean getNumber( int [] a) {