Section 3.7
Introduction to Exceptions and try..catch
In addition to the control structures that determine the normal flow of control in a program, Java has a way to deal with "exceptional" cases that throw the flow of control off its normal track. When an error occurs during the execution of a program, the default behavior is to terminate the program and to print an error message. However, Java makes it possible to "catch" such errors and program a response different from simply letting the program crash. This is done with the try..catch statement. In this section, we will take a preliminary, incomplete look at using try..catch to handle errors. Error handling is a complex topic, which we will return to in Chapter 8.
3.7.1 Exceptions
The term exception is used to refer to the type of error that one might want to handle with a try..catch. An exception is an exception to the normal flow of control in the program. The term is used in preference to "error" because in some cases, an exception might not be considered to be an error at all. You can sometimes think of an exception as just another way to organize a program.
Exceptions in Java are represented as objects of type Exception. Actual exceptions are defined by subclasses of Exception. Different subclasses represent different types of exceptions. We will look at only two types of exception in this section: NumberFormatException and IllegalArgumentException.
A NumberFormatException can occur when an attempt is made to convert a string into a number. Such conversions are done by the functions Integer.parseInt and Double.parseDouble. (See Subsection 2.5.7.) Consider the function call Integer.parseInt(str) where str is a variable of type String. If the value of str is the string "42", then the function call will correctly convert the string into the int 42. However, if the value of str is, say, "fred", the function call will fail because "fred" is not a legal string representation of an int value. In this case, an exception of type NumberFormatException occurs. If nothing is done to handle the exception, the program will crash.
An IllegalArgumentException can occur when an illegal value is passed as a parameter to a subroutine. For example, if a subroutine requires that a parameter be greater than or equal to zero, an IllegalArgumentException might occur when a negative value is passed to the subroutine. How to respond to the illegal value is up to the person who wrote the subroutine, so we can't simply say that every illegal parameter value will result in an IllegalArgumentException. However, it is a common response.
One case where an IllegalArgumentException can occur is in the valueOf function of an enumerated type. Recall from Subsection 2.3.3 that this function tries to convert a string into one of the values of the enumerated type. If the string that is passed as a parameter to valueOf is not the name of one of the enumerated type's values, then an IllegalArgumentException occurs. For example, given the enumerated type
enum Toss { HEADS, TAILS }
Toss.valueOf("HEADS") correctly returns the value Toss.HEADS, while Toss.valueOf("FEET") results in an IllegalArgumentException.
3.7.2 try..catch
When an exception occurs, we say that the exception is "thrown". For example, we say that Integer.parseInt(str) throws an exception of type NumberFormatException when the value of str is illegal. When an exception is thrown, it is possible to "catch" the exception and prevent it from crashing the program. This is done with a try..catch statement. In somewhat simplified form, the syntax for a try..catch is:
try {
statements-1
}
catch ( exception-class-name variable-name ) {
statements-2
}
The exception-class-name could be NumberFormatException, IllegalArgumentException, or some other exception class. When the computer executes this statement, it executes the statements in the try part. If no error occurs during the execution of statements-1, then the computer just skips over the catch part and proceeds with the rest of the program. However, if an exception of type exception-class-name occurs during the execution of statements-1, the computer immediately jumps to the catch part and executes statements-2, skipping any remaining statements in statements-1. During the execution of statements-2, the variable-name represents the exception object, so that you can, for example, print it out. At the end of the catch part, the computer proceeds with the rest of the program; the exception has been caught and handled and does not crash the program. Note that only one type of exception is caught; if some other type of exception occurs during the execution of statements-1, it will crash the program as usual.
By the way, note that the braces, { and }, are part of the syntax of the try..catch statement. They are required even if there is only one statement between the braces. This is different from the other statements we have seen, where the braces around a single statement are optional.
As an example, suppose that str is a variable of type String whose value might or might not represent a legal real number. Then we could say:
try {
double x;
x = Double.parseDouble(str);
System.out.println( "The number is " + x );
}
catch ( NumberFormatException e ) {
System.out.println( "Not a legal number." );
}
If an error is thrown by the call to Double.parseDouble(str), then the output statement in the try part is skipped, and the statement in the catch part is executed.
It's not always a good idea to catch exceptions and continue with the program. Often that can just lead to an even bigger mess later on, and it might be better just to let the exception crash the program at the point where it occurs. However, sometimes it's possible to recover from an error. For example, suppose that we have the enumerated type
enum Day { MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY, SUNDAY }
and we want the user to input a value belonging to this type. TextIO does not know about this type, so we can only read the user's response as a string. The function Day.valueOf can be used to convert the user's response to a value of type Day. This will throw an exception of type IllegalArgumentException if the user's response is not the name of one of the values of type Day, but we can recover from the error easily enough by asking the user to enter another response. Here is a code segment that does this. (Converting the user's response to upper case will allow responses such as "Monday" or "monday" in addition to "MONDAY".)
Day weekday; // User's response as a value of type Day.
while ( true ) {
String response; // User's response as a String.
System.out.print("Please enter a day of the week: ");
response = TextIO.getln();
response = response.toUpperCase();
try {
weekday = Day.valueOf(response);
break;
}
catch ( IllegalArgumentException e ) {
System.out.println( response + " is not the name of a day of the week." );
}
}
// At this point, a legal value has definitely been assigned to weekday.
The break statement will be reached only if the user's response is acceptable, and so the loop will end only when a legal value has been assigned to weekday.
3.7.3 Exceptions in TextIO
When TextIO reads a numeric value from the user, it makes sure that the user's response is legal, using a technique similar to the while loop and try..catch in the previous example. However, TextIO can read data from other sources besides the user. (See Subsection 2.4.5.) When it is reading from a file, there is no reasonable way for TextIO to recover from an illegal value in the input, so it responds by throwing an exception. To keep things simple, TextIO only throws exceptions of type IllegalArgumentException, no matter what type of error it encounters. For example, an exception will occur if an attempt is made to read from a file after all the data in the file has already been read. In TextIO, the exception is of type IllegalArgumentException. If you have a better response to file errors than to let the program crash, you can use a try..catch to catch exceptions of type IllegalArgumentException.
For example, suppose that a file contains nothing but real numbers, and we want a program that will read the numbers and find their sum and their average. Since it is unknown how many numbers are in the file, there is the question of when to stop reading. One approach is simply to try to keep reading indefinitely. When the end of the file is reached, an exception occurs. This exception is not really an error -- it's just a way of detecting the end of the data, so we can catch the exception and finish up the program. We can read the data in a while (true) loop and break out of the loop when an exception occurs. This is an example of the somewhat unusual technique of using an exception as part of the expected flow of control in a program.
To read from the file, we need to know the file's name. To make the program more general, we can let the user enter the file name, instead of hard-coding a fixed file name in the program. However, it is possible that the user will enter the name of a file that does not exist. When we use TextIO.readfile to open a file that does not exist, an exception of type IllegalArgumentException occurs. We can catch this exception and ask the user to enter a different file name. Here is a complete program that uses all these ideas:
/**
* This program reads numbers from a file. It computes the sum and
* the average of the numbers that it reads. The file should contain
* nothing but numbers of type double; if this is not the case, the
* output will be the sum and average of however many numbers were
* successfully read from the file. The name of the file will be
* input by the user.
*/
public class ReadNumbersFromFile {
public static void main(String[] args) {
while (true) {
String fileName; // The name of the file, to be input by the user.
TextIO.put("Enter the name of the file: ");
fileName = TextIO.getln();
try {
TextIO.readFile( fileName ); // Try to open the file for input.
break; // If that succeeds, break out of the loop.
}
catch ( IllegalArgumentException e ) {
TextIO.putln("Can't read from the file \"" + fileName + "\".");
TextIO.putln("Please try again.\n");
}
}
// At this point, TextIO is reading from the file.
double number; // A number read from the data file.
double sum; // The sum of all the numbers read so far.
int count; // The number of numbers that were read.
sum = 0;
count = 0;
try {
while (true) { // Loop ends when an exception occurs.
number = TextIO.getDouble();
count++; // This is skipped when the exception occurs
sum += number;
}
}
catch ( IllegalArgumentException e ) {
// We expect this to occur when the end-of-file is encountered.
// We don't consider this to be an error, so there is nothing to do
// in this catch clause. Just proceed with the rest of the program.
}
// At this point, we've read the entire file.
TextIO.putln();
TextIO.putln("Number of data values read: " + count);
TextIO.putln("The sum of the data values: " + sum);
if ( count == 0 )
TextIO.putln("Can't compute an average of 0 values.");
else
TextIO.putln("The average of the values: " + (sum/count));
}
}