Handling Errors Using Exceptions |
The following error message is one of two similar error messages you will see if you try to compile the class InputFile, because theInputFile
class contains calls to methods that throw exceptions when an error occurs:The Java language requires that methods either catch or specify all checked exceptions that can be thrown within the scope of that method. (Details about what this actually means are covered in the next section, Java's Catch or Specify Requirement.) If the compiler detects a method, such as those inInputFile.java:8: Warning: Exception java.io.FileNotFoundException must be caught, or it must be declared in throws clause of this method. fis = new FileInputStream(filename); ^InputFile
, that doesn't meet this requirement, it issues an error message like the one shown above and refuses to compile the program.Let's look at
InputFile
in more detail and see what's going on.The
InputFile
class wraps aFileInputStream
and provides a method,getLine
, for reading a line from the current position in the input stream.The compiler prints the first error message because of the bold line in the above code listing. The bold line creates a new// Note: This class won't compile by design! import java.io.*; class InputFile { FileInputStream fis; InputFile(String filename) { fis = new FileInputStream(filename); } String getLine() { int c; StringBuffer buf = new StringBuffer(); do { c = fis.read(); if (c == '\n') // UNIX new line return buf.toString(); else if (c == '\r') { // Windows 95/NT new line c = fis.read(); if (c == '\n') return buf.toString(); else { buf.append((char)'\r'); buf.append((char)c); } } else buf.append((char)c); } while (c != -1); return null; } }FileInputStream
object and uses it to open a file whose name is passed into theFileInputStream
constructor.So what should the
FileInputStream
do if the named file does not exist on the file system? Well, that depends on what the program using theFileInputStream
wants to do. The implementers ofFileInputStream
have no idea what theInputFile
class wants to do if the file does not exist. Should theFileInputStream
kill the program? Should it try an alternate filename? Should it just create a file of the indicated name? There's no possible way theFileInputStream
implementers could choose a solution that would suit every user ofFileInputStream
. So, they punted, or rather, threw, an exception. If the file named in the argument to theFileInputStream
constructor does not exist on the file system, the constructor throws ajava.io.FileNotFoundException
. By throwing an exception,FileInputStream
allows the calling method to handle the error in whatever way is most appropriate for it.As you can see from the code, the
InputFile
class completely ignores the fact that theFileInputStream
constructor can throw an exception. However, as stated previously, the Java language requires that a method either catch or specify all checked exceptions that can be thrown within the scope of that method. Because theInputFile
class does neither, the compiler refuses to compile the program and prints an error message.In addition to the first error message shown above, you also see the following similar error message when you compile the
InputFile
class:TheInputFile.java:15: Warning: Exception java.io.IOException must be caught, or it must be declared in throws clause of this method. while ((c = fis.read()) != -1) { ^InputFile
class'sgetLine
method reads from theFileInputStream
that was opened inInputFile
's constructor. TheFileInputStream
read
method throws a java.io.IOException if for some reason it can't read from the file. Again, theInputFile
class makes no attempt to catch or specify this exception. Thus you see the second error message.At this point, you have two options. You can either arrange to catch the exceptions within the appropriate methods in the
InputFile
class, or theInputFile
methods can "duck" and allow other methods further up the call stack to catch them. Either way, theInputFile
methods must do something, either catch or specify the exceptions, before theInputFile
class can be compiled. For the diligent, there's a class, InputFileDeclared, that fixes the bugs inInputFile
by specifying the exceptions.The next section describes in further detail Java's Catch or Specify Requirement. The subsequent sections show you how to comply to the requirement.
Handling Errors Using Exceptions |