Handling Errors Using Exceptions

Your First Encounter with Java Exceptions

The following error message is one of two similar error messages you will see if you try to compile the class InputFile, because the InputFile class contains calls to methods that throw exceptions when an error occurs:

InputFile.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);
              ^

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 in 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 a FileInputStream and provides a method, getLine, for reading a line from the current position in the input stream.

    // 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;
    }
}

The compiler prints the first error message because of the bold line in the above code listing. The bold line creates a new FileInputStream object and uses it to open a file whose name is passed into the FileInputStream 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 the FileInputStream wants to do. The implementers of FileInputStream have no idea what the InputFile class wants to do if the file does not exist. Should the FileInputStream kill the program? Should it try an alternate filename? Should it just create a file of the indicated name? There's no possible way the FileInputStream implementers could choose a solution that would suit every user of FileInputStream. So, they punted, or rather, threw, an exception. If the file named in the argument to the FileInputStream constructor does not exist on the file system, the constructor throws a java.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 the FileInputStream 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 the InputFile 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:

InputFile.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) {
                            ^

The InputFile class's getLine method reads from the FileInputStream that was opened in InputFile's constructor. The FileInputStream read method throws a java.io.IOException if for some reason it can't read from the file. Again, the InputFile 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 the InputFile methods can "duck" and allow other methods further up the call stack to catch them. Either way, the InputFile methods must do something, either catch or specify the exceptions, before the InputFile class can be compiled. For the diligent, there's a class, InputFileDeclared, that fixes the bugs in InputFile 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