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try statement that catches the exception. The
try must provide a handler for the exception, as
described in
Catching and Handling Exceptions.
throws clause
that lists the exception, as described in
Specifying the Exceptions Thrown by a Method.
Not all exceptions are subject to the Catch or Specify Requirement. To understand why, we need to look at the three basic categories of exceptions, only one of which is subject to the Requirement.
java.io.FileReader.
Normally, the user provides the name of an existing, readable file, so
the construction of the FileReader object succeeds,
and the execution of the application proceeds normally. But sometimes
the user supplies the name of a nonexistent file, and the constructor
throws java.io.FileNotFoundException. A well-written
program will catch this exception and notify the user of the mistake,
possibly prompting for a corrected file name.
Checked exceptions are subject to the Catch or Specify
Requirement. All exceptions are checked exceptions, except for those
indicated by Error, RuntimeException, and
their subclasses.
The second kind of exception is the error. These are
exceptional conditions that are external to the application, and that
the application usually cannot anticipate or recover from. For
example, suppose that an application successfully opens a file for
input, but is unable to read the file because of a hardware or system
malfunction. The unsuccessful read will throw
java.io.IOError. An application might choose to catch
this exception, in order to notify the user of the problem — but
it also might make sense for the program to print a stack trace and
exit.
Errors are not subject to the Catch or Specify Requirement.
Errors are those exceptions indicated by Error and its
subclasses.
The third kind of exception is the runtime exception. These are
exceptional conditions that are internal to the application, and that
the application usually cannot anticipate or recover from. These
usually indicate programming bugs, such as logic errors or improper
use of an API. For example, consider the application described
previously that passes a file name to the constructor for
FileReader. If a logic error causes a null
to be passed to the constructor, the constructor will throw
NullPointerException. The application can catch this
exception, but it probably makes more sense to eliminate the bug that
caused the exception to occur.
Runtime exceptions are not subject to the Catch or Specify
Requirement. Runtime exceptions are those indicated by
RuntimeException and its subclasses.
Errors and runtime exceptions are collectively known as unchecked exceptions.
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