The Catch or Specify Requirement
trystatement that catches the exception. The
trymust provide a handler for the exception, as described in Catching and Handling Exceptions.
A method that specifies that it can throw the exception. The method must provide a
throwsclause that lists the exception, as described in Specifying the Exceptions Thrown by a Method.
Code that fails to honor the Catch or Specify Requirement will not compile.
The Three Kinds of Exceptions
The first kind of exception is the checked exception. These are exceptional conditions that a well-written application should anticipate and recover from. For example, suppose an application prompts a user for an input file name, then opens the file by passing the name to the constructor for
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.
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.
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.
Bypassing Catch or Specify
Some programmers consider the Catch or Specify Requirement a serious flaw in the exception mechanism and bypass it by using unchecked exceptions in place of checked exceptions. In general, this is not recommended. The section Unchecked Exceptions — The Controversy talks about when it is appropriate to use unchecked exceptions.