Isn't an unchecked exception that is caught in

问题:

I was told that in Java, unchecked exceptions can be caught in a try block, but if it's caught, isn't it called a checked exception?

回答1:

Unchecked exceptions are exceptions that don't need to be caught in a try-catch block. Unchecked exceptions are subclasses of the RuntimeException or Error classes.

Checked exceptions are exceptions that need to be caught in a try-catch block.

The definition of checked and unchecked exceptions can be found in Section 11.2: Compile-Time Checking of Exceptions of The Java Language Specification:

The unchecked exceptions classes are the class RuntimeException and its subclasses, and the class Error and its subclasses. All other exception classes are checked exception classes.

Just because an unchecked exception is caught in a catch block does not make it a checked exception -- it just means that the unchecked exception was caught, and was handled in the catch block.

One could catch an unchecked exception, and then throw a new checked exception, so any methods calling that method where an unchecked exception could occur and force an method that calls it to handle some kind of exception.

For example, a NumberFormatException which can be thrown when handling some unparsable String to the Integer.parseInt method is an unchecked exception, so it does not need to be caught. However, the method calling that method may want its caller to properly handle such a problem, so it may throw another exception which is checked (not a subclass of RuntimeException.):

public int getIntegerFromInput(String s) throws BadInputException {
    int i = 0;
    try {
        i = Integer.parseInt(s);
    catch (NumberFormatException e) {
        throw new BadInputException();
    }

    return i;
}

In the above example, a NumberFormatException is caught in the try-catch block, and a new BadInputException (which is intended to be a checked exception) is thrown.

Any caller to the getIntegerFromInput method will be forced to catch a BadInputException, and forced to deal with bad inputs. If the NumberFormatException were not to be caught and handled, any callers to this method would have to handle the exception correctly.

(Also, it should be noted, eating an exception and doing something that is not really meaningful is not considered a good practice -- handle exceptions where meaningful exception handling can be performed.)

From The Java Tutorials:

• Lessons: Exceptions
  • The Catch or Specify Requirement - discusses checked exceptions.
  • Chained Exceptions - the practice of catching an exception and throwing a new one, as the example above.
  • Unchecked Exceptions — The Controversy

回答2:

No, it's not called a checked exception just because it is caught. A catch block can be written to catch any kind of Exception or Error. Checked exceptions are those that are subject to the Catch or Specify Requirement, meaning that you are required to either catch them or declare that your method may throw them. You can think of the term checked as meaning that the compiler will check to make sure you adhere to the catch or specify requirement. Errors and RuntimeExceptions are called unchecked exceptions because the compiler does not enforce this requirement on them.

回答3:

I think the distinction is that the compiler will flag uncaught checked exceptions and methods that throw checked exceptions but don't declare them in the method signature at compile time.

Unchecked exceptions don't require declaration or catching, but neither are prohibited. The compiler just doesn't identify them as errors.