已检查与未检查的异常

Question

I've studied that: With an unchecked exception, however, the compiler doesn't force client programmers either to catch the exception or declare it in a throws clause. In fact, client programmers may not even know that the exception could be thrown. eg, StringIndexOutOfBoundsException thrown by String's charAt() method.

what its mean?

according to that code there is no need to put try catch block in code, but i've seen compiler forces to put the code in try catch block.

I'm very confused what they are exactly?

Answer 1

Unchecked exceptions are those that extend RuntimeException class. Compiler will never force you to catch such exception or force you to declare it in the method using throws keyword. All other exception types (that do not extend RuntimeException ) are checked and therefore must be declared to be thrown and/or catched.

Checked exceptions are used when you want the caller of your method (ie the user of your API) to explicitly handle the exceptional case in your API. Checked exceptions are declared when you believe the call will be able to do something meaningful with that exceptional case, like retrying the call, rolling changes back or converting it into some user-readable error message.

If you believe that there is nothing useful the call can do about the exception (especially when it represents a bug, or a wrong usage of your API), then the exception should be unchecked. Also, an API with too many checked exceptions can be annoying to program with (eg try using java reflection API=)

Answer 2

• Checked Exceptions are useful for handling events that occur in the normal operation of a program. An example would be an IOException that is thrown when a file cannot be opened. These exceptions occur even if there is nothing wrong with the program. It is necessary, therefore, to tell the program how to handle the exception.
• Unchecked exceptions are useful for identifying defects in the code. For instance, a NullPointerException is thrown when a value is read on a null object. Thus an Unchecked Exception represents a problem that requires a manual fix by the programmer. It is reasonable for the program to crash in order to avoid erroneous behavior, so a try-catch block is not required (but might be desirable in order to provide mitigation, such as displaying an error to the user).

Answer 3

What is your question exactly? Compilers shouldn't (and won't) enforce you to try/catch unchecked exceptions, that would go against exactly what they are.

The general idea is that checked exceptions are something you may be able to foresee but may be based on input that is out of your control and that you have to deal with. Unchecked exceptions will usually represent bugs in your program.

There's a number of people that think checked exceptions are a mistake in the Java platform and they only use them very sparingly or not at all. You can read more about this debate by searching google.

Answer 4

It is because,

1. Unchecked Exceptions are not a result of programmer's fault. Instead they are the serious consequences where we(programmer) aren't expected to do much with it.
2. In case of Checked Exception, it is an exception generated because of the programmer's fault & often can be resolved by programmer itself.

Check the following links :

Why RunTime Exceptions are unchecked ?
Checked vs Unchecked Exception ?

Answer 5

All Exceptions are part of run time and not compile time. There are two kinds of exceptions checked exceptions and unchecked exceptions. Examples of checked exceptions are IO Exceptions, ClassNotFound Exception and examples of unchecked exceptions are runtime exceptions. In the case of checked exceptions, error or warning message comes at compile time so that the user will take care of it at runtime by using throws keyword, which is used to throw exceptions to the default catch mechanism system. But in case of unchecked exceptions warning is not there at compile time.

Answer 6

**Checked Exceptions

Exceptions which are to be checked or handled or should be taken care during the time of writing the code are called as checked exceptions. For eg: 1. we have FileNotFoundException -->which will be occured when we are writing some code related to file classes. There will e defenetly posibility of non existence of file. In such case in order to handle them , we are forced to handle those exception for sure. 2. one more example is ParseException ,which will be occured when we are dealing with date functions.

UnChecked Exceptions

These are the exceptions that are optional to be handled during the time of coding. Its up to us whether we handle them or not. In case if we fail to handle them, There is a chance of getting runtime errors during the exceution. For eg: We have something called NullPointerException,ArithemeticException,NosSuchElementFoundException and so on. These are like optional things we dont even have to handle them. More over even jvm or compiler will not recommend us to handle them.**

Answer 7

in simple words,

• checked exceptions are those which can be and should be handled by your code(therefore compiler forces you to handle them)
• unchecked exceptions are those which lie beyond programmer's control(therefore compiler doesn't forces you to handle them)

use the same rule even while creating your custom exceptions.