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?
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 (i.e 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 (e.g. try using java reflection API=)
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).
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.
It is because,
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.
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 ?
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.
**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.**
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.
Related
In Java, there are two types of Exceptions, mainly unchecked and checked exceptions. Checked exceptions allow the program to recover when they are caught and handled in a ‘catch’ block. Unchecked exceptions just cause your program to crash, since they are not caught and the error bubbles up all the way got the main method without being caught, if I recall correctly.
That said, in Mono, there are ApplicationExceptions and SystemExceptions and I was wondering if these serve the same purpose in mono.
No they are not. The distinctions are different.
In Java, the checked versus unchecked distinction is about whether Java code needs to deal with the exception:
unchecked exceptions don't need to be dealt with
checked exceptions need to be either caught or declared in the signature of the enclosing method.
(You should chose between declaring an exception as checked or unchecked according to whether you expect / want the caller to handle it. For example, you typically want the app to do something to recover from an IOException, but a NullPointerException is usually a bug and cannot be handled beyond logging and bailing out.)
By contrast, ApplicationExceptions versus SystemExceptions in .NET is about the meaning of the exceptions; see Difference Between Application Exception and System Exception. The ostensible purpose was to allow a program to distinguish between framework and custom exceptions. (But it doesn't really work ... in practice ... because programmers don't follow the guidelines / conventions.)
I'm checking really only to see if the distinction would crash a running program if SystemException was thrown instead of an ApplicationException
You can't make that generalization. The application crashes if either kind of exception (or any other kind) is thrown and not caught1. And that is true for Java exceptions as well.
1 - In Java, this depends on the behavior of the default exception handler.
This question already has answers here:
Difference between java.lang.RuntimeException and java.lang.Exception
(12 answers)
Closed 6 years ago.
I am curious as to how certain exceptions fall into checked exception category whereas others fall on the other ?
I know the basic difference between compile time error (syntactical error etc - this is intuitive and understandable.) and runtime error(null pointer exception etc.).
But I will prefer a more detailed analysis , as to how compile time error is detected and why does IO exception falls in compile time , as Input and output is something that is expected only during runtime ?
Also some discussion on Illegal Argument exception is detected?
I don't think there is a rule. In old times, checked exceptions was way to go, but today people tend to like unchecked exceptions better. You will probably find a lot of discussions around this topic on the internet. You can think of checked exceptions as: "You really need to handle this here or else things will go really bad for you". Personally I usually convert checked exceptions to unchecked exceptions if I need to handle them further up the application stack.
At the VERY core, a "checked" exception is usually caused by external sources e.g. IOException (so at compile time) whereas a "runtime" exception can happen, well, anywhere and is usually dependent on variables (which can change at runtime). It is the programmers duty to prevent all runtime exceptions (checking for null, checking indexes etc) whereas a checked exception can be caused by e.g. a file being renamed/removed. These two have a lot of overlap and hence a specific 'border' is impossible to define.
Some exceptions have been decided to be labeled as runtime because forcing the programmer to catch and handle them each time would obfuscate the code way too much. Checked exceptions usually don't occur too often but when they do, need to handled accordingly (closing files in a finally block etc).
I don't believe there's a definite rule which separates the two, but there has definitely been quite some though process behind it.
How is IllegalArgumentException a runtime exception where IO exception
is a checked exception ?
This is because IllegalArgumentException extends RuntimeException (called unchecked exceptions or runtime exceptions) whereas IOException extends Exception (called as checked exceptions).
In other words, if any exception object (is of type java.lang.Exception) is thrown from a method or code block then it will be verified at compile time whereas if an exception object is of type RuntimeException, then it will NOT be validated at compile time i.e., it will be known only during the actual execution (runtime) of the program.
Also, note that, checked exceptions are meant for compile time validations (i.e., we can provide a recovery mechanism by catching the exception) and the compiler ensures that all the checked exceptions are either thrown using throws keyword or caught using catch which is known as Catch or Specify, you can look here.
Specifically it makes sense for IllegalArgumentException to be unchecked because it's thrown in a large number of core Java APIs. If an exception is thrown practically everywhere making it checked does no good.
I have a hard time interpreting the meaning of these two (seemingly simple) sentences:
"Checked exceptions are checked by the compiler at compile time"
What does this mean? That the compiler checks if all checked exceptions (that are thrown in de code) are caught?
"Unchecked exceptions are checked at runtime, not compile time"
What does "checked" mean in this sentence? I thought unchecked exceptions were just thrown at runtime?
At compilation time
There are two types :
Checked: means if the method thrown to the required exception
Unhandled checked exception: if the method didn't thrown to the required exception, results in a compilation error.
At run time
Called Unchecked Exception occurs at runtime and need not to be explicitly handled.
RuntimeException and its subclasses or Error and its subclasses all fall under Unchecked.
In my opinion, the 2nd statement is incorrect. Or if it is correct, it is using "checked" to mean something different to what it means in the first sentence. And that's an misleading and unhelpful thing to do.
I could maybe be persuaded otherwise if you provided the context (and source) for these sentences.
I thought unchecked exceptions were just thrown at runtime?
They also are (can be) caught at runtime ... and maybe that's what the quote means. But if that is what it means, then it is twisting the normal meaning of "checking" ... see previous note about "misleading and unhelpful".
It might be clearer to read them as this:
"Checked exceptions are (tested to see if there is a matching try/catch) by the compiler at compile time" - if not, the code will not compile
"Unchecked exceptions are (tested to see if there is a matching try/catch) at runtime, not compile time" - if not, the code will crash
I agree that the Unchecked exceptions are checked part is really misleading. The better interpretation would be to divide all Java throwables into three categories:
Checked exceptions
Runtime exceptions
Errors
I personally prefer to think of Checked exceptions as something obligatory to catch in the code, and Runtime exceptions - as something optional to catch. You can process both (e.g. for logging), but you should try to recover just from the Checked ones.
Checked exceptions tend to introduce lots of redundant catch clauses. In some cases it could make sense to catch a Checked exception, wrap it in some Unchecked one and propagate it further.
Always keep in mind Item 41 from Effective Java: Avoid unnecessary use of checked exceptions. In other words, don't use checked exceptions for conditions from which the caller could not possibly recover, or for which the only foreseeable response would be for the program to exit.
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?
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 (i.e 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 (e.g. try using java reflection API=)
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).
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.
It is because,
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.
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 ?
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.
**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.**
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.
I am following this great discussion at SO, titled: The case against checked exceptions , but I am unable to follow where exactly RuntimeException should be used and how it is different from normal Exceptions and its subclasses. Googling gave me a complex answer, that is, it should be used to deal with programming logic errors and should be thrown when no Exception should normally occur, such as in the default block of switch-case construct.
Can you please explain RuntimeException in greater detail here. Thanks.
I am unable to follow where exactly
RuntimeException should be used
That's probably because you are looking at an argument, i.e. people are disagreeing about exactly this point.
and
how it is different from normal
Exceptions and its subclasses.
Very simple: All subclasses of Exception (except for RuntimeException and its subclasses) are checked i.e. the compiler will reject the code unelss you catch or declare them in the method signature. However, subclasses of RuntimeException are unchecked.
Googling gave me a complex answer,
that is, it should be used to deal
with programming logic errors and
should be thrown when no Exception
should normally occur, such as in the
default block of switch-case
construct.
This is the conventional wisdom, which says that for everything that a program can usefully deal with, you should use checked exceptions because then the compiler will force you to deal with them. Conversely, programs can typically not deal usefully with programmer errors, thus they don't have to be checked. This is how the Java Standard API uses RuntimeException.
The discussion you linked to is sparked by the view of some people (this includes me) who think that checked exceptions lead to bad code and should therefore not be used. Since you can't disable exception checking in the compiler, the only way to do this is to use only RuntimeException and its subclasses.
One observation that IMO supports this view is that the conventional wisdom of "use unchecked exceptions only for programmer error" is in fact mainly a rationalization of backwards-reasoning: there is no code safety reason why the compiler should not force you to deal with programmer errors. However, something like NullPointerException and ArrayIndexOutOfBoundsException can crop up almost anywhere, and if those were checked, nobody would ever want to program in Java. Thus, the language designers had to make a, huh, exception for those, and make them unchecked. To explain this, they came up with the "unchecked exceptions are for programmer errors" story.
Quotes from Effective Java 2nd Edition, Item 58: Use checked exceptions for recoverable conditions and runtime exceptions for programming errors
The Java programming language provides three kinds of throwables: checked exceptions, runtime exceptions, and errors. There is some confusion among programmers as to when it is appropriate to use each kind of throwable. While the decision is not always clear-cut, there are some general rules that provide strong guidance.
The cardinal rule in deciding whether to use checked exception or an unchecked one is this:
Use checked exceptions for conditions from which the caller can reasonably be expected to recover. By throwing a checked exception, you force the caller to handle the exception in a catch clause or to propagate it outward. Each checked exception that a method is declared to throw is therefore a potent indication to the API user that associated condition is a possible outcome of invoking the method.
Use runtime exceptions to indicate programming errors. The great majority of runtime exceptions indicate precondition violations. A precondition violation is simply a failure by the client of an API to adhere to the contract specified by the API specification.
Here's an example:
When trying to read a file of arbitrary name, the file may not exists. It's not strictly a programming error when a file does not exist (e.g. perhaps it did before but was then accidentally deleted). Clients may want to recover from this. Thus, FileNotFoundException is a checked exception.
If you give a null string as a filename, then NullPointerException (or perhaps an IllegalArgumentException -- another contentious debate) should be thrown. Client of the API is supposed to provide a valid string value; null isn't. As far as the API is concerned, this is a programmer error, which was easily preventable. Both of these exceptions are runtime exceptions.
Item 59: Avoid unnecessary use of checked exceptions also provides additional guidance:
Checked exceptions are a wonderful feature of the Java programming language. Unlike return codes, they force the programmer to deal with exceptional conditions, greatly enhancing reliability. That said, overuse of checked exceptions can make an API far less pleasant to use. If a method throws one or more checked exceptions, the code that invokes the method must handle the exceptions in one or more catch blocks, or it must declare that it throws the exceptions and let them propagate outward. Either way, it places a nontrivial burden on the programmer.
The burden is justified if:
the exceptional condition cannot be prevented by proper use of the API, and
the programmer using the API can take some useful action once confronted with the exception.
Unless both of these conditions hold, an unchecked exception is more appropriate.
So here's a short summary of the recommendation from Effective Java 2nd Edition:
Preventable exceptions that happen due to API user errors should be unchecked.
Exceptions that can't be handled reasonably should also be unchecked.
Otherwise, the exception should be checked.
See also
Effective Java 2nd Edition
Item 58: Use checked exceptions for recoverable conditions and runtime exceptions for programming errors
Item 59: Avoid unnecessary use of checked exceptions
Item 60: Favor the use of standard exceptions
Item 61: Throw exceptions appropriate to the abstraction
Item 62: Document all exceptions thrown by each method
Technical definition
An unchecked exception is defined as RuntimeException and its subclasses, and Error and its subclasses. They do not have to be declared in a method's throws clause.
References
JLS 11.2 Compile-Time Checking of Exceptions
Related questions
In Java, when should I create a checked exception, and when should it be a runtime exception?
When to choose checked and unchecked exceptions
The case against checked exceptions
IllegalArgumentException or NullPointerException for a null parameter?