when in eclipse i use a method which throws an exception, it usually complains if it is not surrounded by a try/catch or if the exception is not thrown again. But for some exceptions (e.g. Integer.parseInt(string)) eclipse won't complain.
How do i set eclipse to complain for all not handled exceptions??
Thanks!
The simple answer is that you can't.
The longer answer is:
Checked versus unchecked exceptions is a fundamental part of the Java language.
It is not the role of the Eclipse compiler to give compilation errors or warnings for valid and perfectly acceptable Java.
You wouldn't want it to either, considering that the majority of statements could (in theory) throw or propagate exceptions such as NullPointerException, ArrayIndexOutOfBoundsException, OutOfMemoryError, and so on.
Yes, there are one or two "mistakes" ... such as NumberFormatException being an unchecked exception ... but a better way to deal with that would be (for example) to run PMD with some custom rules to pick up exceptions that "ought to be" treated as checked.
This behaviour is defined by the java language definition.
There are two types of Exceptions:
Exceptions must be caught, RuntimeExceptions may occur but need no try/catch block.
RuntimeExceptions do not need to be declared and could be handled by any caller in the stack. Evaluating where that caller is would be impossible at compile-time.
AFAIK, Eclipse uses the same checked exception and unchecked exceptions list that Java does. This is part of the definition of how Java works. I would be surprised if Eclipse allows you to override this.
In Java, all Throwable are checked except subclasses of RuntimeException and Error.
The compiler complains only if the checked exceptions are not handled. Here the Integer.parseInt(string) is expected to throw a numberformatexception which is unchecked or runtime exception.Since it is not complaing.
It is not eclipse that is "set to complain" about exceptions.
See Checked and Unchecked Exceptions in Java.
Related
One of my friends who works on JAVA asked me how do I handle checked and unchecked exceptions in Python. I haven't heard these terms before, so I googled around to find out what is checked and unchecked exception. I didn't find anything related to this kind of exception in Python.
Do we have a concept of checked and unchecked exceptions in Python? If
no, then by default all the exceptions are checked or unchecked?
Thank you in advance!
Checked exceptions are not a thing in Python. But Python does have some similarities in the exception class hierarchy. Here is the Java structure:
source
In Java you have to declare in the method signature if the method will throw an Exception or any class that inherits from Exception, but not the RuntimeException. This rule is called checked exceptions.. it forces the user to consider them or the application will not compile.
In Python we still have a similar concept, but no enforcement. You may have heard before that it's bad style to do a bare except statement, and at the very least you should choose to catch Exception
try:
foo()
except: # bad!
bar()
try:
foo()
except Exception: # somewhat better!
bar()
This is because in Python Exception extends from BaseException and a bare except will catch everything. For example, you probably don't want to catch ctrl+c which raises a KeyboardInterrupt. If you do a bare except:, then KeyboardInterrupt will be caught, but if you do a except Exception: you will let this bubble up and stop the application. That is because KeyboardInterrupt extends BaseException and not Exception and therefore wont be caught!
Java have checked and unchecked exceptions because Java is a complied programming language, checked exception comes in compiling. In python, there is no such exception because Python is an interpreted language.
I see that one definition can be this:
Generally RuntimeExceptions are exceptions that can be prevented
programmatically.
But that is still not the definition of a checked exception. I thought checked exceptions were "exceptions that can be handled at compile-time". Is that correct and/or can you tell me more?
I also read this on the site, can you explain the quote?
Many people say that checked exceptions (i.e. these that you should
explicitly catch or rethrow) should not be used at all.
Java: checked vs unchecked exception explanation
Can I just learn what the definition is? I also read somewhat unexpectedly:
NumberFormatException is unchecked`
But I would think that NumberFormatException is checked since I would handle that at compile-time. Can you please help me understand? I've done some Java programming but I never wrote my own exception class, why would I need that?
Update
A definition is given is the SCJP book by Sierra / Bates:
A checked exception is defined as any subclass of java.lang.Throwable (including Throwable itself) which is not a subclass of java.lang.Error or java.lang.RuntimeException. The guidelines you saw are just that, guidelines, designed to help you understand the intent of runtime exceptions.
See the Java Language Specification, section 11.1.1
The unchecked exception classes are the runtime exception classes and the error
classes.
The checked exception classes are all exception classes other than the unchecked
exception classes. That is, the checked exception classes are all subclasses of
Throwable other than RuntimeException and its subclasses and Error and its
subclasses.
A more broad definition is:
CheckedExceptions are exceptions that you have to deal with explicitly. You either have to declare you can throw it or catch it and deal with it. Cunningham Wiki
As a result RuntimeExceptions are not checked, but NumberFormatException is checked if a method declares that it throws it and you are forced to catch or re-throw.
Checked exceptions allow you to verify at compile time that you are either handling exceptions or declaring them to be thrown.
Some do suggest avoiding checked exceptions. However, this does not eliminate the cost of managing exceptions. It shifts and magnifies it, from compile-time to run time and debug time.
From the Java tutorial:
[P]rogrammers may be tempted to write code that throws only unchecked exceptions or to make all their exception subclasses inherit from RuntimeException. Both of these shortcuts allow programmers to write code without bothering with compiler errors and without bothering to specify or to catch any exceptions. Although this may seem convenient to the programmer, it sidesteps the intent of the catch or specify requirement and can cause problems for others using your classes.
Unchecked exceptions allow you to not have to declare everywhere exceptions that can occur almost anywhere. From the Java tutorial:
Runtime exceptions can occur anywhere in a program, and in a typical one they can be very numerous. Having to add runtime exceptions in every method declaration would reduce a program's clarity. Thus, the compiler does not require that you catch or specify runtime exceptions (although you can).
NumberFormatException is unchecked because it falls within this camp. In many applications, number format exceptions can occur almost anywhere, and be numerous.
I find Java's exception hierarchy confusing. Throwable is divided into Error and Exception, and RuntimeException inherits from Exception.
Error is an unchecked exception. Why doesn't Error inherit from RuntimeException then?
Exception is a checked exception. RuntimeException is an unchecked exception, yet it inherits from Exception. Doesn't this violate the Liskov-Substitution-Principle?
Wouldn't it make more sense if Throwable were divided into Exception (checked) and RuntimeException (unchecked), and Error would inherit from RuntimeExeption?
I find Java's exception hierarchy confusing. Throwable is divided into Error and Exception, and RuntimeException inherits from Exception.
A Throwable is anything that can be used to unwind the call stack. This should include some VM level fault (flagged by an Error) and something application specific (flagged by an Exception)
Error is an unchecked exception. Why doesn't Error inherit from RuntimeException then?
Simply because Errors are not Exceptions. There wouldn't be any point in actually "catching" an Error. For eg. What would you do after catching an OutOfMemoryError. Errors are meant to flag something seriously happened at the VM level which is not necessarily handle-able by the programmer
Exception is a checked exception. RuntimeException is an unchecked exception, yet it inherits from Exception. Doesn't this violate the Liskov-Substitution-Principle?
Not really. What the implementors were trying to say was that Exceptions MUST always be checked. Your code will be cleaner if all your methods declare what sort of application/library Exceptions they throw. RuntimeExceptions should only be thrown in case of more general / dynamic situations such as a NullPointerException where the developer might not have coded for the case but is a serious bug which is not exactly something mentioned in the application spec.
The design of exception handling in the two most popular object-oriented frameworks (Java and .NET) is predicated upon the notion that the question of whether to handle a particular exception should depend primarily upon its type, and that the types of exceptions one will want to catch are going to have a hierarchical class relationship. I think Java and .NET do things that way because C++ did it that way, and C++ did it that way because of a desire to avoid hard-wiring any non-primitive types hard-coded into the language. In the absence of a hard-coded type to which all exceptions may be cast, it's impossible for a catch statement to know anything about any exception type for which it is not explicitly prepared. If it will only make sense to catch exceptions one can decode, a catch statement will be able to sensibly act upon those types, and only those types, which derive from the one in the statement.
In retrospect, it probably would have been better to have the decisions of what exceptions should be acted upon and/or resolved by particular catch statements be determined by some means other than the class hierarchy. Among other things, a single attempt to invoke a method may fail because of multiple problems; if that happens, every catch which is associated with any of those problems should trigger, but only when all of the problems have been resolved should normal execution resume. Unfortunately, neither Java nor .NET has any mechanism to achieve such behavior.
With regard to the top-level layout of the hierarchy, I think there was an assumption that every kind of exception that might be thrown by a method would either always be expected by the immediate calling code or never expected by any calling code. Exceptions of the latter type were classified under Error or RuntimeException, while those of the former type were placed elsewhere. In practice, the question of whether an exception is expected by a method's caller should be independent of its place in the hierarchy or even the exception type. The fact that a method is declared as throws FooException does not mean that calling code is always going to expect that the exception could occur. It's very common for code to call a method which is declared as throwing an exception but believe that the circumstances surrounding the call are such that in practice that the particular call won't ever throw.. If the exception does occur, it should behave like an unexpected exception, even if an outer execution context is expecting to catch an exception of that type. Unfortunately, the exception handling mechanisms are what they are, and I don't expect any major overhaul.
I think that even better hierarchy is
Throwable
Exception
CheckedException
RuntimeException
Error
This hierarchy separates Exceptions and Errors (as #Paarth said) and makes it possible to catch all checked exceptions only (without runtime exceptions). But it seems that James Gosling thought different...
Error is not a subclass of Exception because it is not meant to catch (Once an error is occurred usually the program is no longer expected to function). So error should have more higher place in hierarchy as I believe.
It should not violate Liskov substitution because RuntimeException can be catched if you want, but only it's not enforced.
I am confused with the naming of runtime exception of java.
Those checked exceptions, like SQLexception, also happen during the execution of a program.
Why only those unchecked ones called runtime exception?
It is probably I have misunderstanding of "runtime".
Thank you for any advices.
I can understand your confusion. All exceptions occur at runtime!
The only reason I can come up with for naming the class that way, is that it clarifies that it is an exception that doesn't have to be dealt with at compile time.
As opposed to all other so called "checked" exceptions, RuntimeExceptions doesn't require the programmer to declare the exception to be thrown using a throws clause.
RuntimeException is a subclass of java.lang.Exception. RunTimeExceptions are almost always the result of a programming error or/and invariants not being met (Nulls being passed when they shouldn't), so you do not have to catch them like java.lang.Exception (which is Checked Exceptions). You don't catch them because there's little the Run Time system could do to recover.
I think the phrase runtime just means they happen when the program is running (obviously!!) and crucially the compiler doesn't force checks to be built into the code as with Checked Exceptions. I think it's an example of where it's difficult to name a class appropriately e.g. I guess they could have Exceptions unchecked by default and called it Exception. Then subclassed it to provide CheckedException - everyone calls java.lang.Exception a Checked Exception, but it's not clear from the Class Name. But they didn't and we have:
> java.lang.Exception is referred to as "Checked Exception"
> java.lang.RuntimeException is referred to as "Unchecked Exception"
Yes, the naming is a bit confusing but it reflects the nature of an Exception being handled, not its time of occurrence.
All the exceptions occur at runtime. However, you are forced to handle the behavior of your program yourself for a specific type of exceptions i.e. CheckedExceptions. The reason is that these types of exceptions are more likely to occur. Example FileNotFoundException.
On the other hand, UnchekedExceptions[aka RuntimeExcetions] are less likely to occur and the programmer is not forced to handle those while writing the program. Ex: ArithmeticException.
While reading about exception, I will always come across checked exceptions and unchecked exceptions, So wanted to know how to distinguish that which is what?
Edit: I want to know if i create any exception class then how can i create as a checked or as an unchecked?
and what is the significance of each?
All Throwables except subclasses of java.lang.RuntimeException or java.lang.Error are checked. Properly, in Java, "exceptions" are subclasses of java.lang.Exception, "errors" are subclasses of java.lang.Error and java.lang.Throwable is not usually subclassed directly.
Programs are not supposed to create their own Error subclasses (though the documentation is rather ambiguous on that) so generally you always create Exceptions, using a RuntimeException if you don't want it to be checked.
To know at run-time if you have a checked exception you could use:
if(throwable instanceof Exception && !(throwable instanceof RuntimeException)) {
// this is a checked Exception
}
A checked exception is one which must be either handled in a catch clause, or declared as being thrown in the method signature; the compiler enforces this. Generally, one uses checked exceptions for exceptions which should be handled by the calling code, while unchecked exceptions are for conditions which are the result of a programming error and should be fixed by correcting the code.
That said there is much debate in the Java community about the efficacy of using checked exceptions vs. unchecked exceptions everywhere - a subject way to deep to discuss in this answer.
EDIT 2012-10-23: In response to comments (which are quite valid), to clarify, the following would be what is required to determine if a captured Throwable is a checked Throwable as opposed to a checked Exception:
if(obj instanceof Throwable && !(obj instanceof RuntimeException) && !(obj instanceof Error)) {
// this is a checked Throwable - i.e. Throwable, but not RuntimeException or Error
}
If the object in question is known to be an instance of Throwable (e.g. it was caught), only the second part of the above 'if' is needed (e.g. testing for Throwable is redundant).
See the Java Language Spec, chapter 11:
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. The Java API defines a number of exception classes, both checked and unchecked. Additional exception classes, both checked and unchecked, may be declared by programmers.
You could check this via instanceof at runtime, though I don't really see where this would be useful.
As to the second part of your question:
checked exception represent expected error conditions, which can occur during normal program execution and therefore always have to be handled programatically (which the compiler enforces)
unchecked exception represent unexpected error conditions and signify an abnormal state of your program, due to invalid input, bugs or runtime restrictions (eg memory); the compiler won't force the programmer to handle these, ie you only have to care for them if you know of their occurrence
Java checked vs unchecked exception
Error is internal VM error and usually you can not manage it.
Exception - you are able to catch and handle it
Checked vs Unchecked
checked exception is checked by the compiler and as a programmer you have to handle it using try-catch-finally, throws
unchecked exception is not checked by the compiler but you optionally can manage it explicitly
IntelliJ IDEA's Type Hierarchy tool is useful when you want to find more
If the exception class is a subclass of RuntimeException, it's not checked and does not have to be declared for functions or caught, etc. Error exceptions also do not have to be declared/caught. Is that what you're asking?
Quite sure this will have been asked and answered before, but for the sake of it, it is covered quite nicely here: http://www.javapractices.com/topic/TopicAction.do?Id=129.
Strictly speaking, unchecked exceptions will always extend RuntimeException whereas checked exceptions do not. The mentioned link explains when to use either.
As their name indicates, callers are obliged to deal with checked exceptions, typically handling them (try/catch) or passing them further up the stack. Unchecked exceptions are usually deemed to be caused by elements outside of the caller's control.
package practice;
import java.io.IOException;
class Practice
{
public static void main(String args[])
{
Exception n=new NullPointerException();
if(n instanceof RuntimeException)
{
System.out.println("this is a runtime(unchecked) exception");
}
else
{
System.out.println("this is not a compiletime(checked) exception");
}
}
}