I want to my java script to throw if there is any exception. Problem is, we are extending a super class in my script but in that super class there is an exception caught called ScriptFailureException and super class is in a JAR that we cannot edit. We want to stop that exception from being caught. Is it possible to prevent that exception from being caught or is there any other method to make our script fail in this situation. I tried System.Exit(), etc. methods but they are not working. We are running our script through TestNG.
First of all, Java is not a scripting language and has no notion of a script. You should not use the term script, but the terms "class", "method", "program".
To answer your question, if a method catches an exception, then the exception is caught, and you can't do anything about it. You might throw another exception type which would not be caught by the superclass method, though.
No unless the super class lets you know that a ScriptFailureException occured by an other way than throwing it. One of this other way can be for example a null returned value for a method call.
What actions does the Jar code take when it catches the exception? Can you detect that action after the event? For instance, if the Jar code writes to a log then you could examine the log to see if that exception is recorded there and then throw a new exception of your own.
You can make your Java fail totally by throwing an Error.
throw new Error("Oh dear");
That's messy though.
Try defining your own Exception and throwing that.
public class CustomException extends Exception { }
// somewhere else
throw new CustomException("Some detail");
A little trick is declare ScriptFailureException as an extension of RuntimeException
I think that this isn't the best way to do code, but it works. The RuntimeException objects don't need a explicit try/catch block.
Then, you must declare class StcriptFailureException extends RuntimeException
Regards!
Related
The below picture shows that "Checked" and "Unchecked" Exceptions are subclasses of Exception. I find it confusing that you need to catch an Exception but you don't need to catch a RuntimeException, which directly inherits from Exception. Is there a reason that the devs didn't let us throw Exceptions without needing to catch them?
More specifically: Why can you ignore only RuntimeExceptions and it's children? Why wasn't there a Class introduced called CheckedException extends Exception and you only need to catch it and it's children?
The confusing part is, that you can throw everything below RuntimeException without issue, but when you move up to Exception in the hierarchy, you need to catch it at some point. This is confusing because "abstraction" normally works otherwise. The more you move up, the simpler and more meta everything gets. This is not the case here. The more you move up, the more you have to do (like, putting try/catch after reaching Exception).
If Exception was unchecked then you could implicitly cast checked exceptions to unchecked ones, which would mean that you could throw checked exceptions without catching them like:
public void f() {
Exception e = new IOException();
throw e;
}
and also with overriding methods, if you throw a more specific exception, you can add the requirement to catch the exception that wasn't in the superclass:
public void f() throws Exception {
}
...
#Override
public void f() throws IOException {
}
Suppose they designed it the other way. We have a CheckedException class, and subclasses of that need to be handled, but not other subclasses of Exception.
Now we call a method that might throw an arbitrary Exception:
public static void example() {
functionThatThrowsException();
}
Do we need to handle it? Yes, because that Exception might be a CheckedException. If we didn't need to handle it, we'd be bypassing the checked nature of checked exceptions.
A throwable type with checked descendants must be treated as checked, so checkedness naturally propagates up the inheritance hierarchy. Contrapositively, an unchecked throwable type cannot have checked descendants, so uncheckedness naturally propagates down. This makes it natural to make checkedness the default, and single out specific classes and their descendants as unchecked.
CheckedException (Which does exist) and RuntimeException both extend Exception. Because of this, if something throws a generic Exception (which is always a bad idea), there is no way to tell if the exception could be one or the other, so you have to catch it in case it's a checked one. If you think of the hierarchy in this way, it actually does get simpler the farther up you go.
You seem to have the idea that checked exceptions are more "complex" because you have to do more to work around them. This isn't too healthy a way of thinking about it. Instead, consider this: Exceptions are problems with the program itself - the code. We need to find these exceptions and handle them properly. After already having this concept of exception handling, we discover that there are some problems that we simply can't predict.
"How was I supposed to know the user would enter 'meow' when asked for an integer! I shouldn't have to code around that!" And so, NumberFormatException was born, and you don't have to catch it because it's a "logical error", not an issue caused by bad code (Although, arguably, it might be considered bad code if you don't handle this situation in some way).
In short, reverse your thinking. Exceptions are problems with the program that can be handled. There are some exceptions, however, that are unexpected and are a result of bad design more than incorrect code. Thus there is the addition of RuntimeExceptions which cannot possibly be expected to occur, but certainly can occur.
Perhaps it would help to not think of exception classes in terms of inheritance but simply disjoint sets of classes, one set is checked and other is not. You're right that there could be a CheckedException class allowing us to check only when explicitly intended.
However having the broader/generalized range checked helps in enforcing the catch or specify pattern. Having checked exception allows a reader of the code to figure out quickly that this piece of code needs special attention and enforcing their handling at compile time reducing the runtime bugs.
We can throw any kind of exception, checked or unchecked. If Exception or any super class of RuntimeException were to be set as checked exception then all the sub classes would become checked exceptions. As compiler is most likely checking if an instance of exception or a class in the throws clause derives from a class. It could easily have done that by checking for a specific package which probably would have been more appropriate as being checked or unchecked has simply nothing to do with the inheritance.
In Java, when I decide to throw an exception, is it possible to define this Exception as not having to be handled?
Thanks :-)
(I am on Java 7)
Any Exception that is a child type of RuntimeException is not required to be handled. This is called an unchecked exception.
That said, you can still choose to handle it, should you feel that it is necessary.
It can be achieved by throwing a unchecked exception. RuntimeException are unchecked exception which the calling program need not handle. Any sub-class like ClassCastException etc, are derived from RuntimeException and you need not worry about handling them.
The requirements of the following java assignment are unclear to me. Does anyone understand what is needed?
Assignment: In the previous assignment, you implemented a stack and a list that both inherited from the abstract class ArrayIntCollection. In this next task you are supposed to extend that implementation by making it throw exceptions. If you try to perform operations that are not allowed (for example, if you try to call pop or peek on an empty stack or try to remove an element from a non existing position) you shall create and use the exception class CollectionException of the type UncheckedException. Also write a test program ExceptionMain.java that generates and catches exceptions from your modified methods.
The way I understand it, I need to create an exception class called CollectionException. This class needs to extend UncheckedException. (This part is not working, as I cannot find a class called UncheckedException).
Also, it is apparent whether I am to use try\catch or throw for this task?
Thanks to all!
...by make it throw exceptions...
will mean, yes, you have to use throw.
Also write a test program ExceptionMain.java that generates and catches exceptions from your modified methods.
At least here you will have to use try...catch.
Regarding "UncheckedException": Either there is a class of this name (perhaps in another package). Then you can ask your teacher where it is. Otherwise you can extend from java.lang.RuntimeException (which is an unchecked exception) and ask you teacher if that's OK.
Short form: How do you throw exceptions (or do nice, clean exception handling; or at least dirtily force execution to stop) when the overridden method doesn't throw exceptions?
Context: We have a license for a piece of proprietary software that can be automated using Java "macros". A user-defined macro must be of this form:
public class MyMacro extends SoftwareMacro {
public void execute() {
// user code goes here
}
}
i.e. a class that extents SoftwareMacro and that has a method called execute that overrides the base class' execute. The contents of this overriding execute are what gets... well... executed when the macro is "played".
But the overridden execute method apparently does not throw any exceptions.
execute() in com.mycompany.mypackage.MyMacro cannot implement execute() in
somesoftware.base.SoftwareMacro
overridden method does not throw java.lang.Exception
Maybe this is naïve, but while developing I usually like to have the appropriate exception type bubble up to the top and force execution to stop, so that I can see them and go on to debug. This is apparently not an option here.
Should I resort to throwing RuntimeException instead? (since RuntimeException does not need to be specified) That feels a bit sloppy, and a "violation in spirit" of the base class method contracy.
P.S. No, I can't change the source code of the overriden execute method.
Looks like the intent is that each SoftwareMacro do all its own error handling. Use a big try around your whole execute() method if need be, but don't let any exceptions escape. Do whatever cleanup you need to do inside your execute method, and possibly print an error message for the user, if they provide a way to do that.
You should examine all the APIs they provide -- perhaps there's an error reporting facility you're supposed to use.
It all depends on what the "Macro player" does if it encounters a runtime exception, and on what you want to happen.
If it doesn't handle it at all, but you don't care, throw a RuntimeException.
If it handles them properly, throw a RuntimeException.
If it doesn't handle them properly, and you don't want it to fail miserably, then catch the exceptions that might happen in your execute method, and handle them as you feel is the best: show an error dialog box, output an error message, log some error in the logs...
"Should" implies there's a right answer, which IMO there isn't do what meets your needs.
If the system can tolerate a runtime exception, and it meets your needs, why not?
Not that you have a choice, since you can't throw a checked exception.
(Checked exceptions seem like a failed experiment to me, although I understand the motivation.)
So long as the execute code doesn't absorb the exception, it will still throw it if one is encountered. If the kind of exceptions thrown are either RuntimeException or sub-classes of RuntimeException they don't need to be explicitly declared, mostly because the compiler doesn't enforce their declaration since (as the name implies) they only happen at runtime, and cannot necessarily be predicted at compile time.
If, however, the execute method, which you've said you can't modify, absorbs the exception, and doesn't indicate that exception via a log entry, return value, or some kind of RuntimeException I think you're out of luck.
I would agree with Ernest that it appears that the intent is that the execute method do all its own exception handling.
NOTE: Overridden method signatures don't need to match exactly when it comes to the exceptions they throw - just the name, return type, and list & type of variables.
I am trying to understand the Error class in Java.
I have a good understanding of the Exception class, but can't find examples of code for the Error class. I've tried searching the web and also the java.sun website, but I'm not finding anything useful to help me understand this better.
How do I use the Error class in programs and where do we have to use that?
You don't use Error in your code.
An Error is a specific kind of Throwable, just as Exception is.
Throwable is the base class that defines everything that can be thrown.
Exception is the common case. It is about problems that occur during the execution of your program.
RuntimeException is a special case: it's unchecked (i.e. it need not be declared by a method and the compiler doesn't force you to catch it).
Error is the "rare" case: it signifies problems that are outside the control of the usual application: JVM errors, out of memory, problems verifying bytecode: these are things that you should not handle because if they occur things are already so bad that your code is unlikely to be able to handle it sanely.
You should not attempt to correct the situation that resulted in an Error. You might want to catch it in order to log it and then rethrow it (see the JavaDoc of ThreadDeath for an example on why you need to rethrow it (thanks to #krock for the heads-up)).
There is no other reason to throw any Error (i.e. don't create an Error on your own and throw it, if you think you want to do that, use an Exception or a RuntimeException instead).
If you take a look at the Javadoc here there is a good explanation:
An Error is a subclass of Throwable that indicates serious problems that a reasonable application should not try to catch. Most such errors are abnormal conditions.
Concerning the usage you also have this:
A method is not required to declare in its throws clause any subclasses of Error that might be thrown during the execution of the method but not caught, since these errors are abnormal conditions that should never occur.
Error
Error a subclass of "Throwable" class is thrown by java runtime system and indicate some unrecoverable conditions during the execution of the programs
Once thrown difficult to recover from it and the application get to halt.
Eg..,java.lang.StackOverflowError and java.lang.OutofMemoryError
// An example code which throws StackOverflowError
public class ErrorDemo
{
public void method1()
{
this.method2();
}
public void method2()
{
this.method1();
}
public static void main(String sri[])
{
ErrorDemo k= new ErrorDemo();
k.method1();
}
}
In this code from main method we are calling method1 and from method1 a call was made to method2 and again from method2 we are calling method1 means we created a continous loop which doesn't goes to end and finally a critical error StackOverflowError is being thrown.