Why throws, on a method, is part of its signature? It seems strange to include it. Here is an example where it is in the way:
#Overide
public void foo() {
throw new UnsupportedOperationException();
}
If anyone were to see this method from the outside, they might try to use it without knowing that it is not supported. They would only learn it on trying to run the code.
However, if they could do something like this they would know by looking at the method that it is not supported and if UnsupportedOperationException was not extending RuntimeException, they would get a compilation error. EDIT1: But this is not possible because throws is part of the signature so override will not work.
#Overide
public void foo() throws UnsupportedOperationException {
throw new UnsupportedOperationException();
}
This question concerns Java's design, so I know that it might be hard to answer without one of the people that work on it drops by and answers it, but I was hoping that maybe this question has been asked to them before or that there might be an obvious reason to have it this way to explain why.
The throws part does not indicate that the method is required to throw the mentioned exception(s), not even at particular occasions. It only tells that the function is allowed to do so.
Including throws UnsupportedOperationException will consequently not mean that the method is unsupported. Besides the UnsupportedOperationException is a RuntimeException so a method may throw that anyway.
Now for the reason one would require it in the signature of the method, it boils down to the ability to have checked exceptions at all. For the compiler to be able to decide if a method can only throw the specified exceptions it must be able to decide that the methods that it calls can't throw uncaught exceptions.
This means for example that overriding a method means that you can't add exceptions that might be thrown, otherwise you would break the possibility to verify that a method that calls that method can't throw anything else than it has specified. The other way around would be possible (but I'm not sure if Java supports that), overriding a method that may throw with one that may not throw.
So for example:
class B {
int fubar(int) throws ExceptionA {
}
int frob(int) throws ExceptionA {
return fubar(int);
}
}
class D extends B {
int fubar(int) throws ExceptionB {
}
}
Now frob is specified to possibly throw only ExceptionA, but in calling this.fubar it would open the possibility that something else is thrown, but fubar is defined to possibly only throw ExceptionA. That's why the D.fubar is an invalid override since that would open up the possibility that this.fubar actually throws ExceptionB and the compiler wouldn't be able to guarantee that frob doesn't throw ExceptionB.
Java has two different types of exceptions: checked Exceptions and
unchecked Exceptions.
Unchecked exceptions are subclasses of RuntimeException and you don't have to add a throws declaration. All other exceptions have to be handled in the method body, either with a try/catch statement or with a throws declaration.
Example for unchecked exceptions: IllegalArgumentException that is used sometimes to notify, that a method has been called with illegal arguments. No throws needed.
Example for checked exceptions: IOException that some methods from the java.io package might throw. Either use a try/catch or add throws IOException to the method declaration and delegate exception handling to the method caller.
One thing which no one has mentioned is a very important answer to your question:
Why throws, on a method, is part of its signature?
that throws, is NOT part of the method signature.
JLS 8.4.2. Method Signature makes it quite clear, that:
Two methods or constructors, M and N, have the same signature if they have the same name, the same type parameters (if any) (ยง8.4.4), and, after adapting the formal parameter types of N to the type parameters of M, the same formal parameter types.
If anyone were to see this method from the outside, they might try to use it without knowing that it is not supported.
That's exactly the main reason, why throws is designed in a way to be checked at compile-time for all Checked Exceptions - clients of the method will be aware what this method may possibly throw.
Checked Exceptions are enforced to be either handled or specified to be thrown (but this specification is NOT part of the method signature).
Unchecked Exceptions do not have to be either handled or specified to be thrown, and Oracle's tutorial are good at explaining - why:
Runtime exceptions represent problems that are the result of a programming problem, and as such, the API client code cannot reasonably be expected to recover from them or to handle them in any way. Such problems include arithmetic exceptions, such as dividing by zero; pointer exceptions, such as trying to access an object through a null reference; and indexing exceptions, such as attempting to access an array element through an index that is too large or too small.
Related
This question already has answers here:
Should methods that throw RuntimeException indicate it in method signature?
(7 answers)
Closed 4 years ago.
Let's say I have the following code:
public void methodOne(String argumentOne) {
methodOne(argumentOne, false);
}
public void methodOne(String argumentOne, boolean equality) {
//App logic here
}
And if the app logic throws an exception (say IllegalArgumentException/Parse Exception), and i would like to catch this in the caller method, should the "throws IllegalArgumentException" be added in all the method identifier or only on the base method identifier? Is there any advantage over this?
Throwing a checked exception means you expect the caller to be forced to think about how to handle that exception. You should only do this when:
This is a deliberate, desirable action in your mind.
You have some idea how you expect the caller to deal with a checked exception. Ideally, you have documented what you expect them to do with the exception.
Thus it's a very bad idea to
declare "throws" an exception which is never actually thrown. This just causes confusion and get the developer into the habit of ignoring your exceptions as they have no value.
throw a checked exception when it is unlikely to be a reasonable way to recover from it. An unchecked exception might be better in that case.
I prefer to add throws clauses for unchecked exceptions as a form of documenting what can go wrong and what it means without forcing the caller to handle those exceptions.
Additionally, you should try to add meaningful messages to each Exception/Error to make it easier for a developer to work out how to fix the code or configuration.
i would like to catch this in the caller method, should the "throws IllegalArgumentException" be added in all the method identifier or only on the base method identifier?
Only add it to the methods which can actually throw the exception.
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.
When I throw checked exceptions from a method should I just declare the super class of the exceptions in the method signature or all the different types? If I have the following exceptions:
private class SuperException extends Exception {
}
private class SubExceptionOne extends SuperException {
}
private class SubExceptionTwo extends SuperException {
}
Should the method signature be:
void confirmAccount() throws SubExceptionOne, SubExceptionTwo;
or
void confirmAccount() throws SuperException;
In the last method signature, how do I tell other developers what exceptions that could be thrown from the method? If the different sub types need different handling?
The interface should be as stable as possible. So probably Super. Many libraries use the "Super" strategy, because exception specs cause far more annoyance in maintainability than readability or safety they add. Even the IOException is a Super that nearly all Java library code uses instead of declaring more specific exceptions. (But when they do declare more specific exceptions, it's because the contract is that more general IOExceptions won't be thrown. Read on.)
You might list Sub1 and Sub2 if you really do want to say each of those exceptions can be thrown, but don't want to say that any derivative of Super can be thrown. Perhaps Sub1 is NumberCrunchException and your method calls crunchNumbers() and users of your method can be assured that's the only exception-ful thing your method does. In that case the specific strategy is better.
If the different sub types need different handling, then definitely declare the two different exceptions. Never expect the developer using your method to guess that you are actually throwing different types of exceptions.
If you declare two distinct exceptions, and the user knows from the Javadoc that they are actually descendents of the same class, the user may choose to catch them both with a catch (SuperException e) rather than two individual catch clauses. But it depends on the user's choice.
If you don't declare them separately, your IDE is not going to add the appropriate #Throws to your Javadoc comment. And your Javadoc will therefore only indicate that you're throwing SuperException, which will leave the user in the dark. Solving this by just putting it in the text of the comment is not a real solution. If any tool is using reflection to determine what your method throws, it will not see the individual exceptions in the array returned from Method.getExceptionTypes().
If the functionality expected of the different exceptions is more or less the same and it's just a matter of how they will appear in the logs, it may be better to just use the parent exception, with different messages.
The throws clause is there to convey useful information to the calling method about what might go wrong during invocation of this method. That means that how specific you are will depend on how much information you want to convey; and that will be application-dependent.
For instance, declaring throws Exception is almost always a bad idea: the information this conveys is just "something might go wrong", which is too vague to be useful. But whether calling classes are going to need perfectly fine-grained information in the throws clause is something you need to decide by looking at your program. There's no set answer.
This is my code:
public static void main(String[] args) throws Exception {
Scanner s = new Scanner(new File("story.txt"));
someFunction(s);
}
The story.txt is in the the project root, but without the throws Exception the code doesn't run.
Why?
You don't need throws Exception, but it won't hurt any if you leave it like that.
What you do need is a throws FileNotFoundException because the Scanner constructor you are using is declared to throw that exception, and you don't catch any exceptions.
You should throw a FileNotFoundException, because the Scanner constructor can throw one. Since it is a checked exception, you must either pass it further up the stack or handle it somehow.
I asked my professor, and he explained that Java makes you handle exceptions.
You must be ready for them if a function might throw them.
You can either use:
try {...
} catch { ...
}
to handle exceptions yourself, or you can use:
throws Exceptions
to "pass them to the next level" (handle them later).
Thanks for the help!
If a method or constructor either throws an exception which isn't derived from RuntimeException, or calls any method whose declaration indicates it must do so, then it must either catch that exception or declare that the exception may be thrown from it. This language feature is almost a good thing, but in its present form has been widely acknowledged as a mistake. Nonetheless, it is sufficiently firmly established as part of the Java language that it's not going anywhere.
Conceptually, it's good to make a distinction between exceptions which are likely to have special meaning to a method's immediate caller and those which would not. Unfortunately, the fact that an exception would have a special meaning to its immediate caller doesn't mean the immediate caller is going to be interested in it. The way Java implements checked exceptions, if Foo is declared as throwing a checked exception and Bar calls Foo but isn't prepared to handle that exception, it must either declare itself as throwing that exception (even though it's unlikely to have any special meaning to its caller), or else catch the exception it has no hope of handling (perhaps rethrowing as a type derived from RuntimeException). Catching and rethrowing as a type derived from RuntimeException is semantically the best approach, but ends up being the most verbose, especially if one wants to avoid making RuntimeException objects.
Note also that because there's no way a chained constructor call can be wrapped in a try block, any constructor which is chained through a constructor that is declared as throwing any exceptions is required to declare itself as throwing those same exceptions. Things are implemented that way for a reason, but it unfortunately makes it difficult for a class constructor to distinguish exceptions which got thrown as part of the base-class construction process and those which got thrown in the construction of the derived class.
Because, Scanner s = new Scanner(new File("story.txt")); throws checked FileNotFoundException. You should throws or catch checked exception for compilation.
So here is the quote from the book:
The overriding method must NOT throw checked exceptions that are new or broader than those declared by the overridden method. For example, a method that declares a FileNotFoundException cannot be overridden by a method that declares a SQLException, Exception, or any other non-runtime exception unless it's a subclass of FileNotFoundException.
Now here is my question, if the method in the superclass throws an exception, then can the overriding method NOT throw an exception at all?
Because I just tried this in Java, where the overriding method did not throw any exceptions, and there was no error.
Please explain.
You can declare an overriding method as throwing less types of exceptions than the superclass, you just can't introduce new ones. The subclass method has to be compatible with the behavior of the superclass method. More exactly, you have to be able to substitute objects of the subclass for objects of the superclass without breaking anything (where adding a new checked exception to the throws clause would mean things calling it would have to change their code to handle it).
(The idea behind this is the Liskov Substitution Principle: a program should be able to deal with objects at a high level without getting bogged down in details about everything's exact type. If subclasses can introduce changes that mean the program has to pick them out and handle them differently then it defeats the purpose of abstraction.)
So an overriding method can be declared as throwing no checked exceptions at all (by omitting the throws clause entirely), because that doesn't require changes to any callers.
There are examples in the JDK, such as in java.io, where the subclass can't possibly throw an exception declared by the super class (see the ByteArrayOutputStream close method). Here the close method could have had its throws clause removed, since it never throws IOException. (Maybe it was left on the chance someone would want to subclass it with a version that did throw IOException?)