Two interrelated issues being discussed here:. We don't allow type annotations on catch clauses because there's really no way to know what type an exception will have.
Pattern matching 101. C# 7 introduced the following patterns: the const pattern, the type pattern, the var pattern and the discard pattern. Patterns can be used in is-expressions and in case blocks. The implementation of the const pattern in is-expression for value types is far from perfect from the performance point of view.
You can throw objects of any type and system generated exceptions (such as out of memory exception) can technically happen at any time. Even if we had a Java-like notion of throws annotations it is not clear that we could ever really rely on them.
(And, it is also not clear that they work all that well in Java, but that's another discussion.). We don't narrow any in type guards unless we know the exact type you're narrowing to. For example, if you check typeof x 'string' we will indeed narrow an any to string because we know for sure that is the exact type of x. That isn't the case with instanceof.
Specifically, you might check x instanceof Base, but x might actually be an instance of a class derived from Base. If we narrowed x to type Base you'd see errors on access to properties of the derived class even though there is nothing wrong with the code. Now, before you argue those errors would be fine, keep in mind that the declared type of x is any, so you've already said you want no checking. We take that as an indication that we should issue errors only when we're 100% sure they're errors.
And we only know that if we know the exact type. It would be very nice if we could type-annotate the catch clause variable with ex: Error since it's perfectly sound to do so in many if not most cases.
Once the exception caught what is that that you are going to do with it? A rhetorical question, but at the end of the day your options are limited:. an unexpected exception which is absolutely nothing you can do about, extremely hard to reliably recover, takes special coding discipline to do so, just fail fast and loud. an expected exception, with a chance for graceful recovery, if so, pardon my curiosity, why would you use exceptions (that are terribly supported by the JS runtime) if you can make it a legit part of the result value type? The point of this issue is not about allowing a potentially confusing annotation, it's about supporting the practice of checking what was thrown in the catch clause, so that for example recoverable errors can be handled and everything else re-thrown.This is made difficult in TypeScript at present due to the fact that there's currently no way to make type guards work on a variable of type any, which is what the catch variable always is.
![Error Error](https://community.ptc.com/legacyfs/online/34080_Matrix+Calc.png)
It would be fine to leave the catch variable unannotated if there was some way of narrowing that variable in type guards in the catch block, but presently there isn't. Unless you count the workaround of introducing a second dummy variable with an annotation, which both adds the 'misleading' annotation you mentioned, as well as introducing an extra variable that is useless at runtime.
You're absolutely correct that this suffers from the same problems.The implication is that instanceof is not reliable. Personally, I think it is an anti-pattern.The problem I have with 's suggested solution is that it bakes an unreliable pattern into the language, encouraging its use by providing manifest syntax, and providing a false sense of security to the programmer.With a restriction is being lifted, but you still have to choose to shoot yourself in the foot and nothing encourages you to do so.
.The Exception Handling in Java is one of the powerful mechanism to handle the runtime errors so that normal flow of the application can be maintained.In this page, we will learn about Java exceptions, its type and the difference between checked and unchecked exceptions. What is Exception in JavaDictionary Meaning: Exception is an abnormal condition.In Java, an exception is an event that disrupts the normal flow of the program.
It is an object which is thrown at runtime. What is Exception HandlingException Handling is a mechanism to handle runtime errors such as ClassNotFoundException, IOException, SQLException, RemoteException, etc.
What is the difference between checked and unchecked exceptions?. What happens behind the code int data=50/0;?. Why use multiple catch block?.
Is there any possibility when finally block is not executed?. What is exception propagation?.
What is the difference between throw and throws keyword?. What are the 4 rules for using exception handling with method overriding?Hierarchy of Java Exception classesThe java.lang.Throwable class is the root class of Java Exception hierarchy which is inherited by two subclasses: Exception and Error.
A hierarchy of Java Exception classes are given below: Types of Java ExceptionsThere are mainly two types of exceptions: checked and unchecked. Here, an error is considered as the unchecked exception. According to Oracle, there are three types of exceptions:.
Checked Exception. Unchecked Exception. ErrorDifference between Checked and Unchecked Exceptions 1) Checked ExceptionThe classes which directly inherit Throwable class except RuntimeException and Error are known as checked exceptions e.g. IOException, SQLException etc. Checked exceptions are checked at compile-time. 2) Unchecked ExceptionThe classes which inherit RuntimeException are known as unchecked exceptions e.g. ArithmeticException, NullPointerException, ArrayIndexOutOfBoundsException etc.
Unchecked exceptions are not checked at compile-time, but they are checked at runtime.3) ErrorError is irrecoverable e.g. OutOfMemoryError, VirtualMachineError, AssertionError etc. Java Exception KeywordsThere are 5 keywords which are used in handling exceptions in Java. KeywordDescriptiontryThe 'try' keyword is used to specify a block where we should place exception code. The try block must be followed by either catch or finally.
It means, we can't use try block alone.catchThe 'catch' block is used to handle the exception. It must be preceded by try block which means we can't use catch block alone. It can be followed by finally block later.finallyThe 'finally' block is used to execute the important code of the program.
It is executed whether an exception is handled or not.throwThe 'throw' keyword is used to throw an exception.throwsThe 'throws' keyword is used to declare exceptions. It doesn't throw an exception. It specifies that there may occur an exception in the method. It is always used with method signature.Java Exception Handling ExampleLet's see an example of Java Exception Handling where we using a try-catch statement to handle the exception.