All exception classes in Kotlin inherit the
Throwable class. Every exception has a message, a stack trace, and an optional cause.
To throw an exception object, use the
To catch an exception, use the
There may be zero or more
catch blocks, and the
finally block may be omitted. However, at least one
finally block is required.
Try is an expression
try is an expression, which means it can have a return value:
The returned value of a
try expression is either the last expression in the
try block or the last expression in the
catch block (or blocks). The contents of the
finally block don't affect the result of the expression.
Kotlin does not have checked exceptions. There are many reasons for this, but we will provide a simple example that illustrates why it is the case.
The following is an example interface from the JDK implemented by the
This signature says that every time I append a string to something (a
StringBuilder, some kind of a log, a console, etc.), I have to catch the
IOExceptions. Why? Because the implementation might be performing IO operations (
Writer also implements
Appendable). The result is code like this all over the place:
And that's not good. Just take a look at Effective Java, 3rd Edition, Item 77: Don't ignore exceptions.
Bruce Eckel says this about checked exceptions:
And here are some additional thoughts on the matter:
Java's checked exceptions were a mistake (Rod Waldhoff)
The Trouble with Checked Exceptions (Anders Hejlsberg)
If you want to alert callers about possible exceptions when calling Kotlin code from Java, Swift, or Objective-C, you can use the
@Throws annotation. Read more about using this annotation for Java and for Swift and Objective-C.
The Nothing type
throw is an expression in Kotlin, so you can use it, for example, as part of an Elvis expression:
throw expression has the type
Nothing. This type has no values and is used to mark code locations that can never be reached. In your own code, you can use
Nothing to mark a function that never returns:
When you call this function, the compiler will know that the execution doesn't continue beyond the call:
You may also encounter this type when dealing with type inference. The nullable variant of this type,
Nothing?, has exactly one possible value, which is
null. If you use
null to initialize a value of an inferred type and there's no other information that can be used to determine a more specific type, the compiler will infer the
Please see the section on exceptions in the Java interoperability page for information about Java interoperability.