All exception classes in Kotlin inherit the class
Throwable. 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
finally block may be omitted. However, at least one
finally block should be present.
Try is an expression
try is an expression; thus, 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). Contents of the
finally block do not 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.
The following is an example interface of the JDK implemented by
What does this signature say? It says that every time I append a string to something (a
StringBuilder, some kind of a log, a console, etc.) I have to catch those
IOExceptions. Why? Because it might be performing IO (
Writer also implements
Appendable )... So it results in this kind of code all over the place:
And this is no good, see Effective Java, 3rd Edition, Item 77: Don't ignore exceptions.
Bruce Eckel says about checked exceptions:
Other citations of this sort:
Java's checked exceptions were a mistake (Rod Waldhoff)
The Trouble with Checked Exceptions (Anders Hejlsberg)
If you want to alert callers of 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 as well as 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:
The type of the
throw expression is the special type
Nothing. The 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:
Another case where you may encounter this type is 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 section for information about Java interoperability.