Exceptions
Exception classes
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 throw
expression:
To catch an exception, use the try
... catch
expression:
There may be zero or more catch
blocks. finally
block may be omitted. However, at least one catch
or 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.
Checked exceptions
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 StringBuilder
class:
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 Nothing?
type:
Java interoperability
Please see the section on exceptions in the Java Interoperability section for information about Java interoperability.