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