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exceptions.md

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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:

fun main() {
//sampleStart
    throw Exception("Hi There!")
//sampleEnd
}

{kotlin-runnable="true" kotlin-min-compiler-version="1.3"}

To catch an exception, use the try...catch expression:

try {
    // some code
} catch (e: SomeException) {
    // handler
} finally {
    // optional finally block
}

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:

val a: Int? = try { input.toInt() } catch (e: NumberFormatException) { null }

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:

Appendable append(CharSequence csq) throws IOException;

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:

try {
    log.append(message)
} catch (IOException e) {
    // Must be safe
}

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:

Examination of small programs leads to the conclusion that requiring exception specifications could both enhance developer productivity and enhance code quality, but experience with large software projects suggests a different result – decreased productivity and little or no increase in code quality.

And here are some additional thoughts on the matter:

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:

val s = person.name ?: throw IllegalArgumentException("Name required")

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:

fun fail(message: String): Nothing {
    throw IllegalArgumentException(message)
}

When you call this function, the compiler will know that the execution doesn't continue beyond the call:

val s = person.name ?: fail("Name required")
println(s)     // 's' is known to be initialized at this point

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:

val x = null           // 'x' has type `Nothing?`
val l = listOf(null)   // 'l' has type `List<Nothing?>

Java interoperability

Please see the section on exceptions in the Java interoperability page for information about Java interoperability.