Exceptions

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:

throw Exception("Hi There!")

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 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, unlike Java.

Appendable append(CharSequence csq) throws IOException;

The Nothing type

throw is an expression in Kotlin, so you can use it, for example, as part of an Elvis expression:

val str = 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

Since fail has a return type of Nothing, the compiler knows that the program cannot continue beyond the call to fail. Therefore, the println(s) statement will not be executed if person?.name is null.

Code comment means that, When println(s) is executed, it's known that s has been initialized and no exception was thrown.

Last modified: 22 February 2024