How to throw custom exception in Kotlin?

Exception is an important aspect of any programming language. It prevents our code from generating incorrect output at runtime. There are two types of exceptions −

• Checked exceptions
• Unchecked exceptions

Checked Exceptions

Checked exceptions are those which are checked at the compile time. As per example, FileNotFoundException() or IOException. In the following example, we will see how we can generate a checked Exception.

Example

import java.io.File
import java.io.InputStream

fun main(args: Array<String>) {
   try {
      val inputStream: InputStream = File("Hello.txt").inputStream()
   } catch(e:Exception) {
      e.printStackTrace();
   }
}

Output

Once you execute this code, it will generate the following output in the result section. You could see that while compiling, we have got the checked exception as an output.

java.io.FileNotFoundException: Hello.txt (No such file or directory)
   at java.io.FileInputStream.open0(Native Method)
   at java.io.FileInputStream.open(FileInputStream.java:195)
   at java.io.FileInputStream.<init>(FileInputStream.java:138)
   at MainKt.main(main.kt:6)

Unchecked Exceptions

Unchecked exceptions are those which are not checked at the compile time; rather they will be thrown at the runtime. As per example, we can consider any of the ArithmeticException, NumberFormatException. In the following example, we will see how we can generate an unchecked Exception.

Example

fun main(args: Array<String>) {
   try {
      val myVar:Int = 12;
      val v:String = "Tutorialspoint.com";
      v.toInt();
   } catch(e:Exception) {
      e.printStackTrace();
   } finally {
       println("Exception Handeling in Kotlin");
   }
}

Output

It will produce the following output −

Exception Handeling in Kotlin
java.lang.NumberFormatException: For input string: "Tutorialspoint.com"
   at java.lang.NumberFormatException.forInputString(NumberFormatException.java:65)
   at java.lang.Integer.parseInt(Integer.java:580)
   at java.lang.Integer.parseInt(Integer.java:615)
   at MainKt.main(main.kt:5)

Custom Exception in Kotlin

The concept of exception in Kotlin is very much same as it is in Java. All the exceptions in Kotlin are the descendants of the Throwable class. In Kotlin, developers do have the privilege to create their own custom Exception. Custom Exceptions are a part of unchecked exception which means they will be thrown at the runtime.

Example of Custom Exception

We will create our own custom exception using a very simple example. In this example, we are declaring a variable and checking whether that variable value is less than 50. Depending on the outcome, we will throw a custom exception using Kotlin built-in functionality.

fun main(args: Array<String>) {
   val sampleNumber:Int;
   sampleNumber = 100;

   if(sampleNumber > 50) {
      // throwing custom exception instead of checked Exception
      throw myCustomException("Invalid Input. Enter a correct number")
   } else {
      println("Welcome!! You have entered a correct value")
   }
}

// declaring custom exception class
class myCustomException(message: String) : Exception(message)

Output

When we execute this code, it will yield the following output. You can observe that we are throwing our custom exception along with the passed message.

Exception in thread "main" myCustomException: Invalid Input. Enter a correct number
   at MainKt.main(main.kt:7)