Exception

Like in many other languages, some unexpected situations may occur in Kotlin. Imagine, you've written your program and now you are trying to run it. Suddenly, the Kotlin compiler says that code isn't correct and marks errors in the code. However, no compiler is almighty, so the Kotlin one can't prevent all the possible errors. Therefore, an error can happen while a program is running, even though it's syntactically correct and has been compiled without any problem. Such situations are called Exceptions in Kotlin and we are going to explore this concept.

Causing An Exception

First of all, let's describe an exception. Obviously, it's an unexpected and non-standard event, which occurs in different situations and may be annoying for any programmer. Let's look at the example below:

fun readNextInt(): Int {
    return readln().toInt()
}

fun runIncrementer() {
    val increment = 1 + readNextInt()
    println(increment)
}

fun main() {
    runIncrementer()
}

We've just created a function to read from the standard input. It's quite a simple example: we just read a line, convert it to a number and return the result. Any possible problems? Well, here they are: we can't be sure that a user won't type something like this:

> Hi :)

Oops! If you run the program and enter this line, an exception will happen and the program will crash and output something like this:

Exception in thread "main" java.lang.NumberFormatException: For input string: "Hi :)"
	at java.base/java.lang.NumberFormatException.forInputString(NumberFormatException.java:67)
	at java.base/java.lang.Integer.parseInt(Integer.java:661)
	at java.base/java.lang.Integer.parseInt(Integer.java:777)
	at org.example.MainKt.readNextInt(Main.kt:4)
	at org.example.MainKt.runIncrementer(Main.kt:8)
	at org.example.MainKt.main(Main.kt:13)
	at org.example.MainKt.main(Main.kt)

It's mean that you get a NumberFormatException in the function readNextInt() at line 4.

What Is An Exception

So, as you can see, some problems in your code won't stop the program from running, but the program will crash when it tries to execute a line with an error called an exception. Exceptions are errors found during the program's execution (runtime), while syntax errors are found when the program is being compiled. An exception interrupts the normal execution of the program.

Exceptions can happen for different reasons, such as performing an invalid arithmetic operation or receiving incorrect input as in example above. These errors indicate that something is behaving unexpectedly, and you need to validate your input data or change how you process it. Let's look at the another example and see how you can avoid exceptions.

Division By Zero

For example, you have a simple task. You bought some milk, you know how much money you've spent and how many packs you've got. You want to know the price of each pack. Well, it's quite easy:

fun itemPrice(total: Int, amount: Int): Int {
    return total / amount
}

But what happens if you call this function with amount equal to 0? As you know, you can't divide by zero. You will get something like this:

Exception in thread "main" java.lang.ArithmeticException: / by zero
	at org.example.MainKt.itemPrice(Main.kt:4)
	at org.example.MainKt.main(Main.kt:8)
	at org.example.MainKt.main(Main.kt)	at org.example.MainKt.itemPrice(Main.kt:4)

It's mean that you get an ArithmeticException in the function itemPrice() at line 4.

Well, we met another exception. In this case, you can prevent this error quite simply:

fun itemPrice(total: Int, amount: Int): Int {
    if (amount == 0) {
        println("Division by zero")
        return 0
    }
    return total / amount
}

Often you can prevent exceptions with additional validation, so don't forget to think about corner cases.

Conclusion

Let's recap. Exceptions don't stop a program from being compiled and run, but the program will crash when it tries to execute a line that causes an exception. Control statements can help you avoid some exceptions in your programs. Later, you'll learn a general method to handle exceptions and even throw them yourself. Good luck with your tasks!