Exception Handling with try

This lesson introduces the try control structure and teaches you how it can be used for exception handling.

We'll cover the following

Exception Handling

When running a program, we sometimes encounter unexpected conditions which give us errors and warnings. These conditions are known as exceptions. While exceptions are unexpected, they need to be anticipated by the programmer and only then can they be dealt with in a timely manner. The process of skillfully handling exceptions is known as exception handling.

Let’s take a real-life example to better visualize exception handling.

Imagine you own a courier service which delivers packages to people’s homes on bikes. During one such delivery, one of the bikes breaks down unexpectedly. While this was unexpected, you still anticipated bikes to break down and kept a spare as back up. Because you handled the situation, the customer was able to receive their package in a timely manner. This is exception handling; handling exceptions in a program to prevent premature termination of the code.

svg viewer

In Scala, we handle exceptions using the try control structure. Let’s see how it works below.

try

try is used to try particular parts of a code that might throw exceptions. If an exception is thrown, it is caught using catch. When the compiler catches an exception, it chooses how to handle it from a list of provided cases.

Let’s look at the syntax of try below.

Syntax

The block of code in which you anticipate an exception is wrapped in a try expression. This is followed by the catch statement which catches the exception and runs different cases until it finds the correct exception. The statement after => tells the compiler what to do in place of executing the block of code.

Scala reuses Java’s exceptions.

Let’s look at a very common exception: dividing by zero.

Using try

Dividing a number by another number is a simple enough task, but sometimes we end up diving by zero which causes the program to crash and pre-maturely terminate.

Press + to interact
val dividend = 10
val divisor = 0
val quotient = dividend/divisor
println(quotient)

Running the above code gives an ArithmeticException as it is thrown during an error while doing an arithmetic calculation. How would we handle this using try?

Press + to interact
val dividend = 10
val divisor = 0
try { 
  val quotient = dividend/divisor
  println(quotient)
}
catch {
  case ex: ArithmeticException => println("Dividing by zero is not allowed")
}

The code above will print "Dividing by zero is not allowed" instead of crashing like it did in the first program. Try changing divisor to a non-zero value and see what happens.

Let’s move on to our final control structure: match.