Return default or raise an exception?

Question

I am confused about what is considered a good practice - is this decision language dependent? Suppose I have the following Java code:

public class Stack {
    public Integer pop() {
      if (isEmpty()) return null; // or some exception maybe?
          // else get and return the top item in the stack.
      };
    }
}

The client of the pop method expects some Integer value, so what would be the best approach to let the client know that the stack is empty?

Answer 1

Returning null or a default value is usually a bad practice and an exception should be preferred. The reason is that you should always strive to fail as fast as you can when something goes wrong. If you return null instead, you will get errors happening in some other place in the code and users of your API will have trouble finding where the problem was. This is called Fail Fast.

An exception to this rule of thumb is when your API will make users rely on exceptions for flow control, so if your stack does not support isEmpty(), an exception is not a good idea here. If you have a stack that can allow only peek(), pop(), and add() - for some reason, isEmpty() is not allowed to be part of the API.

What will happen to the code of your users for both approaches?

Option 1 - Using null:

Integer x = stack.pop();
if (x != null) { 
    //do something with x
}

Option 2 - Using exceptions:

Integer x = null;
try { 
    x = stack.pop();
} catch (MyException e) { }
//do something with x

The second is actually using the exceptions mechanism for flow control - which is a big flaw in API design.