12.2 Exceptions

In this section, we will learn how to throw exceptions to effectively handle errors that may arise in our code.

Our ArraySet implementation from the previous section has a small error. When we add null to our ArraySet, we get a NullPointerException.

The probelm lies in the contains method where we check items[i].equals(x). If the value at items[i] is null, then we are calling null.equals(x) -> NullPointerException.

Exceptions cause normal flow of control to stop. We can in fact choose to throw our own exceptions. In python you may have seen this with the raise keyword. In Java, Exceptions are objects and we throw exceptions using the following format:

throw new ExceptionObject(parameter1, ...)

Let's throw an exception when a user tries to add null to our ArraySet. We'll throw an IllegalArgumentException which takes in one parameter (a String message).

Our updated add method:

/* Associates the specified value with the specified key in this map.
   Throws an IllegalArgumentException if the key is null. */
public void add(T x) {
    if (x == null) {
        throw new IllegalArgumentException("can't add null");
    }
    if (contains(x)) {
        return;
    }
    items[size] = x;
    size += 1;
}

We get an Exception either way - why does this better?

1. We have control of our code: we consciously decide at what point to stop the flow of our program

2. More useful Exception type and helpful error message for those using our code

However, it would be better if the program doesn't crash at all. There are different things we could do in this case. Here are some below:

Approach 1: Don't add null to the array if it is passed into add

Approach 2: Change the contains method to account for the case if items[i] == null.

Whatever you decide, it is important that users know what to expect. That is why documentation (such as comments about your methods) is very important.