Java Puzzle 1 - Autoboxing

Use collections with care in your code or else you will get unwanted side effects. On this article, I show the likely problem that requires extra caution when working with collections.

The above code snippet is really easy and straightforward to understand.

Line 11 we are creating a collection HashSet object.

Line 13 to 15 we are adding elements into to the Set.

Line 17 to 19 we are removing elements from the Set.

Line 18 has no effect because s - 0 has no effect on the result, that is, the arithmetic operation results in the same value. Therefore line 21 should print Zero as size. Yes! We got it. If you agree with me, we are wrong :)

Let's explain why below.

First, we are adding a short primitive on line 14. However, owing to the concept of Auto-boxing added since Java 1.5 or Java 5, the short primitive is converted to it's corresponding wrapper type which is a Short. This one is fine.

Now coming to line 18 where we are removing the elements, observe that it is as though we are removing the same short we added on previous line. However, we are not! Let's break it down.

When you perform an arithmetic operation on variables of one of the following primitive types (byte, char, short), the resulting type is always an int. Even if it is say (short + short). So, s - 0 results in an int type, because before remove() actually execute, s - 0 must be evaluated first. Evaluation of it results in an int type. Now, auto-boxing the type to corresponding wrapper type, we end up with Integer object. But our Set contains elements of type Short. This is where the problem lies.

Although s1 and i1 are equivalent below, they are not equal!

Short s1 = 1;

Integer i1 = 1;

These two classes are just siblings to the same parent (Number) but they are not the same. An implementation of equals method for every wrapper class first checks if the Object passed as an argument is of the same type to this object. If not, it returns false. That is why s1 and i2 are not equal. Therefore, the code above should print 10 on the console.

I have created a short video to explain this concept practically, you can watch the video from the link below.