When should I use primitives instead of wrapping objects?

Question

Actually here is a similar topic with little practical value. As far as I understand, primitives perform better and should be used everywhere except for the cases where Object-related features (e.g. null check) are needed. Right?

Answer 1

Do not forget that, since creating a new wrapper for every boxing occurrence is quite expensive, especially considering it usually being used at a single scope of a method, Autoboxing uses a pool of common wrappers.

This is in fact an implementation of the flyweight design pattern. When a boxing occurs for a well-known value, instead of creating a new wrapper instance, a pre-created instance is fetched from a pool and returned.

One consequence is: it’s still not recommended to use autoboxing for scientific calculations. For example, the code d = a * b + c is using Integer classes for a, b, c and d, and the generated code is d.valueOf(a.intValue() * b.intValue() + c.intValue()). All these method invocations have their own overhead, so it’s usually recommended to use autoboxing when needed to store primitives in collections.

And even then, if you have a huge collection of Integer wrapping int, the overhead can implies longer execution times, up to 20 times longer, as reported in this article.

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Jb adds this important comment:

Also Wrapper.valueOf(primitive) uses pool of wrappers. So prefer Integer.valueOf(5) to new Integer(5)