JDK primitive wrappers are immutable classes. Although this have its advantages, sometimes it is a drawback, especially regarding performance. In heavy calculations, creating new objects all the times may be time and memory consuming, enough to unnecessary slow down the application. Therefore, Jodd provides mutable versions of all wrappers. They are very similar to immutable, extend Number, but have a value property and attribute that can be changed, as well as empty constructor. Jodd recognize mutable classes and they can be used instead mutable without any further change.