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A boxing conversion converts the value of a primitive type to the corresponding value of the reference type, for instance, from {{int}} to the type {{Integer}} \[[JLS 2005, Section 5.1.7|http://java.sun.com/docs/books/jls/third_edition/html/conversions.html#5.1.7], "Boxing Conversions"\]. This is convenient in many cases where an object parameter is desired, such as with collection classes like {{Map}} and {{List}}. Another use case is to pass object references to methods, as opposed to primitive types that are always passed by value. The resulting wrapper types also help to reduce clutter in code.

This noncompliant code example prints {{100}} as the size of the {{HashSet}} while it is expected to print {{1}}. The combination of values of types {{short}} and {{int}} in the operation {{i-1}} leads to autoboxing of the result into an object of type {{Integer}}. (See guideline [EXP05-J. Be aware of integer promotions in binary operators].) The {{HashSet}} contains values of only one type {{Short}}, whereas the code attempts to remove objects of the (different) type {{Integer}}. As a result, the remove operation is equivalent to a _No Operation_ (NOP). The compiler enforces type checking so that only {{Short}} values are inserted; however, a programmer is free to remove an object of any type without triggering any exceptions because {{Collections<E>.remove()}} accepts an argument of type {{Object}} and not {{E}}. Such behavior can result in unintended object retention or memory leaks \[[Techtalk 2007|AA. Bibliography#Techtalk 07]\].