C#

It is best to avoid using value types in situations where they must be boxed a high number of times, for example in non-generic collections classes such as System.Collections.ArrayList. You can avoid boxing of value types by using generic collections such as System.Collections.Generic.List. Boxing and unboxing are computationally expensive processes. When a value type is boxed, an entirely new object must be created. This can take up to 20 times longer than a simple reference assignment. When unboxing, the casting process can take four times as long as an assignment.

Java

Converting a primitive value (an int, for example) into an object of the corresponding wrapper class (Integer) is called autoboxing. The Java compiler applies autoboxing when a primitive value is:

Passed as a parameter to a method that expects an object of the corresponding wrapper class.
Assigned to a variable of the corresponding wrapper class.

Converting an object of a wrapper type (Integer) to its corresponding primitive (int) value is called unboxing. The Java compiler applies unboxing when an object of a wrapper class is:

Passed as a parameter to a method that expects a value of the corresponding primitive type.
Assigned to a variable of the corresponding primitive type.

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