Autoboxing

Autoboxing is the automatic conversion between primitive types and their corresponding object wrapper classes by the compiler or runtime. In languages such as Java, these conversions allow primitive values to be used where objects are required, and conversely enable objects to be treated as primitives in certain contexts. For Java, the common pairs are int with Integer, long with Long, boolean with Boolean, char with Character, byte with Byte, short with Short, float with Float, and double with Double. Unboxing is the reverse process.

In Java, autoboxing is implemented by the compiler and can involve calls to valueOf for boxing and unboxing operations for converting back to primitives. This feature is essential for using primitives in generic types and collections, such as List<Integer>, where type parameters must be reference types. Java also implements caching for frequently used values (for example, Integer.valueOf(-128) to valueOf(127) are cached), which reduces the cost of boxing in common cases.

In C#, boxing and unboxing apply when a value type is treated as an object or as an interface type. Boxing wraps the value in a reference-type object on the heap, while unboxing extracts the value through a type cast. Boxing is necessary for non-generic collections and for interoperability with APIs that expect objects, but it incurs allocation and later garbage collection overhead. Unboxing requires an explicit or implicit cast and can fail at runtime if the value is not of the expected type.

Common issues include null values and reference equality. Unboxing a null reference in Java yields a NullPointerException, and comparing wrapper objects with == may compare references rather than values. Performance-wise, unnecessary boxing can add overhead; in performance-critical code, it is often preferable to minimize boxing or to use primitive-focused facilities where available.