What is autoboxing and unboxing in Java?

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Autoboxing is the automatic conversion between primitives ( int , double ) and their corresponding wrapper classes ( Integer , Double ). Unboxing is the reverse. Why — collections and generics work only with objects, not primitives. Compiler magic — the compiler inserts Integer.valueOf() / intValue() calls. Caching — Integer.valueOf(-128..127) returns cached instances; == can compare equal for those but not larger values. Pitfalls — performance overhead in loops; NullPointerException if you unbox a null wrapper. The performance trap in loops is the key practical point. Know the NPE risk from unboxing null. The Integer cache (-128 to 127) is a related topic. Recommend primitives for performance, wrappers for nullability and collections.