Java: Double (class) vs double (primitive)

A Double is a class, or a reference type, defined in the standard library. It stores a reference to an object containing a value (a "box").

A double on the other hand, is a primitive type and part of the language itself. It stores an actual value.

We say that Double is the wrapper type for double, and objects of type Double are boxed values.

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Java: Primitives vs Objects and References
Java: Wrapper Types