Java: Double (class) vs double (primitive)
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").
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.
Continue reading here:
Java: Primitives vs Objects and References
Java: Wrapper Types