The Double class wraps a value of the primitive type double in an object. An object of type Double contains a single field whose type is double." link. As @Fess mentioned and because Double is reference type it can be null. If you want you can explictly convert from Double to double with .doubleValue() method and viceverrsa with new Double(1.0).
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The church has doubled its membership in the last five years. double the size/number/amount etc (of something) A promise was given to double the number of police on duty. see thesaurus at increase 2 [ transitive] (also double over/up) to fold something in half Take a sheet of paper and double it over. 3 [ intransitive] to hit the ball far