嚴格講既不是基礎類型也不是對象類型，是一個特殊的類型，并可以看作允許cast成任意類型的對象。
Java Language Specification Chapter 4

There is also a special null type, the type of the expression null, which has no name. Because the null type has no name, it is impossible to declare a variable of the null type or to cast to the null type. The null reference is the only possible value of an expression of null type. The null reference can always be cast to any reference type. In practice, the programmer can ignore the null type and just pretend that null is merely a special literal that can be of any reference type.