严格讲既不是基础类型也不是对象类型，是一个特殊的类型，并可以看作允许cast成任意类型的对象。
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.
不是对象, 是一个特殊的值.
String s = (String)null; // no error.
((String)null).valueOf(0); // 调用String.valueOf(int)
Integer i = (Integer)null; // no error.
int i2 = null; // error.
