If null were an Object, it would support the methods of java.lang.Object such as equals(). However, this is not the case - any method invocation on a null results in a NullPointerException.
And this is what the Java Language Specification has to say on this topic:
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.
I think this can be boiled down to "null is special".
Answer from Michael Borgwardt on Stack OverflowIf null were an Object, it would support the methods of java.lang.Object such as equals(). However, this is not the case - any method invocation on a null results in a NullPointerException.
And this is what the Java Language Specification has to say on this topic:
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.
I think this can be boiled down to "null is special".
According to the Java spec, null is a type that can be assigned to an object variable (as a value as noted in the comment). You cannot instantiate or create variables of this type though, you must use the literal null provided by the compiler.
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Unfortunately, in many tutorials, books and other resources about Java, the terms "object" and "variable" are used in a sloppy way, which can lead to confusion similar to what you describe in your question.
For example, look at this line of code:
CopyString message = "Hello World";
People will often say "I have a String object named message here with the value "Hello World". This is wrong and it makes beginners think that objects and variables are the same thing, and that objects have names.
Accurate would be: "I have a variable named message here which is a reference to a String object with the value "Hello World".
Variables and objects are not the same thing. A variable* is not an object, it's a reference to an object (it's a pointer to an object somewhere in memory).
Variables have names. In the code above, there is a variable named message. Objects do not have names.
There is also not a one-to-one correspondence between variables and objects. Multiple variables may refer to the same object.
The value null is a special value that you can assign to variables* that means "this variable refers to no object". It's just like a null pointer in languages such as C and C++.
It doesn't really make sense to talk about null being an object. It's not an object, it's a special reference value to make variables refer to "no object".
If null can be assigned to any Object type
This is why I suspect you're confused about the difference between objects and variables: you do not assign null to an object, but to a variable (of "any object type" which is the same as "of a non-primitive type").
*: we're talking about variables of non-primitive types here
For a more advanced understanding:
The value null has a bit of a bad reputation (it's the cause of many bugs). You might want to use java.util.Optional to help prevent some of the problems that null causes.
See also the null object pattern, which is a design pattern about the idea of having a special object that represents "the empty value". This might be useful in special situations.
Null could have been defined to be a unique Object with behavior very similar to null references. It could be a singleton with a unique name and otherwise uninstantiable; it could compare equal to itself with == and unequal to other non-null objects; it could have a unique class, say Null.
This is, in fact, a common design pattern known as the null object pattern.
So why isn't it that way?
An
Objectvariable can refer to an object of any class. But it doesn't work the other way around. You can't stuff anObjectinto any variable you like. I can't write:CopyString str = new Object();Null objects would still require special language support. If
nullwere a singleton object of classObjector classNullyou could assign it to anObjectvariable, sure, but you wouldn't be able to assign it to aStringorArrayListvariable. Java would require different type safety rules, making an exception for this null object so it could be assigned to any type.The null object pattern is typically class-specific. You make a null object for a specific class and then implement sensible behavior for that class's methods on the null object. For example, you could create a "null" list that behaves like an empty list, returning no elements when iterated over and returning a size of
0when queried.That wouldn't work with a generic null object. A null object of class
Objector classNullwouldn't have implementations of the methods available on arbitrary subclasses. Ifnullwere an object, what would happen if you callednull.open("...")on aFilereference ornull.start()on aThread? Those methods wouldn't be defined.Again, there'd have to be special language support. Maybe those calls could return
nullthemselves? (What if they returnvoid, though?) Or maybe they could throwNullObjectExceptions?At some point you're just reinventing null references.
Finally, the simplest answer is: it's not that way because it's another way. Null could be a reference that points nowhere, or it could be a reference to a null object. Even if both were equally good solutions it can't be both. You're asking why it's "A" instead of "B". If it were "B", somebody would be asking why it's not "A".