What is the difference between NULL in C++ and null in Java?

Question

I've been trying to figure out why C++ is making me crazy typing NULL. Suddenly it hits me the other day; I've been typing null (lower case) in Java for years. Now suddenly I'm programming in C++ and that little chunk of muscle memory is making me crazy.

Wikiperipatetic defines C++ NULL as part of the stddef:

A macro that expands to a null pointer constant. It may be defined as ((void*)0), 0 or 0L depending on the compiler and the language.

Sun's docs tells me this about Java's "null literal":

The null type has one value, the null reference, represented by the literal null, which is formed from ASCII characters. A null literal is always of the null type.

So this is all very nice. I know what a null pointer reference is, and thank you for the compiler notes. Now I'm a little fuzzy on the idea of a literal in Java so I read on...

A literal is the source code representation of a fixed value; literals are represented directly in your code without requiring computation.

There's also a special null literal that can be used as a value for any reference type. null may be assigned to any variable, except variables of primitive types. There's little you can do with a null value beyond testing for its presence. Therefore, null is often used in programs as a marker to indicate that some object is unavailable.

Ok, so I think I get it now. In C++ NULL is a macro that, when compiled, evaluates to the null pointer constant. In Java, null is a fixed value that any non-primitive can be assigned too; great for testing in a handy if statement.

Java does not have pointers, so I can see why they kept null a simple value rather than anything fancy. But why did java decide to change the all caps NULL to null?

Furthermore, am I missing anything here?

Answer 1

Java's null is more like C++0x's nullptr. NULL in C++ is just 0 and can end up resolving to int rather than a pointer like you'd want. Consider:

void f(int);
void f(char*);

...

f(NULL); // which f()?

C++0x has nullptr, which fixes that problem but it's still not going to be totally equivalent to Java's null. They're just different languages.

Oh, and another diff is that Java has no pointers (or so it says). In Java you can legitimately assign null to a reference, in C++ you can't do that without having already used an ill-formed construct. Admittedly, Java would be next to useless without this ability but it's definitely another important difference.

Answer 2

• NULL is a preprocessor directive identifier, according to convention, those should be all caps.

• null is a language litteral representing a constant value and should according to convention be all lower (just as true or false).

Answer 3

Use 0 instead of NULL.

From "The C++ Programming Language" book by the creator of the language, Bjarne Stroustrup:

In C, it has been popular to define a macro NULL to represent the zero pointer. Because of C++’s tighter type checking, the use of plain 0 , rather than any suggested NULL macro, leads to fewer problems. If you feel you must define NULL, use const int NULL = 0;

Answer 4

In C++ NULL is a macro that, when compiled, defines the null pointer constant.

This is where you got it wrong. In C++, NULL is 0, and nothing else. Both pointers and integers can be NULL.

In java on the other hand, null is a null pointer, meaning that only objects can be null.

Answer 5

Well, I don't know WHY java desided to change NULL to null, however uppercase is more troublesom to type (at least in my consideration). But you can probably always ask sun :-P. Anyway, saying that java doesn't have pointers isn't compleately true, because normal references in java or c# or the likes works more like pointers than refferences in c++. For instance, in c++ a reference can't be NULL, but a reference in java can. And a reference in c++ can't change what variable it's pointing at (I might be mistaken at this point), but java reference can.

Answer 6

No, you've pretty much got it. In C++, a null pointer is any literal zero value that is assigned to a pointer. NULL is just a macro for 0, and so ptr = 0 and ptr = NULL are identical. It's included only for convenience/readability. The reason it is uppercase is because there is a longstanding convention in C and C++ that names of macros should be uppercase.

In Java, null is a built-in literal, and since it is not a macro (and Java has no concept of a macro anyway), there is no compelling reason to make it uppercase.

Answer 7

Regarding your last question, I'm not sure what the design rationale is behind lowercase null, but I think that the goal was to make null a language-level literal (like true or false) rather than a constant.

Keywords in java are generally lowercased and so are named literals. Constants (like Integer.MAX_VALUE) are typically uppercased.

Answer 8

Java doesn't have pointers, as you said yourself, which is why the concept of "null" is completely incomparable between Java and C++.

Your question makes as much sense as "what is the difference between an angle and an apple", i.e. it can't be answered in any other way besides the obvious: NULL in C++ and null in Java are two totally and completely unrelated concepts.

Java never "decided" to change all caps NULL to null, because, once again, Java's null has no relation whatsoever to C++'s NULL.

Answer 9

Pointers are basically memory addresses. And there is only one well-known and established memory address, 0. The c++ NULL is capitalized because it is a preprocessor macro and they are always capitalized by convention. The pointer concept maps directly to the concept of an address space in hardware.

Java uses references and it has it's own internal way of representing them. There is one well-known reference, null. It's in lower case because it is basically another symbol, albeit referring to a literal value.. I don't think they "changed" anything from C to Java. I think they just wanted the null concept so they called it "null".

In both cases, the null allows a function to return additional information. it can return the address of the result and the fact that maybe the operation was unsuccessful.

Answer 10

The Java null keyword is used to identify a variable as not referencing any object. The null keyword cannot be assigned to a variable that is declared with a primitive data type. The C++ NULL value is a constant that is defined as 0.