Nullable types are a feature of some programming languages which allow the value to be set to the special value NULL instead of the usual possible values of the data type. In statically-typed languages, a nullable type is an option type, while in dynamically-typed languages, equivalent behavior is provided by having a single null value. NULL is frequently used to represent a missing value or invalid value, such as from a function that failed to return or a missing field in a database, as in NULL in SQL. Primitive types such as integers and Booleans cannot generally be null, but the corresponding nullable types can also assume the NULL value.
Example
An integer variable may represent integers, but 0 is a special case because 0 in many programming languages can mean "false". Also this doesn't give us any notion of saying that the variable is empty, a need for which occurs in many circumstances. This need can be achieved with a nullable type. In programming languages like C# 2.0, a nullable integer, for example, can be declared by a question mark. In programming languages like C# 1.0, nullable types can be defined by an external library as new types. A Boolean variable makes the effect more clear. Its values can be either "true" or "false", while a nullable boolean may also contain a representation for "undecided". However, the interpretation or treatment of a logical operation involving such a variable depends on the language.
In contrast, object pointers can be set to NULL by default in most common languages, meaning that the pointer or reference points to nowhere, that no object is assigned. Nullable references were invented by C. A. R. Hoare in 1965 as part of the Algol W language. Hoare later described his invention as a "billion-dollar mistake". This is because object pointers that can be NULL require the user to check the pointer before using it and require specific code to handle the case when the object pointer is NULL. Java has classes that correspond to scalar values, such as Integer, Boolean and Float. Combined with autoboxing, this effectively allows nullable variables for scalar values.
Compared with option types
Nullable type implementations usually adhere to the null object pattern. There is a more general and formal concept that extend the nullable type concept, it comes from option types, which enforce explicit handling of the exceptional case. Option type implementations usually adhere to the Special Case pattern.
Language support
The following programming languages support nullable types. Statically typed languages with native null support include:
