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In object-oriented computer programming, a null object is an object with no referenced value or with defined neutral (null) behavior.The null object design pattern, which describes the uses of such objects and their behavior (or lack thereof), was first published as "Void Value" [1] and later in the Pattern Languages of Program Design book series as "Null Object".
In computing, a null pointer or null reference is a value saved for indicating that the pointer or reference does not refer to a valid object. Programs routinely use null pointers to represent conditions such as the end of a list of unknown length or the failure to perform some action; this use of null pointers can be compared to nullable types ...
The language does not provide any explicit pointer manipulation operators. It is still possible for code to attempt to dereference a null reference (null pointer), however, which results in a run-time exception being thrown. The space occupied by unreferenced memory objects is recovered automatically by garbage collection at run-time. [20]
Null device, a virtual file that discards data written to it, on Unix systems /dev/null Null pointer or reference (sometimes written NULL , nil , or None ), an object pointer (or reference) not currently set to point (or refer) to a valid object
In a 2009 talk, Tony Hoare traced the invention of the null pointer to his design of the ALGOL W language and called it a "mistake": I call it my billion-dollar mistake. It was the invention of the null reference in 1965. At that time, I was designing the first comprehensive type system for references in an object oriented language (ALGOL W).
Branch to target if value is a non-null object reference (alias for brtrue). Base instruction 0x2D brinst.s <int8 (target)> Branch to target if value is a non-null object reference, short form (alias for brtrue.s). Base instruction 0x39 brnull <int32 (target)> Branch to target if value is null (alias for brfalse). Base instruction 0x2C
When a variable of an object goes out of scope the reference is broken and when there are no references left the object gets marked as garbage. The garbage collector will then soon collect and destroy it. A reference variable is null when it does not reference any object.
Reference types differ from these in that the entities they refer to are always accessed via references; for example, whereas in C++ it's possible to have either a std:: string and a std:: string *, where the former is a mutable string and the latter is an explicit pointer to a mutable string (unless it's a null pointer), in Java it is only ...