A keyword used in Java to indicate a method does
not return a value. It is not used to indicate a method requires no
parameters. Constructors don’t
use the void keyword even though they don’t return an
Void is also a dummy class.
Void also refers generically to the various flavours of
hollow Strings, namely: blank (i.e x.length() != 0 && x.trim().length() == 0, e.g. ), empty (i.e. x.length() == 0, e.g.
"") and null (i.e.
x == null). One of the most common sources of error in
Java programs comes from confusing the three
different flavours of void strings. Java code often has a maintenance timebomb
ticking in it in the form of inconsistent representation of void strings and objects.
It is unwise to just let NullPointerException find your
problems for you. The problem may surface many kilometers from the true source of the
problem. There will always be one more bug. It is better to decide on your canonical
void representation and be fanatically rigid about it.
Which void representation should you use?
If you don’t use a canonical representation and consistent checking for
the various void forms you will have two classes of bug:
- null is fast to test for, e.g. if ( it == null ),
however, you can’t run any methods on it. e.g anull. equals( somethingElse) will generate a NullPointerException.
- "" will work as a parameter to most
methods expecting a String.
These bugs can be a bitch to track down because the variant voids or void itself
are often rare for many data fields. It may require a particular improbable
constellation of data for the bug to surface.
- Void Strings mistakenly treated as if they were non-void. The effect can be
indirect and very hard to track down, e. g. You may test for the presence of A and
if it exists do some operation on B.
- Non-void Strings are mistakenly treated as if they were void. Data just
disappear or are ignored.
There are two plausible canonical representations for void namely null and empty (""). Normally you
combine either of them with canthappen. The four ways you
might represent void Strings are:
You are just asking for trouble if you use a variety of void representations.
The code may be clear to you, but will drive people who come after you maintaining
the code crazy.
Check for all forms of void parameters and throw an
IllegalArgumentException or NullPointerException if Java won’t throw one all by itself
soon. It is far easier to prevent bad data (i.e. inconsistent void
representations) getting into your objects and databases that to deal with it
once it gets in. You might wrap this code in if
(debugging) so it can be turned off for production speed. Unfortunately
Java has no design-by-contract
features to do this more elegantly. You can catch these with a neverNull method.
Make sure all void inputs are converted to null using a possiblyNull method.
Make sure all void inputs are converted to "" using a
In a similar way you can get into trouble returning null instead of an empty Collection.
One convention, used by File.list, is to return an empty
array or Collection for no elements and null to represent the result of an invalid request.
Note to C programmers
There are a few differences between C and Java that may
- Java Strings don’t have a terminating 0 byte.
They don’t have a terminating 0 character either. Java Strings are 16-bit characters. Instead,
they have a hidden 32-bit count field to track how long
- NULL in C is just
a synonym for 0. null in Java is a magic value that can be assigned to
pointers/references to say they don’t currently point to anything. Instance
and static references are automatically initialised to
null for you in Java. Usually this value is represented by 0 at the hardware
level, but there is no way you could find out just by writing a Java program.
- In C, there is a difference between NULL and a
string consisting only of a single 0. In Java, the equivalents are null and "" which makes a
There are three ways you might initialise
- No initialisation at all: advantage: compiler catches you if you fail to
provide a proper value later (for local variables).
- null: advantage: fast comparison test. Run time
catches you if you fail to provide a proper value later. Happens automatically
for static and instance variables.
- "": advantage: The value will be
processed just like any other String without incident.
This may be quite suitable if an empty String is a
IntelliJ IDE (Integrated Development Environment) has custom annotations
@Nullable and @NotNull that you can use to mark a parameter or return value as
potentially null, or must be not null, respectively. You must import to make the
The IntelliJ annotations.jar file must be available to
javac.exe on the processorpath (usually the classpath) for
them to work. You must specify the specific jar, not just the directory.
Oracle’s Javadoc on Void
class : available: