void : Java Glossary

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?

• 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.

1. 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.
2. 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:

1. canthappen

   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.

2. null

   Make sure all void inputs are converted to null using a possiblyNull method.

3. empty

   Make sure all void inputs are converted to "" using a possiblyEmpty method.

4. blank

   Not recommended.

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

• 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 they are.
• 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 similar distinction.