Use of array of zero length

An example. Say, you have a function

public String[] getFileNames(String criteria) {

to get some filenames. Imagine that you don't find any filenames satisfying criteria. What do you return? You have 2 choices - either return null, or 0-sized array.

The variant with 0-sized array is better, because your caller doesn't need to check for NULL and can process the array in a consistent way - say, in a loop (which would be empty in this case).

There's a chapter on this in Effective Java, Item 27

It's easier to work with than null in many cases, where null is the obvious alternative.

Suppose you want to return an Iterable<String> containing (say) a list of relevant filenames... but there aren't any for some reason. You could return null to indicate that, but then the caller has to special-case that. Instead, if you return an empty collection, the caller can still use an enhanced for loop:

for (String file : getFiles())

So why use an empty array instead of an empty ArrayList or something similar? Arrays are a fixed size, so an empty array is effectively immutable. That means you can keep a single value and return it to whoever you like, knowing they can't possibly do anything with it. That can be very useful in some situations.

it is a replacement for null since you don't need to check for null before use it. More formally it is a special case of special case design pattern (check also Null Object).

Another idiomatic use is collections toArray:

List<String> list = new ... ;
// fill the list
String[] array = list.toArray(new String[0]);