What does [:] mean in groovy?

Question

While reading some groovy code of another developer I encountered the following definition:

def foo=[:]

What does it mean?

Answer 1

Quoting the doc:

Notice that [:] is the empty map expression.

... which is the only Map with size() returning 0. ) By itself, it's rarely useful, but you can add values into this Map, of course:

def emptyMap = [:]
assert emptyMap.size() == 0
emptyMap.foo = 5
assert emptyMap.size() == 1
assert emptyMap.foo == 5