What does [:] mean in groovy?

[:] creates an empty Map. The colon is there to distinguish it from [], which creates an empty List.

This groovy code:

def foo = [:]

is roughly equivalent to this java code:

Object foo = new java.util.LinkedHashMap();

[:] is shorthand notation for creating a Map.

You can also add keys and values to it:

def foo = [bar: 'baz']

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