Question

I’m fairly new to groovy, looking at some existing code, and I see this:

def timestamp = event.timestamp[]

I don’t understand what the empty square brackets are doing on this line. Note that the timestamp being def’d here should receive a long value.

In this code, event is defined somewhere else in our huge code base, so I’m not sure what it is. I thought it was a map, but when I wrote some separate test code using this notation on a map, the square brackets result in an empty value being assigned to timestamp. In the code above, however, the brackets are necessary to get correct (non-null) values.

Some quick Googling didn’t help much (hard to search on “[]”).

EDIT: Turns out event and event.timestamp are both zero.core.groovysupport.GCAccessor objects, and as the answer below says, the [] must be calling getAt() on these objects and returning a value (in this case, a long).

Answer 1

The square brackets will invoke the underlying getAt(Object) method of that object, so that line is probably invoking that one.

I made a small script:

class A { 
    def getAt(p) { 
        println "getAt: $p"
        p 
    }
}
def a = new A()
b = a[]
println b.getClass()

And it returned the value passed as a parameter. In this case, an ArrayList. Maybe that timestamp object has some metaprogramming on it. What does def timestamp contains after running the code?

Also check your groovy version.