Question

I ran into a bad chunk of code today that equates to this:

[["asdf"]].each {String str -> println str}

println (["asdf"] as String)

Put this into groovy console (groovysh) and you’ll see this come out:

asdf

[asdf]

Can anyone explain why there’s a difference in the outputs?

Answer 1

Groovy will unwrap a List if it means it will fit into the types you have defined in your closure.

If you don’t define the type (or set it as List), it will behave as you expect:

// Both print '[a]'
[['a']].each { it -> println it }
[['a']].each { List it -> println it }

If however you define a type, it will attempt to unwrap the list and apply the contents to the defined parameters, so:

// Prints 'a'
[['a']].each { String it -> println it }

Think of it like varargs in java, or calling the closure with closure( *it ) in Groovy

It actually comes in pretty useful as you can do things like:

// Tokenize the strings into 2 element lists, then call each with these
// elements in separate variables
['a=b', 'b=c']*.tokenize( '=' )
               .each { key, value ->
  println "$key = $value"
}