Square brackets [] create a list that implements the List interface.

By default it is of type ArrayList though you can create a list of type LinkedList as well.

examples/groovy/lists.groovy


def a_mess = ["question", 42, 'answer']
println a_mess              // [question, 42, answer]
println a_mess.size()       // 3
println a_mess[0]           // question
println a_mess.getClass()   // class java.util.ArrayList

def names = ['foo', 'bar', 'qux'] as LinkedList
println names               // [foo, bar, qux]
println names.size()        // 3
println names[1]            // bar
println names.getClass()    // class java.util.LinkedList

for (i = 0; i < names.size(); i++) {
   println i + ') ' + names[i]
}
// 0) foo
// 1) bar
// 2) qux


names.each {
    println it
}
// foo
// bar
// qux

names.each { name ->
    println name
}
// foo
// bar
// qux

names.eachWithIndex { name, i ->
    println i +  '] ' + name
}
// 0] foo
// 1] bar
// 2] qux

There are several ways to iterate over the elements of a list.

There is the old-school way of using a for loop. This is like in most of the other languages uses a variable to go over the indexes from 0 to the size-1 and then use this index to access the real value in the list. The name of this internal variable is typically i or j and it does not need to be declared using def.

The

each method will also iterate over the elements of the list. For each iteration it will put the current value in a varaible called it and then execute the block that follows the each statement. The it is similar to to $_ of Perl and the _ of Python.

Optionally we can set the name of this internal variable by providing it right after the opening { of the block, followed by an arrow.

Finally, we can also use the eachWithIndex in which case we need to define the name of two variables. The first one will get the current value, the second one will get the index of the current value.

Read more about lists in Groovy.