What is the Groovy 'it'?

Question

I have a collection which I process with removeIf {} in Groovy. Inside the block, I have access to some it identifier. What is this and where is it documented?

Answer 1

it is an implicit variable that is provided in closures. It's available when the closure doesn't have an explicitly declared parameter.

When the closure is used with collection methods, such as removeIf, it will point to the current iteration item.

It's like you declared this:

List<Integer> integers = [1, 2, 3]
for(Integer it: integers) {print(it)}

When you use each, instead (and that's an example), you can get it implicitly provided:

integers.each{print(it)} //it is given by default

Or

integers.removeIf{it % 2 == 0} //it is the argument to Predicate.test()

it will successively take the values 1, 2, and 3 as iterations go.

You can, of course, rename the variable by declaring the parameter in the closure:

integers.each{myInteger -> print(myInteger)}

In this case, Groovy doesn't supply the implicit it variable. The documentation has more details

Answer 2

If you create a closure without an explicit argument list, it defaults to having a single argument named it. Here's an example that can be run in the Groovy console

Closure incrementBy4 = { it + 4 }

// test it
assert incrementBy4(6) == 10

In the example above the closure is identical to

Closure incrementBy4 = { it -> it + 4 }

Here's another example that uses removeIf

Closure remove2 = { it == 2 }

def numbers = [1, 2, 3]
numbers.removeIf(remove2)

// verify that it worked as expected
assert numbers == [1, 2]