Groovy - Different ways to invoke closure or method with closure as parameter

Closure is one of the most powerful features of Groovy. In Groovy, if a method takes closure as a parameter, there are several different ways to invoke the method. Also, there are different ways to invoke a closure. I'll also show some cases which you might expect to work, but don't. These can sometimes be confusing for a beginner. I'll show them with examples here.

Invoking method with closure as parameter:

The below method takes closure as it's second parameter.

def getOddNumbers(n, closure) {
for (int i=1; i<=n; i+=2) {
println "in method ${closure(i)}"
}
}

There are many ways (i.e syntaxes) to invoke this method:

1) getOddNumbers(10, {
println it
it
})

The closure is the second parameter passed to method. All it does is print the number and return that number. Now, we'll see the other ways.

2) getOddNumbers 10, {
println it
it
}

This syntax is same as 1, except that the open and close braces are optional. This really has nothing to do with closure, but I wanted to show this other syntax also.

3) getOddNumbers(10) {
println it
it
}

If closure is the last parameter to the method, then it can be passed as a separate code block like above. This is very elegant and widely used way to pass closure to a method.

4) printingClosure = {
println it
it
}
getOddNumbers(10, printingClosure)

We can assign closure to variable and use that in our method call later. Also,
getOddNumbers 10, printingClosure will work too i.e we can omit braces here. But getOddNumbers(10) printingClosure does not work though.

Invoking Closure:

Also, there are different ways to invoke the closure. In our above method, we used closure(i) to invoke the closure. We could also do closure i to invoke the method. This works as long as we don't use the return value. But, if we do
x = closure i or println closure i, this fails. closure(i) works fine in these cases and is the common way to invoke closure.