66. Parameters

We just saw that closures have an in-built it parameter but we can specify a parameter when we declare the closure:

def cls = { name -> println "Hello, $name" }
cls('Jerry')

In the example above I declare the name parameter and this replaces it - in fact, if I called it within the closure I’d get an exception.

The syntax of closures is starting to become evident:

1. Start the closure with {
2. List parameters, followed by ->
3. Write a set of statements for the closure body
4. End the closure with }

Each parameter is separated using a comma (,):

def cls = { name, pet -> println "Hello, $name. How is your $pet?" }
cls('Jerry', 'cat')

As the closure gets more complicated I like to break it up over a number of lines. I start the closure and declare the parameters on the first line. The body of the closure then follows much the same as a method body and a final line closes off the closure:

def cls = { name, pet ->
    println "Hello, $name. How is your $pet?"
}

cls('Barry', 'Lemur')

Closure parameters let me do the same things I can do with method parameters:

1. Use data types for parameters
2. Provide default values
3. Varargs
4. Named parameters

Parameter data types:

def cls = { String name, String pet ->
    println "Hello, $name. How is your $pet?"
}

cls('Sally', 'Echidna')

Default values:

def cls = { name, pet = 'dog' ->
    println "Hello, $name. How is your $pet?"
}

cls('Barry')

Varargs:

def cls = { name, ... pets ->
    println "Hello, $name. How are your $pets?"
}

cls('Herman', 'cat', 'dog', 'spider')

Named parameters:

def cls = { grades, name ->
    println "This year $name received the following grades: $grades"
}

cls(maths: 'D', science: 'C', art: 'A', 'Albert')

So closures and methods are rather similar - there’s no black magic going on.