56. Parameters

Let’s look at the last example from the previous chapter:

def determineAverage(list) {
    return list.sum() / list.size()
}

def scores = [2, 7, 4, 3]
def result = determineAverage(scores)
println result

You might be wondering what happened to the scores variable once it was passed to determineAverage as a parameter. Basically, Groovy gave it another name (list) for use within the method. Inside the method, list is just another variable. This means that if determineAverage changes list in some way, this is reflected in the scores variable used in the main script:

def scores = [2, 7, 4, 3]
def result = determineAverage(scores)
println result
println scores

def determineAverage(list) {
    list << 9
    return list.sum() / list.size()
}

The code above is very poorly behaved - it modifies list by adding a new item. Unless you provided documentation that explicitly states that you will change a parameter, most developers will assume that their parameters will be safely untouched by your method.

Declaring data types for parameters

Groovy lets you designate a data type for your parameters:

def determineAverage(List list) {
    return list.sum() / list.size()
}

As you start to develop classes and larger programs, methods create your Application Programming Interface (API). Such methods can be called by other people’s code and they could be using another JVM language (such as Java). It can make their life a little easier if you indicate the data types you’re expecting for your parameters. Alternatively, you can stay true to dynamic typing and let people know through your documentation.

Multiple parameters

Let’s look at another method - one that needs several parameters:

def callFriend(name, phone, message) {
    println "Dialling $name on $phone"
    println "Hi, $name - $message"
}

Either of these calls would work - it just depends if you want to use the parentheses:

callFriend('Barry', '0400 123 456', 'Did you see that local sporting team?')

callFriend 'Alex', '07 3344 0000', 'Could you please check on my pets whilst I\'\
m away?'

Each parameter may be typed if needed:

def callFriend(String name, String phone, String message) {..}

You can provide a mix of typed and untyped parameters but this is a little messy and I think it’s bad form so I can’t be bothered encouraging such an action by providing an example.