Optional parameter

One of the uses of his facility is to have some of the parameters become optional.

In the following example we have a simple function called prompt that would receive a text and a number. It would prompt the user with the text and check the input against some value. (In our example it is a simple variable. In a more realistic situation we would have the password hashed and probably stored in a database.)

If the user makes a mistake s/he is prompted again and again.

The number of times the user can try is set by the second parameter.

In our example we also have a second definition of the prompt function that only expects a string. It then calls the same name again (prompt passing to it the receives text and the number 3.

examples/groovy/prompt.groovy

def prompt(text, Integer cnt) {
    def pw = 'secret'
    while (cnt > 0) {
        res = System.console().readLine text
        if (res == 'secret') {
            return true
        }
        cnt--
    }
    return false
}

def prompt(text) {
    prompt(text, 3)
}

// println(prompt('Password: ', 2))
println(prompt('Password: '))

If we call prompt with a text and a number, the first version will be executed.

If we call prompt with only a string, the second version will be executed. It will call prompt with the same string and the number 3. This time Groovy will execute the first version of the function.

Same function with different types

In this example you can see how we declared the same function name 3 times. Each time expecting different types of arguments. Then we call the function and we can see Groovy calls the right implementation in each case. The order of the definition is not important, you could put the general case earlier as well, but it seems to be clearer to me to first have the special cases declared and only later the more generic ones.

examples/groovy/add_functions.groovy

def add(Integer x, Integer y) {
    println("Add integers")
}

def add(String x, String y) {
    println("Add strings")
}

def add(x, y) {
    println("Add any two things")
}



add(2, 3)             // Add integers
add("abc", "def")     // Add strings
add(2.3, 4)           // Add any two things
add('a', 4)           // Add any two things