In Groovy we can add a method named call to a class and then invoke the method without using the name call. We would simply just type the parentheses and optional arguments on an object instance. Groovy calls this the call operator: (). This can be especially useful in for example a DSL written with Groovy. We can add multiple call methods to our class each with different arguments. The correct method is invoked at runtime based on the arguments.

In the following example we have User class with three call method implementations. Next we see how we invoke the call methods, but without typing the method name and just use the parenthesis and arguments:

import groovy.transform.ToString
@ToString(includeNames = true)
class User {

    String name
    String alias
    String email
    String website

    // Set name.
    def call(final String name) {
        this.name = name
        this
    }

    // Use properties from data to assign
    // values to properties.
    def call(final Map data) {
        this.name = data.name ?: name
        this.alias = data.alias ?: alias
        this.email = data.email ?: email
        this.website = data.website ?: website
        this
    }

    // Run closure with this object as argument.
    def call(final Closure runCode) {
        runCode(this)
    }

}


def mrhaki =
    new User(
        name: 'Hubert Klein Ikkink',
        alias: 'mrhaki',
        email: 'mrhaki@email.nl',
        website: 'https://www.mrhaki.com')

// Invoke the call operator with a String.
// We don't have to explicitly use the
// call method, but can leave out the method name.
// The following statement is the same:
// mrhaki.call('Hubert A. Klein Ikkink')
mrhaki('Hubert A. Klein Ikkink')

// Of course parentheses are optional in Groovy.
// This time we invoke the call method
// that takes a Map arguemnt.
mrhaki email: 'h.kleinikkink@email.nl'

assert mrhaki.name == 'Hubert A. Klein Ikkink'
assert mrhaki.alias == 'mrhaki'
assert mrhaki.email == 'h.kleinikkink@email.nl'
assert mrhaki.website == 'https://www.mrhaki.com'


// We can pass a Closure to the call method where
// the current instance is an argument for the closure.
// By using the call operator we have a very dense syntax.
mrhaki { println it.alias }  // Output: mrhaki

// Example to transform the user properties to JSON.
def json =  mrhaki {
    new groovy.json.JsonBuilder(\[vcard: \[name: it.name, contact: it.email, online: it.website\]\]).toString()
}

assert json == '{"vcard":{"name":"Hubert A. Klein Ikkink","contact":"h.kleinikkink@email.nl","online":"https://www.mrhaki.com"}}'

Written with Groovy 2.4.8.

Original blog post

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