Over a million developers have joined DZone.

Groovy Goodness: Using Implicit call() Method

DZone's Guide to

Groovy Goodness: Using Implicit call() Method

· Java Zone
Free Resource

Microservices! They are everywhere, or at least, the term is. When should you use a microservice architecture? What factors should be considered when making that decision? Do the benefits outweigh the costs? Why is everyone so excited about them, anyway?  Brought to you in partnership with IBM.

In Groovy we can invoke an implicit call() method on a Groovy object. We can leave out the call method name and just use (). We can use meta programming to add an implementation for the call() method to a class. In the following example script we add an implementation for the call() method with a single parameter to the String class. The implementation returns the element found at the range specified by the argument when we invoke the method:

String.metaClass.call = { range ->

def value = 'Groovy is Gr8'
assert value(0) == 'G'
assert value(10) == 'G'
assert value(4) == value[4]
assert value.call(1) == value(1)
assert value(0..5) == 'Groovy'

Inspired by the examples http://groovyconsole.appspot.com/view.groovy?id=21006 and http://groovyconsole.appspot.com/script/21005 we can also write our own class and implement the call() method. This can for example be used in DSLs.

class StringConverter {
    def value
    def value(s) {
        value = s

    /** Convert characters in value property if cond is true */    
    def upper(cond) {
        value = value.collect { cond(it) ? it.toUpperCase() : it }.join()

    def call(callable) {

def converter = new StringConverter()
converter.with {
    value 'mrhaki' upper { it < 'm' }
    // Equivalent to:
    // value('mrhaki') upper { it < 'm' }
    // or
    // value('mrhaki').call(upper { it < 'm' })
    // or
    // value('mrhaki').call(upper({ it < 'm' }))
assert converter.value == 'mrHAKI'

converter.with {
    value('jdriven') upper { it == 'j' || it == 'd' }

    assert value == 'JDriven'

(Code written with Groovy 2.0.4)





Discover how the Watson team is further developing SDKs in Java, Node.js, Python, iOS, and Android to access these services and make programming easy. Brought to you in partnership with IBM.


Published at DZone with permission of Hubert Klein Ikkink, DZone MVB. See the original article here.

Opinions expressed by DZone contributors are their own.


Dev Resources & Solutions Straight to Your Inbox

Thanks for subscribing!

Awesome! Check your inbox to verify your email so you can start receiving the latest in tech news and resources.


{{ parent.title || parent.header.title}}

{{ parent.tldr }}

{{ parent.urlSource.name }}