[ZZ]Groovy Goodness: Using the GroupBy Method

In Groovy we can group the elements of a Collection type in a map. We define the rule for grouping with a closure. The result is a map where the key is the grouping condition and the value contains the elements of the Collection type belonging to the key. Let's see the groupBy method in action:

<!--<br /> <br /> Code highlighting produced by Actipro CodeHighlighter (freeware)<br /> http://www.CodeHighlighter.com/<br /> <br /> --> class  User {
    String name
    String city
    Date birthDate
     public  String toString() {  " $name "  }
}

def users  =  [
     new  User(name: ' mrhaki ' , city: ' Tilburg ' , birthDate: new  Date( 73 , 9 , 7 )),
     new  User(name: ' bob ' , city: ' New York ' , birthDate: new  Date( 63 , 3 , 30 )),
     new  User(name: ' britt ' , city: ' Amsterdam ' , birthDate: new  Date( 80 , 5 , 12 )),
     new  User(name: ' kim ' , city: ' Amsterdam ' , birthDate: new  Date( 83 , 3 , 30 )),
     new  User(name: ' liam ' , city: ' Tilburg ' , birthDate: new  Date( 109 , 3 , 6 ))
]

//  Helper closure for asserts.
def userToString  =  { it.toString() }

//  Group by city property of user object:
def usersByCity  =  users.groupBy({ user  ->  user.city })
assert   2   ==  usersByCity[ " Tilburg " ].size()
assert  [ ' mrhaki ' ,  ' liam ' ]  ==  usersByCity[ " Tilburg " ].collect(userToString) 
assert  [ ' bob ' ]  ==  usersByCity[ " New York " ].collect(userToString)
assert  [ ' britt ' ,  ' kim ' ]  ==  usersByCity[ " Amsterdam " ].collect(userToString)

//  Group by year of birthdate property of user object:
def byYear  =  { u  ->  u.birthDate[Calendar.YEAR] }
def usersByBirthDateYear  =  users.groupBy(byYear)
assert  [ ' mrhaki ' ]  ==  usersByBirthDateYear[ 1973 ].collect(userToString)

//  Just a little fun with the closure:
def groupByGroovy  =  { 
     if  (it  =~   / y / ) {
         " Contains y "
    }  else  {
         " Doesn't contain y "
    }
}
assert  [ " Contains y " :[ " Groovy " ],  " Doesn't contain y " :[ " Java " ,  " Scala " ]]  ==  [ ' Groovy ' ,  ' Java ' ,  ' Scala ' ].groupBy(groupByGroovy)