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June 12, 2018

Groovy Goodness: Intersect Collections With Custom Comparator

In a previous post we learned about the intersect method added to collections in Groovy. Since Groovy 2.5.0 we can supply a custom Comparator to the intersect method to define our own rules for the intersection.

In the following example we first apply the intersect method with the default Comparator. Then we create a new Comparator using a closure where we check if the value is in both collections and if the value starts with the letter M:

def stuff = ['Groovy', 'Gradle', 'Grails', 'Spock', 'Micronaut', 'Ratpack'] as Set
def micro = ['Ratpack', 'Micronaut', 'SpringBoot', 'Microservice']

// Using default comparator to get values
// that are in both collections.
assert stuff.intersect(micro) == ['Ratpack', 'Micronaut'] as Set
assert micro.intersect(stuff) == ['Micronaut', 'Ratpack']

// Comparator to check if value is in
// both collection and starts with a 'M'.
def microName = { a, b -> 
    def comp = a <=> b
    comp == 0 ? a[0] == 'M' ? 0 : -1 : comp
} as Comparator

// This time we use the Comparator and
// end up with all elements in both
// collections that start with a 'M'.
assert stuff.intersect(micro, microName) == ['Micronaut'] as Set
assert micro.intersect(stuff, microName) == ['Micronaut']

Written with Groovy 2.5.0.