Groovy Goodness: Removing Duplicate Elements in a Collection

We can easily remove duplicate elements in a collection with the different unique() methods. Without any parameters the default comparator for the objects is used to determine duplicate elements. But we can also use a closure to define the rules for duplicate elements. The closure can have one parameter and then we must return a value that is used for comparison, or the closure has two parameters and then we must return an Integer value where 0 means the items are not unique. Finally we can pass our own Comparator implementation to the unique() method where we define when an element is unique or not.

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00.class User implements Comparable {
01.String username, email
02.boolean active
03.
04.int compareTo(Object other) {
05.username <=> other.username
06.}
07.}
08.
09.def mrhaki1 = new User(username: 'mrhaki', email: 'mrhaki@localhost', active: false)
10.def mrhaki2 = new User(username: 'mrhaki', email: 'user@localhost', active: true)
11.def hubert1 = new User(username: 'hubert', email: 'user@localhost', active: false)
12.def hubert2 = new User(username: 'hubert', email: 'hubert@localhost', active: true)
13.
14.// Caution: unique() changes the original collection, so
15.// for the sample we invoke unique each time on a fresh new
16.// user list.
17.def uniqueUsers = [mrhaki1, mrhaki2, hubert1, hubert2].unique()
18.assert 2 == uniqueUsers.size()
19.assert [mrhaki1, hubert1] == uniqueUsers
20.
21.// Use closure with one parameter.
22.def uniqueEmail = [mrhaki1, mrhaki2, hubert1, hubert2].unique { user ->
23.user.email
24.}
25.assert 3 == uniqueEmail.size()
26.assert [mrhaki1, mrhaki2, hubert2] == uniqueEmail
27.
28.// Use closure with two parameters.
29.def uniqueActive = [mrhaki1, mrhaki2, hubert1, hubert2].unique { user1, user2 ->
30.user1.active <=> user2.active
31.}
32.assert 2 == uniqueActive.size()
33.assert [mrhaki1, mrhaki2] == uniqueActive
34.
35.// Use a comparator.
36.def emailComparator = [
37.equals: { delegate.equals(it) },
38.compare: { first, second ->
39.first.email <=> second.email
40.}
41.] as Comparator
42.def unique = [mrhaki1, mrhaki2, hubert1, hubert2].unique(emailComparator)
43.assert 3 == unique.size()
44.assert [mrhaki1, mrhaki2, hubert2] == unique
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