5
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I have written a function that returns me the duplicates of a list:

static List<SomeClass> findDuplicateElements(List<SomeClass> myObjects) {
    List<SomeClass> duplicates = myObjects
    .groupBy { it.someAttribute }
    .findAll { it.value.size() > 1 }
    .collect { it.value }
    .flatten()

    assert duplicates.size() != 1
    duplicates
}

The list cannot have the size 1 because if no duplicates are found it must be empty else if there are duplicates it must me at least 2 elements.

Without that check I would do it this way without having to create any new variables:

static List<SomeClass> findDuplicateElements(List<SomeClass> myObjects) {
    myObjects
    .groupBy { it.someAttribute }
    .findAll { it.value.size() > 1 }
    .collect { it.value }
    .flatten()
}

When using assert, is there any way to avoid saving the result in the duplicates variable and returning it at the end?

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2
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You can use with to do things inline without declaring another variable:

List list = [
  [first: "foo", last: "bar"],
  [first: "baz", last: "qux"],
  [first: "baz", last: "bar"]
]

List findDuplicateElements(myObjects) {
   myObjects
   .groupBy { it.last }
   .findAll { it.value.size() > 1 }
   .collect { it.value }
   .flatten()
   .with {
     assert it.size() != null
     it
   }
}

assert findDuplicateElements(list).size() == 2
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