Lists

A list is an ordered collection. [...] creates a java.util.ArrayList.

def fruits = ['apple', 'banana', 'cherry']

println fruits[0]
println fruits[-1]
println fruits.size()
println fruits.first() + ' / ' + fruits.last()
println fruits.contains('banana')
println fruits.indexOf('cherry')

apple
cherry
3
apple / cherry
true
2

Indexes start at 0, and negative indexes count from the end.

Changing a list

def list = [3, 1, 2]

list << 4                 // append (left-shift operator)
list.add(5)
list[0] = 30              // replace
list.remove(Integer.valueOf(2)) // remove by value
list.removeAt(0)          // remove by index
println list

list.addAll([9, 8])
list.clear()
println list.isEmpty()

[1, 4, 5]
true

remove(2) on a list of integers removes the item at index 2 — Java's overloading picks the index version. Use removeAt(i) for indexes and remove(Integer.valueOf(x)) or removeElement(x) for values, so the intent is clear.

Operators on lists

def a = [1, 2, 3]
println a + [4, 5]          // new list
println a - [2]             // remove all 2s
println a * 2               // repeat
println([[1, 2], [3, [4]]].flatten())
println([3, 1, 2].sort())
println([1, 2, 2, 3, 3].unique())
println([1, 2, 3].reverse())
println([5, 3, 9].max() + ' ' + [5, 3, 9].min() + ' ' + [5, 3, 9].sum())

[1, 2, 3, 4, 5]
[1, 3]
[1, 2, 3, 1, 2, 3]
[1, 2, 3, 4]
[1, 2, 3]
[1, 2, 3]
[3, 2, 1]
9 3 17

Notice the parentheses in println([3, 1, 2].sort()). Without them, println [3, 1, 2] is read as println[3, 1, 2] — indexing a property called println — and fails with a MissingPropertyException. When a method's first argument starts with [, always use parentheses.

Mutating vs. non-mutating

def nums = [3, 1, 2]
def sorted = nums.sort(false)   // false: return a sorted copy
println "nums=$nums sorted=$sorted"

nums.sort()                     // sorts in place!
println "nums=$nums"

def frozen = [1, 2].asImmutable()
try {
    frozen << 3
} catch (UnsupportedOperationException e) {
    println 'cannot change an immutable list'
}

nums=[3, 1, 2] sorted=[1, 2, 3]
nums=[1, 2, 3]
cannot change an immutable list

sort() and unique() change the original list (and return it). Pass false to get a copy instead. asImmutable() wraps a list so nobody can change it.

Practice

Start with [5, 3, 8, 1], append 7, remove the smallest number, and print the list sorted from largest to smallest.

def nums = [5, 3, 8, 1]
nums << 7
nums.removeElement(nums.min())
println nums.sort(false).reverse()

[8, 7, 5, 3]