JN1015-Collections

Lists

A list is an ordered collection of objects:

def list = [5, 6, 7, 8]
assert list.size == 4
assert list.size() == 4
assert list.class == ArrayList //the specific kind of list being used

assert list[2] == 7 //indexing starts at 0
assert list.getAt(2) == 7 //equivalent method to []
assert list.get(2) == 7 //alternative method

list[2] = 9
assert list == [5, 6, 9, 8, ] //trailing comma OK

list.putAt(2, 10) //equivalent method to [] when value being changed
assert list == [5, 6, 10, 8]
assert list.set(2, 11) == 10 //alternative method that returns old value
assert list == [5, 6, 11, 8]

assert [ 'a', 1, 'a', 'a', 2.5, 2.5f, 2.5d, 'hello', 7g, null, 9 as byte ]
                    //objects can be of different types; duplicates allowed

assert [1,2,3,4,5][-1] == 5 //use negative indices to count from the end
assert [1,2,3,4,5][-2] == 4
assert [1,2,3,4,5].getAt(-2) == 4 //getAt() available with negative index...
try{ [1,2,3,4,5].get(-2); assert 0 } //...but not get()
catch(e){ assert e instanceof ArrayIndexOutOfBoundsException }

Lists can be evaluated as a boolean value:

assert ! [] //an empty list evaluates as false
assert [1] && ['a'] && [0] && [0.0] && [false] && [null]
    //all other lists, irrespective of contents, evaluate as true

We can use [] to assign a new empty list and << to append items to it:

def list = []; assert list.size() == 0
list << 5; assert list.size() == 1
list << 7 << 'i' << 11; assert list == [5, 7, 'i', 11]
list << ['m', 'o']; assert list == [5, 7, 'i', 11, ['m', 'o'] ]
assert ( [1,2] << 3 << [4,5] << 6 ) == [1,2,3, [4, 5], 6]
    //first item in chain of << is target list
assert ([1,2,3] << 4) == ([1,2,3].leftShift(4))
    //using this method is equivalent to using <<

We can add to a list in many ways:

assert [1,2] + 3 + [4,5] + 6 == [1, 2, 3, 4, 5, 6]
assert [1,2].plus(3).plus([4,5]).plus(6) == [1, 2, 3, 4, 5, 6]
    //equivalent method for +
def a= [1,2,3]; a += 4; a += [5,6]; assert a == [1,2,3,4,5,6]
assert [1, *[222, 333], 456] == [1, 222, 333, 456]
assert [ *[1,2,3] ] == [1,2,3]
assert [ 1, [2,3,[4,5],6], 7, [8,9] ].flatten() == [1, 2, 3, 4, 5, 6, 7, 8, 9]

def list= [1,2]
list.add(3) //alternative method name
list.addAll([5,4]) //alternative method name
assert list == [1,2,3,5,4]

list= [1,2]
list.add(1,3) //add 3 just before index 1
assert list == [1,3,2]
list.addAll(2,[5,4]) //add [5,4] just before index 2
assert list == [1,3,5,4,2]

list = ['a', 'b', 'z', 'e', 'u', 'v', 'g']
list[8] = 'x'
assert list == ['a', 'b', 'z', 'e', 'u', 'v', 'g', null, 'x']
    //nulls inserted if required

We can use the each and eachWithIndex methods to execute code on each item in a list:

[1, 2, 3].each{ println "Item: $it" }
['a', 'b', 'c'].eachWithIndex{ it, i -> println "$i: $it" }

We can construct a list using another's elements as a template:

def list1= ['a','b','c']
def list2 = new ArrayList( list1 )
    //construct a new list, seeded with the same items as in list1
assert list2 == list1 // == checks that each corresponding element is the same
def list3 = list1.clone()
assert list3 == list1

We can perform a closure on each item of a list and return the result:

assert [1, 2, 3].collect{ it * 2 } == [2, 4, 6]
    //simple call gives single result
assert [1, 2, 3]*.multiply(2) == [1, 2, 3].collect{ it.multiply(2) }
    //shortcut syntax instead of collect

def list= []
assert [1, 2, 3].collect( list ){ it * 2 } == [2, 4, 6]
    //this style of call gives two identical results
assert list == [2, 4, 6]

Other methods on a list return a value:

assert [1, 2, 3].find{ it > 1 } == 2
assert [1, 2, 3].findAll{ it > 1 } == [2, 3]
assert ['a','b','c','d','e'].findIndexOf{ it in ['c','e','g'] } == 2
    //find first item that satisfies closure
assert [1, 2, 3].every{ it < 5 }
assert ! [1, 2, 3].every{ it < 3 }
assert [1, 2, 3].any{ it > 2 }
assert ! [1, 2, 3].any{ it > 3 }

   // sum anything with a plus() method
assert [1,2,3,4,5,6].sum() == 21
assert ['a','b','c','d','e'].sum{
  it=='a'?1: it=='b'?2: it=='c'?3: it=='d'?4: it=='e'?5: 0
} == 15
assert ['a','b','c','d','e'].sum{ (char)it - (char)'a' } == 10
assert ['a','b','c','d','e'].sum() == 'abcde'
assert [['a','b'], ['c','d']].sum() == ['a','b','c','d']
   // an initial value can be provided
assert [].sum(1000) == 1000
assert [1, 2, 3].sum(1000) == 1006

assert [1, 2, 3].join('-') == '1-2-3'
assert [1, 2, 3].inject('counting: '){ str, item -> str + item } ==
    'counting: 123'
assert [1, 2, 3].inject(0){ count, item -> count + item } == 6

We can find the maximum and minimum in a collection:

def list= [9, 4, 2, 10, 5]
assert list.max() == 10
assert list.min() == 2
assert Collections.max( list ) == 10
assert Collections.min( list ) == 2
assert ['x', 'y', 'a', 'z'].min() == 'a'
    //we can also compare single characters

def list2= ['abc', 'z', 'xyzuvw', 'Hello', '321']
assert list2.max{ it.size() } == 'xyzuvw'
    //we can use a closure to spec the sorting behaviour
assert list2.min{ it.size() } == 'z'

We can use a "Comparator" to define the comparing behaviour:

def mc= [compare:{a,b-> a.equals(b)? 0: a<b? -1: 1}] as Comparator
    //this syntax to be explained in a later tutorial
def list= [7,4,9,-6,-1,11,2,3,-9,5,-13]
assert list.max( mc ) == 11
assert list.min( mc ) == -13
assert Collections.max( list, mc ) == 11
assert Collections.min( list, mc ) == -13

def mc2= [
  compare: {a,b-> a.equals(b)? 0: Math.abs(a)<Math.abs(b)? -1: 1 }
] as Comparator
    //we should always ensure a.equals(b) returns 0, whatever else we do,
    //to avoid inconsistent behaviour in many contexts

assert list.max( mc2 ) == -13
assert list.min( mc2 ) == -1
assert list.max{a,b-> a.equals(b)? 0: Math.abs(a)<Math.abs(b)? -1: 1 } == -13
assert list.min{a,b-> a.equals(b)? 0: Math.abs(a)<Math.abs(b)? -1: 1 } == -1

We can remove elements from a list by referring to the element/s to be removed:

assert ['a','b','c','b','b'] - 'c' == ['a','b','b','b']
    //remove 'c', and return resulting list
assert ['a','b','c','b','b'] - 'b' == ['a','c']
    //remove all 'b', and return resulting list
assert ['a','b','c','b','b'] - ['b','c'] == ['a']
    //remove all 'b' and 'c', and return resulting list
assert ['a','b','c','b','b'].minus('b') == ['a','c']
    //equivalent method name for -
assert ['a','b','c','b','b'].minus( ['b','c'] ) == ['a']
def list= [1,2,3,4,3,2,1]
list -= 3
assert list == [1,2,4,2,1] //use -= to remove 3, permanently
assert ( list -= [2,4] ) == [1,1] //remove 2's and 4's, permanently

We can remove an element by referring to its index:

def list= [1,2,3,4,5,6,2,2,1]
assert list.remove(2) == 3 //remove the third element, and return it
assert list == [1,2,4,5,6,2,2,1]

We can remove the first occurrence of an element from a list:

def list= ['a','b','c','b','b']
assert list.remove('c') //remove 'c', and return true because element removed
assert list.remove('b')
    //remove first 'b', and return true because element removed
assert ! list.remove('z') //return false because no elements removed
assert list == ['a','b','b']

We can clear a list of all elements:

def list= ['a',2,'c',4]
list.clear()
assert list == []

We can pop the last item from a list, and use the list as a simple stack:

def stack= [1,2,4,6]
stack << 7
assert stack == [1,2,4,6,7]
assert stack.pop() == 7
assert stack == [1,2,4,6]

Other useful operators and methods:

assert 'a' in ['a','b','c']
assert ['a','b','c'].contains('a')
assert [1,3,4].containsAll([1,4])

assert [].isEmpty()
assert [1,2,3,3,3,3,4,5].count(3) == 4

assert [1,2,4,6,8,10,12].intersect([1,3,6,9,12]) == [1,6,12]

assert [1,2,3].disjoint( [4,6,9] )
assert ! [1,2,3].disjoint( [2,4,6] )
assert Collections.disjoint( [1,2,3], [4,6,9] ) //alternative method name

There's various ways of sorting:

assert [6,3,9,2,7,1,5].sort() == [1,2,3,5,6,7,9]

def list= ['abc', 'z', 'xyzuvw', 'Hello', '321']
assert list.sort{ it.size() } == ['z', 'abc', '321', 'Hello', 'xyzuvw']

def list2= [7,4,-6,-1,11,2,3,-9,5,-13]
assert list2.sort{a,b-> a.equals(b)? 0: Math.abs(a)<Math.abs(b)? -1: 1 } ==
    [-1, 2, 3, 4, 5, -6, 7, -9, 11, -13]
def mc= [
  compare: {a,b-> a.equals(b)? 0: Math.abs(a)<Math.abs(b)? -1: 1 }
] as Comparator
assert list2.sort(mc) == [-1, 2, 3, 4, 5, -6, 7, -9, 11, -13]

def list3= [6,-3,9,2,-7,1,5]
Collections.sort(list3)
assert list3 == [-7,-3,1,2,5,6,9]
Collections.sort(list3, mc)
assert list3 == [1,2,-3,5,6,-7,9]

We can repeat a list or element:

assert [1,2,3] * 3 == [1,2,3,1,2,3,1,2,3]
assert [1,2,3].multiply(2) == [1,2,3,1,2,3]
assert Collections.nCopies( 3, 'b' ) == ['b', 'b', 'b']
    //nCopies works differently
assert Collections.nCopies( 2, [1,2] ) == [ [1,2], [1,2] ] //not [1,2,1,2]

We can find the first or last index of items in a list:

assert ['a','b','c','d','c','d'].indexOf('c') == 2 //index returned
assert ['a','b','c','d','c','d'].indexOf('z') == -1
    //index -1 means value not in list
assert ['a','b','c','d','c','d'].lastIndexOf('c') == 4

Some very common methods are:

def list= [], list2= []
[1,2,3,4,5].each{ list << it*2 }
assert list == [2,4,6,8,10]
[1,2,3,4,5].eachWithIndex{item, index-> list2 << item * index }
    //closure supplied must have 2 params
assert list2 == [0,2,6,12,20]

A list may contain itself, but equals() may not always be consistent. Consider this:

def list, list2, list3
list= [1, 2, list, 4]
list2= [1, 2, list2, 4]
assert list.equals(list2)
list3= [1, 2, list, 4]
assert ! list.equals(list3)

Ranges and List-Slicing

Ranges are consecutive lists of sequential values like Integers, and can be used just like a List:

assert 5..8 == [5,6,7,8] //includes both values
assert 5..<8 == [5, 6, 7] //excludes specified top value

They can also be used with single-character strings:

assert ('a'..'d') == ['a','b','c','d']

Ranges are handy with the each method:

def n=0
(1..10).each{ n += it }
assert n == 55

We can define lists using a range or ranges within a list. This is called slicing:

assert [*3..5] == [3,4,5]
assert [ 1, *3..5,  7, *9..<12 ] == [1,3,4,5,7,9,10,11]

Lists can be used as subscripts to other lists:

assert ('a'..'g')[ 3..5 ] == ['d','e','f']
assert ('a'..'g').getAt( 3..5 ) == ['d','e','f'] //equivalent method name

assert ('a'..'g')[ 1, 3, 5, 6 ] == ['b','d','f','g']
assert ('a'..'g')[ 1, *3..5 ] == ['b','d','e','f']
assert ('a'..'g')[ 1, 3..5 ] == ['b','d','e','f']
    //range in subscript flattened automatically
assert ('a'..'g')[-5..-2] == ['c','d','e','f']
assert ('a'..'g').getAt( [ 1, *3..5 ] ) == ['b','d','e','f']
    //equivalent method name
assert ('a'..'g').getAt( [ 1, 3..5 ] ) == ['b','d','e','f']

We can view a sublist of a list:

def list=[1,2,3,4,5], sl= list.subList(2,4)
sl[0]= 9 //if we change the sublist...
assert list == [1,2,9,4,5] //...backing list changes...
list[3]= 11
assert sl == [9,11] //...and vice versa

We can perform the same methods on the subscripted lists as we can on the lists they're produced from:

assert ['a','b','c','d','e'][1..3].indexOf('c') == 1
    //note: index of sublist, not of list

We can update items using subscripting too:

def list = ['a','b','c','d','e','f','g']
list[2..3] = 'z'
assert list == ['a', 'b', 'z', 'e', 'f', 'g'] //swap two entries for one
list[4..4]= ['u','v']
assert list == ['a', 'b', 'z', 'e', 'u', 'v', 'g'] //swap one entry for two

def list= ['a', 'b', 'z', 'e', 'u', 'v', 'g']
list[0..1]= []
assert list == ['z', 'e', 'u', 'v', 'g'] //remove entries from index range
list[1..1]= []
assert list == ['z', 'u', 'v', 'g'] //remove entry at index

We can also use a method instead of [] with ranges:

def list = ['a','b','c','d','e','f','g']
list.putAt(2..3, 'z')
assert list == ['a', 'b', 'z', 'e', 'f', 'g']
list.putAt(4..4, ['u','v'])
assert list == ['a', 'b', 'z', 'e', 'u', 'v', 'g']
list.putAt(1..<3, [])
assert list == ['a', 'e', 'u', 'v', 'g']
list.putAt( 0..<0, 'm' ) //
assert list == ['m', 'a', 'e', 'u', 'v', 'g']
list.removeRange(1,3) //another method to do similar, means: list[1..<3]= []
list[1..2].clear()
assert list == ['m', 'g']

More List Utilities

To reverse a list:

assert [1,2,3].reverse() == [3,2,1]

def list= ['a','b','c','d','e']
Collections.reverse( list )
assert list == ['e','d','c','b','a']
use(Collections){ list.reverse() }
    //alternative syntax for null-returning Collections.reverse(List)
assert list == ['a','b','c','d','e']

def list2= []
[1,2,3,4,5].reverseEach{ list2 << it*2 }
    //same as, but more efficient than: [...].reverse().each{...}
assert list2 == [10,8,6,4,2]

assert [1,2,3,4,5,6][3..1] == [4,3,2]
    //use backwards range to reverse returned sublist

def list3 = [1, 2, -3, 5, 6, -7, 9]
def rmc= Collections.reverseOrder()
Collections.sort(list3, rmc)
assert list3 == [9, 6, 5, 2, 1, -3, -7]

def list4 = [1, 2, -3, 5, 6, -7, 9]
def mc= [
  compare: {a,b-> a.equals(b)? 0: Math.abs(a)<Math.abs(b)? -1: 1}
] as Comparator
def rmc2= Collections.reverseOrder( mc )
Collections.sort(list4, rmc2)
assert list4 == [9, -7, 6, 5, -3, 2, 1]

We can perform a binary search on a sorted list:

assert Collections.binarySearch([2,5,6,7,9,11,13,26,31,33], 26) == 7
    //list must already be sorted
assert Collections.binarySearch([2,5,6,7,9,11,13,31,33], 26) == -8
    //if key not there, give negative of one plus the index before which key
    //would be if it was there

def mc= [
  compare: {a,b-> a.equals(b)? 0: Math.abs(a)<Math.abs(b)? -1: 1 }
] as Comparator
assert Collections.binarySearch([2,-5,-6,7,9,-11,13,26,31,-33], 26, mc) == 7
    //give comparator list sorted by

We can remove or retain elements in bulk. retainAll() gives the intersection of two lists; removeAll() gives the assymmetric difference.

def list= ['a','b','c','b','b','e','e']
assert list.removeAll( ['b','z'] )
    //remove 'b' and 'z', return true because list changed
assert list == ['a','c','e','e']
assert ! list.removeAll( ['b','z'] )
    //return false because list didn't change
assert list == ['a','c','e','e']
assert list.retainAll( ['a','e'] )
    //retain only 'a' and 'e', return true because list changed
assert list == ['a','e','e']
assert ! list.retainAll( ['a','e'] )
    //retain only 'a' and 'e', return true because list didn't change
assert list == ['a','e','e']

Some miscellaneous methods:

def list= ['a', 7, 'b', 9, 7, 7, 2.4, 7]
Collections.replaceAll( list, 7, 55)
assert list == ['a', 55, 'b', 9, 55, 55, 2.4, 55]

list= ['a', 7, 'b', 9, 7, 7, 2.4, 7]
use(Collections){ list.replaceAll(7, 55) } //alternative syntax
assert list == ['a', 55, 'b', 9, 55, 55, 2.4, 55]

list= ['a',2,null,4,'zyx',2.5]
use(Collections){ list.fill( 'g' ) } //or: Collections.fill( list, 'g' )
assert list == ['g', 'g', 'g', 'g', 'g', 'g']

list= ['a', 'e', 'i', 'o', 'u', 'z']
use(Collections){ list.swap(2, 4) } //or: Collections.swap(list, 2, 4)
assert list == ['a', 'e', 'u', 'o', 'i', 'z']

assert Collections.frequency(['a','b','a','c','a','a','d','e'], 'a') == 4
use(Collections){
  assert ['a','b','a','c','a','a','d','e'].frequency('a') == 4
}

list= ['a','b','c','d','e']
Collections.rotate(list, 3)
assert list == ['c','d','e','a','b']
use(Collections){ list.rotate(-2) }
assert list == ['e','a','b','c','d']

list= [1,2,3,4,5]
Collections.shuffle(list, new Random())
    //we can supply our own random number generator...
assert list != [1,2,3,4,5]

list= [1,2,3,4,5]
Collections.shuffle(list) //...or use the default one
assert list != [1,2,3,4,5]

assert [3,5,5,5,2].unique() == [3,5,2]
def mc= [ compare:
  {a,b-> a.equals(b) || a.equals(-b)? 0: Math.abs(a)<Math.abs(b)? -1: 1 }
] as Comparator

assert [3,5,5,-5,2,-7].unique(mc) == [3,5,2,-7]
    //remove subsequent items comparator considers equal
assert [3,5,5,-5,2,-7].unique{a, b->
  a == b || a == -b? 0: Math.abs(a)<Math.abs(b)? -1: 1
} == [3,5,2,-7]

list= [1,2,3]
Collections.copy( list, [9,8,7] )
assert list == [9,8,7] //overwrites original data
Collections.copy( list, [11,12] ) //source list shorter...
assert list == [11,12,7] //...which leaves remaining entries unchanged
try{ Collections.copy( list, [21,22,23,24] ); assert 0 } //source list too long
    catch(e){ assert e instanceof IndexOutOfBoundsException }

list= [1,8,8,2,3,7,6,4,6,6,2,3,7,5]
assert Collections.indexOfSubList( list, [2,3,7] ) == 3
assert Collections.lastIndexOfSubList( list, [2,3,7] ) == 10
assert Collections.indexOfSubList( list, [9,9,13] ) == -1
    //if sublist doesn't exist

Sets

A set is an unordered collection of objects, with no duplicates. It can be considered as a list with restrictions, and is often constructed from a list:

def s1= [1,2,3,3,3,4] as Set,
    s2= [4,3,2,1] as Set,
    s3= new HashSet( [1,4,2,4,3,4] )
assert s1.class == HashSet && s2.class == HashSet
    //the specific kind of set being used
assert s1 == s2
assert s1 == s3
assert s2 == s3
assert s1.asList() && s1.toList()
    //a choice of two methods to convert a set to a list
assert ( ([] as Set) << null << null << null ) == [null] as Set

A set should not contain itself as an element.

Most methods available to lists, besides those that don't make sense for unordered items, are available to sets.

[ { it[1] }, { it.getAt(1) }, { it.putAt(1,4) }, { it.reverse() } ].each{
  try{ it([1,2,3] as Set); assert 0 }
  catch(e){ assert e instanceof MissingMethodException }
}

The add() and addAll() methods return false if the set wasn't changed as a result of the operation:

def s= [1,2] as Set
assert s.add(3)
assert ! s.add(2)
assert s.addAll( [5,4] )
assert ! s.addAll( [5,4] )
assert s == [1,2,3,5,4] as Set

Examples with Lists and Sets

For small numbers of items, it's common in Groovy to use a list for set processing, and only convert it to a set when necessary, eg, for output.

Though the uniqueness of set items is useful for some processing, for example, if we want to separate the unique and duplicating items in a list:

list=[1,2,7,2,2,4,7,11,5,2,5]
def uniques= [] as Set, dups= [] as Set
list.each{ uniques.add(it) || dups.add(it) }
uniques.removeAll(dups)
assert uniques == [1,4,11] as Set && dups == [2,5,7] as Set

To calculate the symmetric set difference of two sets non-destructively:

def s1=[1,2,3,4,5,6], s2=[4,5,6,7,8,9]
def diff = (s1 as Set) + s2
tmp = s1 as Set
tmp.retainAll(s2)
diff.removeAll(tmp)
assert diff == [1,2,3,7,8,9]

Sorted Sets

A sorted set is one with extra methods that utilize the sorting of the elements. It's often more efficient than doing the same with lists.

def list= [3,2,3,3,1,7,5]
assert new TreeSet(list) == new TreeSet([1,1,1,2,5,7,3,1])
assert new TreeSet(list).toList() == list.unique().sort()

assert new TreeSet(list).first() == list.unique().min()
assert new TreeSet(list).last() == list.unique().max()

We can construct a TreeSet by giving a comparator to order the elements in the set:

def c= [ compare:
  {a,b-> a.equals(b)? 0: Math.abs(a)<Math.abs(b)? -1: 1 }
] as Comparator
def ts= new TreeSet( c )
ts<< 3 << -7 << 9 << -2 << -4
assert ts == new TreeSet( [-2, 3, -4, -7, 9] )
assert ts.comparator() == c //retrieve the comparator

The range-views, headSet() tailSet() and subSet(), are useful views of the items in a sorted set. These range-views remain valid even if the backing sorted set is modified directly. The sorted set returned by these methods will throw an IllegalArgumentException if the user attempts to insert an element out of the range.

def ss= new TreeSet(['a','b','c','d','e'])

def hs= ss.headSet('c')
assert hs == new TreeSet(['a','b'])
    //return all elements < specified element
hs.remove('a')
assert ss == new TreeSet(['b','c','d','e'])
    //headset is simply a view of the data in ss

def ts= ss.tailSet('c')
assert ts == new TreeSet(['c','d','e'])
    //return all elements >= specified element
ts.remove('d')
assert ss == new TreeSet(['b','c','e'])
    //tailset is also a view of data in ss

def bs= ss.subSet('b','e')
assert bs == new TreeSet(['b','c'])
    //return all elements >= but < specified element
bs.remove('c')
assert ss == new TreeSet(['b','e'])
    //subset is simply a view of the data in ss

ss << 'a' << 'd'
assert hs == new TreeSet(['a','b'])
    //if backing sorted set changes, so do range-views
assert ts == new TreeSet(['d','e'])
assert bs == new TreeSet(['b','d'])

For a SortedSet of strings, we can append '\0' to a string to calculate the next possible string:

def dic= new TreeSet(
  ['aardvark', 'banana', 'egghead', 'encephalograph', 'flotsam', 'jamboree']
)
assert dic.subSet('banana', 'flotsam').size() == 3
    //incl 'banana' but excl 'flotsam'
assert dic.subSet('banana', 'flotsam\0').size() == 4 //incl both
assert dic.subSet('banana\0', 'flotsam').size() == 2 //excl both
dic.subSet('e', 'f').clear()
assert dic == new TreeSet(
   ['aardvark', 'banana', 'flotsam', 'jamboree']
)//clear all words beginning with 'e'

To go one element backwards from an element elt in a SortedSet:

Object predecessor = ss.headSet( elt ).last()

Immutable Collections

We can convert a list or set into one that can't be modified:

def imList= ['a', 'b', 'c'].asImmutable()
try{ imList<< 'd'; assert 0 }
catch(e){ assert e instanceof UnsupportedOperationException }

imList= Collections.unmodifiableList( ['a', 'b', 'c'] ) //alternative way
try{ imList<< 'd'; assert 0 }
catch(e){ assert e instanceof UnsupportedOperationException }

def imSet= (['a', 'b', 'c'] as Set).asImmutable()
try{ imSet<< 'd'; assert 0 }
catch(e){ assert e instanceof UnsupportedOperationException }

imSet= Collections.unmodifiableSet( ['a', 'b', 'c'] as Set ) //alternative way
try{ imSet<< 'd'; assert 0 }
catch(e){ assert e instanceof UnsupportedOperationException }

def imSortedSet= ( new TreeSet(['a', 'b', 'c']) ).asImmutable()
try{ imSortedSet<< 'd'; assert 0 }
catch(e){ assert e instanceof UnsupportedOperationException }

imSortedSet= Collections.unmodifiableSortedSet( new TreeSet(['a', 'b', 'c']) )
                                                        //alternative way
try{ imSortedSet<< 'd'; assert 0 }
catch(e){ assert e instanceof UnsupportedOperationException }

We can create an empty list or set that can't be modified:

def list= Collections.emptyList()
assert list == []
try{ list<< 'a'; assert 0 }
catch(e){ assert e instanceof UnsupportedOperationException }
list= Collections.EMPTY_LIST
assert list == []
try{ list<< 'a'; assert 0 }
catch(e){ assert e instanceof UnsupportedOperationException }

def set= Collections.emptySet()
assert set == [] as Set
try{ set<< 'a'; assert 0 }
catch(e){ assert e instanceof UnsupportedOperationException }
set= Collections.EMPTY_SET
assert set == [] as Set
try{ set<< 'a'; assert 0 }
catch(e){ assert e instanceof UnsupportedOperationException }

We can create a single-element list that can't be modified:

def singList= Collections.singletonList('a')
assert singList == ['a']
try{ singList<< 'b'; assert 0 }
catch(e){ assert e instanceof UnsupportedOperationException }

def singSet = Collections.singleton('a')
assert singSet == ['a'] as Set
try{ singSet<< 'b'; assert 0 }
catch(e){ assert e instanceof UnsupportedOperationException }