java.util
Class List

addAll

public boolean addAll(int index, Object[] items)

 

Modifies this list by inserting all of the elements in the specified array into the list at the specified position. Shifts the element currently at that position (if any) and any subsequent elements to the right (increases their indices). The new elements will appear in this list in the order that they occur in the array. The behavior of this operation is undefined if the specified array is modified while the operation is in progress. See also plus for similar functionality with copy semantics, i.e. which produces a new list after adding the additional items at the specified position but leaves the original list unchanged.

Parameters:

items - array containing elements to be added to this collection.

index - index at which to insert the first element from the specified array.

Returns:

true if this collection changed as a result of the call

Since:

1.7.2

See:

List#addAll(int, Collection).

asImmutable

public List asImmutable()

 

A convenience method for creating an immutable list

Returns:

an immutable List

Since:

1.0

See:

Collections#unmodifiableList.

asSynchronized

public List asSynchronized()

 

A convenience method for creating a synchronized List.

Returns:

a synchronized List

Since:

1.0

See:

Collections#synchronizedList.

collate

public List collate(int size)

 

Collates this list into sub-lists of length size. Example:

def list = [ 1, 2, 3, 4, 5, 6, 7 ]
def coll = list.collate( 3 )
assert coll == [ [ 1, 2, 3 ], [ 4, 5, 6 ], [ 7 ] ]

Parameters:

size - the length of each sub-list in the returned list.

Returns:

a List containing the data collated into sub-lists

Since:

1.8.6

collate

public List collate(int size, int step)

 

Collates this list into sub-lists of length size stepping through the code step elements for each subList. Example:

def list = [ 1, 2, 3, 4 ]
def coll = list.collate( 3, 1 )
assert coll == [ [ 1, 2, 3 ], [ 2, 3, 4 ], [ 3, 4 ], [ 4 ] ]

Parameters:

size - the length of each sub-list in the returned list.

step - the number of elements to step through for each sub-list.

Returns:

a List containing the data collated into sub-lists

Since:

1.8.6

collate

public List collate(int size, boolean keepRemainder)

 

Collates this list into sub-lists of length size. Any remaining elements in the list after the subdivision will be dropped if keepRemainder is false. Example:

def list = [ 1, 2, 3, 4, 5, 6, 7 ]
def coll = list.collate( 3, false )
assert coll == [ [ 1, 2, 3 ], [ 4, 5, 6 ] ]

Parameters:

size - the length of each sub-list in the returned list.

keepRemainder - if true, any rmeaining elements are returned as sub-lists. Otherwise they are discarded.

Returns:

a List containing the data collated into sub-lists

Since:

1.8.6

collate

public List collate(int size, int step, boolean keepRemainder)

 

Collates this list into sub-lists of length size stepping through the code step elements for each sub-list. Any remaining elements in the list after the subdivision will be dropped if keepRemainder is false. Example:

def list = [ 1, 2, 3, 4 ]
assert list.collate( 3, 1, true  ) == [ [ 1, 2, 3 ], [ 2, 3, 4 ], [ 3, 4 ], [ 4 ] ]
assert list.collate( 3, 1, false ) == [ [ 1, 2, 3 ], [ 2, 3, 4 ] ]

Parameters:

size - the length of each sub-list in the returned list.

step - the number of elements to step through for each sub-list.

keepRemainder - if true, any rmeaining elements are returned as sub-lists. Otherwise they are discarded.

Returns:

a List containing the data collated into sub-lists

Since:

1.8.6

drop

public List drop(int num)

 

Drops the given number of elements from the head of this list if they are available.

def strings = [ 'a', 'b', 'c' ]
assert strings.drop( 0 ) == [ 'a', 'b', 'c' ]
assert strings.drop( 2 ) == [ 'c' ]
assert strings.drop( 5 ) == []

Similar to Iterable#drop(int) except that it attempts to preserve the type of the original list.

Parameters:

num - the number of elements to drop from this list.

Returns:

a list consisting of all elements of this list except the first num ones, or else the empty list, if this list has less than num elements.

Since:

1.8.1

dropWhile

public List dropWhile(Closure condition)

 

Returns a suffix of this List where elements are dropped from the front while the given Closure evaluates to true. Similar to Iterable#dropWhile except that it attempts to preserve the type of the original list.

def nums = [ 1, 3, 2 ]
assert nums.dropWhile{ it < 4 } == []
assert nums.dropWhile{ it < 3 } == [ 3, 2 ]
assert nums.dropWhile{ it != 2 } == [ 2 ]
assert nums.dropWhile{ it == 0 } == [ 1, 3, 2 ]

Parameters:

condition - the closure that must evaluate to true to continue dropping elements.

Returns:

the shortest suffix of the given List such that the given closure condition evaluates to true for each element dropped from the front of the List

Since:

1.8.7

equals

public boolean equals(Object[] right)

 

Determines if the contents of this list are equal to the contents of the given array in the same order. This returns false if either collection is null.

assert [1, "a"].equals( [ 1, "a" ] as Object[] )

Parameters:

right - the Object[] being compared to.

Returns:

true if the contents of both collections are equal

Since:

1.5.0

equals

public boolean equals(List right)

 

Compare the contents of two Lists. Order matters. If numbers exist in the Lists, then they are compared as numbers, for example 2 == 2L. If both lists are null, the result is true; otherwise if either list is null, the result is false.

assert ["a", 2].equals(["a", 2])
assert ![2, "a"].equals("a", 2)
assert [2.0, "a"].equals(2L, "a") // number comparison at work

Parameters:

right - the List being compared to.

Returns:

boolean true if the contents of both lists are identical, false otherwise.

Since:

1.0

execute

public Process execute()

 

Executes the command specified by the given list. The toString() method is called for each item in the list to convert into a resulting String. The first item in the list is the command the others are the parameters.

For more control over Process construction you can use java.lang.ProcessBuilder (JDK 1.5+).

Returns:

the Process which has just started for this command line representation.

Since:

1.0

execute

public Process execute(String[] envp, File dir)

 

Executes the command specified by the given list, with the environment defined by envp and under the working directory dir. The first item in the list is the command; the others are the parameters. The toString() method is called on items in the list to convert them to Strings.

For more control over Process construction you can use java.lang.ProcessBuilder (JDK 1.5+).

Parameters:

envp - an array of Strings, each member of which has environment variable settings in the format name=value, or null if the subprocess should inherit the environment of the current process..

dir - the working directory of the subprocess, or null if the subprocess should inherit the working directory of the current process..

Returns:

the Process which has just started for this command line representation.

Since:

1.7.1

execute

public Process execute(List envp, File dir)

 

Executes the command specified by the given list, with the environment defined by envp and under the working directory dir. The first item in the list is the command; the others are the parameters. The toString() method is called on items in the list to convert them to Strings.

For more control over Process construction you can use java.lang.ProcessBuilder (JDK 1.5+).

Parameters:

envp - a List of Objects (converted to Strings using toString), each member of which has environment variable settings in the format name=value, or null if the subprocess should inherit the environment of the current process..

dir - the working directory of the subprocess, or null if the subprocess should inherit the working directory of the current process..

Returns:

the Process which has just started for this command line representation.

Since:

1.7.1

first

public Object first()

 

Returns the first item from the List.

def list = [3, 4, 2]
assert list.first() == 3
// check original is unaltered
assert list == [3, 4, 2]

Returns:

the first item from the List

Since:

1.5.5

getAt

public List getAt(Range range)

 

Support the range subscript operator for a List.

def list = [1, "a", 4.5, true]
assert list[1..2] == ["a", 4.5]

Parameters:

range - a Range indicating the items to get.

Returns:

a new list instance based on range borders

Since:

1.0

getAt

public List getAt(EmptyRange range)

 

Support the range subscript operator for a List.

def list = [true, 1, 3.4]
assert list[0..<0] == []

Parameters:

range - a Range indicating the items to get.

Returns:

a new list instance based on range borders

Since:

1.0

getAt

public List getAt(Collection indices)

 

Select a List of items from a List using a Collection to identify the indices to be selected.

def list = [true, 1, 3.4, false]
assert list[1,0,2] == [1, true, 3.4]

Parameters:

indices - a Collection of indices.

Returns:

a new list of the values at the given indices

Since:

1.0

getAt

public Object getAt(int idx)

 

Support the subscript operator for a List.

def list = [2, "a", 5.3]
assert list[1] == "a"

Parameters:

idx - an index.

Returns:

the value at the given index

Since:

1.0

head

public Object head()

 

Returns the first item from the List.

def list = [3, 4, 2]
assert list.head() == 3
assert list == [3, 4, 2]

Returns:

the first item from the List

Since:

1.5.5

last

public Object last()

 

Returns the last item from the List.

def list = [3, 4, 2]
assert list.last() == 2
// check original is unaltered
assert list == [3, 4, 2]

Returns:

the last item from the List

Since:

1.5.5

minus

public List minus(Collection removeMe)

 

Create a List composed of the elements of the first list minus every occurrence of elements of the given Collection.

assert [1, "a", true, true, false, 5.3] - [true, 5.3] == [1, "a", false]

Parameters:

removeMe - a Collection of elements to remove.

Returns:

a List with the given elements removed

Since:

1.0

minus

public List minus(Iterable removeMe)

 

Create a List composed of the elements of the first list minus every occurrence of elements of the given Iterable.

class AbcIterable implements Iterable {
    Iterator iterator() { "abc".iterator() }
}
assert "backtrack".toList() - new AbcIterable() == ["k", "t", "r", "k"]

Parameters:

removeMe - an Iterable of elements to remove.

Returns:

a List with the supplied elements removed

Since:

1.8.7

minus

public List minus(Object removeMe)

 

Create a new List composed of the elements of the first list minus every occurrence of the given element to remove.

assert ["a", 5, 5, true] - 5 == ["a", true]

Parameters:

removeMe - an element to remove from the list.

Returns:

the resulting List with the given element removed

Since:

1.0

permutations

public Set permutations()

 

Finds all permutations of a collection.

Example usage:

def result = [1, 2, 3].permutations()
assert result == [[3, 2, 1], [3, 1, 2], [1, 3, 2], [2, 3, 1], [2, 1, 3], [1, 2, 3]] as Set

Returns:

the permutations from the list

Since:

1.7.0

permutations

public List permutations(Closure function)

 

Finds all permutations of a collection, applies a function to each permutation and collects the result into a list.

Example usage:

Set result = [1, 2, 3].permutations { it.collect { v -> 2*v }}
assert result == [[6, 4, 2], [6, 2, 4], [2, 6, 4], [4, 6, 2], [4, 2, 6], [2, 4, 6]] as Set

Parameters:

function - the function to apply on each permutation.

Returns:

the list of results of the application of the function on each permutation

Since:

2.2.0

plus

public List plus(int index, Object[] items)

 

Creates a new List by inserting all of the elements in the specified array to the elements from the original List at the specified index. Shifts the element currently at that index (if any) and any subsequent elements to the right (increasing their indices). The new elements will appear in the resulting List in the order that they occur in the original array. The behavior of this operation is undefined if the list or array operands are modified while the operation is in progress. The original list and array operands remain unchanged.

def items = [1, 2, 3]
def newItems = items.plus(2, 'a'..'c' as String[])
assert newItems == [1, 2, 'a', 'b', 'c', 3]
assert items == [1, 2, 3]

See also addAll for similar functionality with modify semantics, i.e. which performs the changes on the original list itself.

Parameters:

items - array containing elements to be merged with elements from the original list.

index - index at which to insert the first element from the specified array.

Returns:

the new list

Since:

1.8.1

See:

List#plus(int, List).

plus

public List plus(int index, List additions)

 

Creates a new List by inserting all of the elements in the given additions List to the elements from the original List at the specified index. Shifts the element currently at that index (if any) and any subsequent elements to the right (increasing their indices). The new elements will appear in the resulting List in the order that they occur in the original lists. The behavior of this operation is undefined if the original lists are modified while the operation is in progress. The original lists remain unchanged.

def items = [1, 2, 3]
def newItems = items.plus(2, 'a'..'c')
assert newItems == [1, 2, 'a', 'b', 'c', 3]
assert items == [1, 2, 3]

See also addAll for similar functionality with modify semantics, i.e. which performs the changes on the original list itself.

Parameters:

additions - a List containing elements to be merged with elements from the original List.

index - index at which to insert the first element from the given additions List.

Returns:

the new list

Since:

1.8.1

plus

public List plus(int index, Iterable additions)

 

Creates a new List by inserting all of the elements in the given Iterable to the elements from this List at the specified index.

Parameters:

additions - an Iterable containing elements to be merged with the elements from the original List.

index - index at which to insert the first element from the given additions Iterable.

Returns:

the new list

Since:

1.8.7

See:

List#plus(int, List).

pop

public Object pop()

 

Removes the last item from the List. Using add() and pop() is similar to push and pop on a Stack.

def list = ["a", false, 2]
assert list.pop() == 2
assert list == ["a", false]

Returns:

the item removed from the List

Since:

1.0

push

public boolean push(Object value)

 

Appends an item to the List. Synonym for add().

def list = [3, 4, 2]
list.push("x")
assert list == [3, 4, 2, "x"]

Parameters:

value - element to be appended to this list..

Returns:

true (as per the general contract of the Collection.add method).

Since:

1.5.5

putAt

public void putAt(int idx, Object value)

 

A helper method to allow lists to work with subscript operators.

def list = [2, 3]
list[0] = 1
assert list == [1, 3]

Parameters:

idx - an index.

value - the value to put at the given index.

Since:

1.0

putAt

public void putAt(EmptyRange range, Object value)

 

A helper method to allow lists to work with subscript operators.

def list = ["a", true]
list[1..<1] = 5
assert list == ["a", 5, true]

Parameters:

range - the (in this case empty) subset of the list to set.

value - the values to put at the given sublist or a Collection of values.

Since:

1.0

putAt

public void putAt(EmptyRange range, Collection value)

 

A helper method to allow lists to work with subscript operators.

def list = ["a", true]
list[1..<1] = [4, 3, 2]
assert list == ["a", 4, 3, 2, true]

Parameters:

range - the (in this case empty) subset of the list to set.

value - the Collection of values.

Since:

1.0

See:

List#putAt.

putAt

public void putAt(IntRange range, Collection col)

 

List subscript assignment operator when given a range as the index and the assignment operand is a collection. Example:

def myList = [4, 3, 5, 1, 2, 8, 10]
myList[3..5] = ["a", true]
assert myList == [4, 3, 5, "a", true, 10]

Items in the given range are replaced with items from the collection.

Parameters:

range - the subset of the list to set.

col - the collection of values to put at the given sublist.

Since:

1.5.0

putAt

public void putAt(IntRange range, Object value)

 

List subscript assignment operator when given a range as the index. Example:

def myList = [4, 3, 5, 1, 2, 8, 10]
myList[3..5] = "b"
assert myList == [4, 3, 5, "b", 10]

Items in the given range are replaced with the operand. The value operand is always treated as a single value.

Parameters:

range - the subset of the list to set.

value - the value to put at the given sublist.

Since:

1.0

putAt

public void putAt(List splice, List values)

 

A helper method to allow lists to work with subscript operators.

def list = ["a", true, 42, 9.4]
list[1, 4] = ["x", false]
assert list == ["a", "x", 42, 9.4, false]

Parameters:

splice - the subset of the list to set.

values - the value to put at the given sublist.

Since:

1.0

putAt

public void putAt(List splice, Object value)

 

A helper method to allow lists to work with subscript operators.

def list = ["a", true, 42, 9.4]
list[1, 3] = 5
assert list == ["a", 5, 42, 5]

Parameters:

splice - the subset of the list to set.

value - the value to put at the given sublist.

Since:

1.0

reverse

public List reverse()

 

Creates a new List with the identical contents to this list but in reverse order.

def list = ["a", 4, false]
assert list.reverse() == [false, 4, "a"]
assert list == ["a", 4, false]

Returns:

a reversed List

Since:

1.0

See:

List#reverse(boolean).

reverse

public List reverse(boolean mutate)

 

Reverses the elements in a list. If mutate is true, the original list is modified in place and returned. Otherwise, a new list containing the reversed items is produced.

def list = ["a", 4, false]
assert list.reverse(false) == [false, 4, "a"]
assert list == ["a", 4, false]
assert list.reverse(true) == [false, 4, "a"]
assert list == [false, 4, "a"]

Parameters:

mutate - true if the list itself should be reversed in place and returned, false if a new list should be created.

Returns:

a reversed List

Since:

1.8.1

reverseEach

public List reverseEach(Closure closure)

 

Iterate over each element of the list in the reverse order.

def result = []
[1,2,3].reverseEach { result << it }
assert result == [3,2,1]

Parameters:

closure - a closure to which each item is passed..

Returns:

the original list

Since:

1.5.0

subsequences

public Set subsequences()

 

Finds all non-null subsequences of a list.

Example usage:

def result = [1, 2, 3].subsequences()
assert result == [[1, 2, 3], [1, 3], [2, 3], [1, 2], [1], [2], [3]] as Set

Returns:

the subsequences from the list

Since:

1.7.0

tail

public List tail()

 

Returns the items from the List excluding the first item.

def list = [3, 4, 2]
assert list.tail() == [4, 2]
assert list == [3, 4, 2]

Returns:

a list without its first element

Since:

1.5.6

take

public List take(int num)

 

Returns the first num elements from the head of this list.

def strings = [ 'a', 'b', 'c' ]
assert strings.take( 0 ) == []
assert strings.take( 2 ) == [ 'a', 'b' ]
assert strings.take( 5 ) == [ 'a', 'b', 'c' ]

Similar to Iterable#take(int) except that it attempts to preserve the type of the original list.

Parameters:

num - the number of elements to take from this list.

Returns:

a list consisting of the first num elements of this list, or else the whole list if it has less then num elements.

Since:

1.8.1

takeWhile

public List takeWhile(Closure condition)

 

Returns the longest prefix of this list where each element passed to the given closure condition evaluates to true. Similar to Iterable#takeWhile except that it attempts to preserve the type of the original list.

def nums = [ 1, 3, 2 ]
assert nums.takeWhile{ it < 1 } == []
assert nums.takeWhile{ it < 3 } == [ 1 ]
assert nums.takeWhile{ it < 4 } == [ 1, 3, 2 ]

Parameters:

condition - the closure that must evaluate to true to continue taking elements.

Returns:

a prefix of the given list where each element passed to the given closure evaluates to true

Since:

1.8.7

toSpreadMap

public SpreadMap toSpreadMap()

 

Creates a spreadable map from this list.

Returns:

a newly created SpreadMap

Since:

1.8.0

See:

SpreadMap#SpreadMap.

Map#toSpreadMap.

transpose

public List transpose()

 

Adds GroovyCollections#transpose(List) as a method on lists. A Transpose Function takes a collection of columns and returns a collection of rows. The first row consists of the first element from each column. Successive rows are constructed similarly.

Example usage:

def result = [['a', 'b'], [1, 2]].transpose()
assert result == [['a', 1], ['b', 2]]

def result = [['a', 'b'], [1, 2], [3, 4]].transpose()
assert result == [['a', 1, 3], ['b', 2, 4]]

Returns:

a List of the transposed lists

Since:

1.5.0

See:

GroovyCollections#transpose.

withDefault

public List withDefault(Closure init)

 

An alias for withLazyDefault which decorates a list allowing it to grow when called with index values outside the normal list bounds.

Parameters:

init - a Closure with the target index as parameter which generates the default value.

Returns:

the decorated List

Since:

1.8.7

See:

List#withLazyDefault.

List#withEagerDefault.

withEagerDefault

public List withEagerDefault(Closure init)

 

Decorates a list allowing it to grow when called with a non-existent index value. When called with such values, the list is grown in size and a default value is placed in the list by calling a supplied init Closure. Null values can be stored in the list.

How it works: The decorated list intercepts all calls to getAt(index) and get(index). If an index greater than or equal to the current size() is used, the list will grow automatically up to the specified index. Gaps will be filled by calling the init Closure. If generating a default value is a costly operation consider using withLazyDefault.

Example usage:

def list = [0, 1].withEagerDefault{ 42 }
assert list[0] == 0
assert list[1] == 1
assert list[3] == 42   // default value
assert list == [0, 1, 42, 42]   // gap filled with default value

// illustrate using the index when generating default values
def list2 = [5].withEagerDefault{ index -> index * index }
assert list2[3] == 9
assert list2 == [5, 1, 4, 9]

// illustrate what happens with null values
list2[2] = null
assert list2[2] == null
assert list2 == [5, 1, null, 9]

Parameters:

init - a Closure with the target index as parameter which generates the default value.

Returns:

the wrapped List

Since:

1.8.7

withLazyDefault

public List withLazyDefault(Closure init)

 

Decorates a list allowing it to grow when called with a non-existent index value. When called with such values, the list is grown in size and a default value is placed in the list by calling a supplied init Closure. Subsequent retrieval operations if finding a null value in the list assume it was set as null from an earlier growing operation and again call the init Closure to populate the retrieved value; consequently the list can't be used to store null values.

How it works: The decorated list intercepts all calls to getAt(index) and get(index). If an index greater than or equal to the current size() is used, the list will grow automatically up to the specified index. Gaps will be filled by {@code null}. If a default value should also be used to fill gaps instead of {@code null}, use withEagerDefault. If getAt(index) or get(index) are called and a null value is found, it is assumed that the null value was a consequence of an earlier grow list operation and the init Closure is called to populate the value.

Example usage:

def list = [0, 1].withLazyDefault{ 42 }
assert list[0] == 0
assert list[1] == 1
assert list[3] == 42   // default value
assert list == [0, 1, null, 42] // gap filled with null

// illustrate using the index when generating default values
def list2 = [5].withLazyDefault{ index -> index * index }
assert list2[3] == 9
assert list2 == [5, null, null, 9]
assert list2[2] == 4
assert list2 == [5, null, 4, 9]

// illustrate what happens with null values
list2[2] = null
assert list2[2] == 4

Parameters:

init - a Closure with the target index as parameter which generates the default value.

Returns:

the decorated List

Since:

1.8.7