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ArraySet

public final class ArraySet
extends Object implements Collection<E>, Set<E>

java.lang.Object
   ↳ android.util.ArraySet<E>


ArraySet is a generic set data structure that is designed to be more memory efficient than a traditional HashSet. The design is very similar to ArrayMap, with all of the caveats described there. This implementation is separate from ArrayMap, however, so the Object array contains only one item for each entry in the set (instead of a pair for a mapping).

Note that this implementation is not intended to be appropriate for data structures that may contain large numbers of items. It is generally slower than a traditional HashSet, since lookups require a binary search and adds and removes require inserting and deleting entries in the array. For containers holding up to hundreds of items, the performance difference is not significant, less than 50%.

Because this container is intended to better balance memory use, unlike most other standard Java containers it will shrink its array as items are removed from it. Currently you have no control over this shrinking -- if you set a capacity and then remove an item, it may reduce the capacity to better match the current size. In the future an explicit call to set the capacity should turn off this aggressive shrinking behavior.

Summary

Public constructors

ArraySet()

Create a new empty ArraySet.

ArraySet(int capacity)

Create a new ArraySet with a given initial capacity.

ArraySet(ArraySet<E> set)

Create a new ArraySet with the mappings from the given ArraySet.

Public methods

boolean add(E value)

Adds the specified object to this set.

boolean addAll(Collection<? extends E> collection)

Perform an add(Object) of all values in collection

void addAll(ArraySet<? extends E> array)

Perform a add(Object) of all values in array

void clear()

Make the array map empty.

boolean contains(Object key)

Check whether a value exists in the set.

boolean containsAll(Collection<?> collection)

Determine if the array set contains all of the values in the given collection.

void ensureCapacity(int minimumCapacity)

Ensure the array map can hold at least minimumCapacity items.

boolean equals(Object object)

Indicates whether some other object is "equal to" this one.

This implementation returns false if the object is not a set, or if the sets have different sizes.

int hashCode()

Returns a hash code value for the object.

int indexOf(Object key)

Returns the index of a value in the set.

boolean isEmpty()

Return true if the array map contains no items.

Iterator<E> iterator()

Return an Iterator over all values in the set.

boolean remove(Object object)

Removes the specified object from this set.

boolean removeAll(Collection<?> collection)

Remove all values in the array set that exist in the given collection.

boolean removeAll(ArraySet<? extends E> array)

Perform a remove(Object) of all values in array

E removeAt(int index)

Remove the key/value mapping at the given index.

boolean retainAll(Collection<?> collection)

Remove all values in the array set that do not exist in the given collection.

int size()

Return the number of items in this array map.

<T> T[] toArray(T[] array)

Returns an array containing all of the elements in this collection; the runtime type of the returned array is that of the specified array.

Object[] toArray()

Returns an array containing all of the elements in this collection.

String toString()

Returns a string representation of the object.

This implementation composes a string by iterating over its values.

E valueAt(int index)

Return the value at the given index in the array.

Inherited methods

From class java.lang.Object
From interface java.util.Collection
From interface java.util.Set
From interface java.lang.Iterable

Public constructors

ArraySet

added in API level 23
ArraySet ()

Create a new empty ArraySet. The default capacity of an array map is 0, and will grow once items are added to it.

ArraySet

added in API level 23
ArraySet (int capacity)

Create a new ArraySet with a given initial capacity.

Parameters
capacity int

ArraySet

added in API level 23
ArraySet (ArraySet<E> set)

Create a new ArraySet with the mappings from the given ArraySet.

Parameters
set ArraySet

Public methods

add

added in API level 23
boolean add (E value)

Adds the specified object to this set. The set is not modified if it already contains the object.

Parameters
value E: the object to add.

Returns
boolean true if this set is modified, false otherwise.

Throws
ClassCastException when the class of the object is inappropriate for this set.

addAll

added in API level 23
boolean addAll (Collection<? extends E> collection)

Perform an add(Object) of all values in collection

Parameters
collection Collection: The collection whose contents are to be retrieved.

Returns
boolean true if this collection changed as a result of the call

addAll

added in API level 23
void addAll (ArraySet<? extends E> array)

Perform a add(Object) of all values in array

Parameters
array ArraySet: The array whose contents are to be retrieved.

clear

added in API level 23
void clear ()

Make the array map empty. All storage is released.

contains

added in API level 23
boolean contains (Object key)

Check whether a value exists in the set.

Parameters
key Object: The value to search for.

Returns
boolean Returns true if the value exists, else false.

containsAll

added in API level 23
boolean containsAll (Collection<?> collection)

Determine if the array set contains all of the values in the given collection.

Parameters
collection Collection: The collection whose contents are to be checked against.

Returns
boolean Returns true if this array set contains a value for every entry in collection, else returns false.

ensureCapacity

added in API level 23
void ensureCapacity (int minimumCapacity)

Ensure the array map can hold at least minimumCapacity items.

Parameters
minimumCapacity int

equals

added in API level 23
boolean equals (Object object)

Indicates whether some other object is "equal to" this one.

The equals method implements an equivalence relation on non-null object references:

  • It is reflexive: for any non-null reference value x, x.equals(x) should return true.
  • It is symmetric: for any non-null reference values x and y, x.equals(y) should return true if and only if y.equals(x) returns true.
  • It is transitive: for any non-null reference values x, y, and z, if x.equals(y) returns true and y.equals(z) returns true, then x.equals(z) should return true.
  • It is consistent: for any non-null reference values x and y, multiple invocations of x.equals(y) consistently return true or consistently return false, provided no information used in equals comparisons on the objects is modified.
  • For any non-null reference value x, x.equals(null) should return false.

The equals method for class Object implements the most discriminating possible equivalence relation on objects; that is, for any non-null reference values x and y, this method returns true if and only if x and y refer to the same object (x == y has the value true).

Note that it is generally necessary to override the hashCode method whenever this method is overridden, so as to maintain the general contract for the hashCode method, which states that equal objects must have equal hash codes.

This implementation returns false if the object is not a set, or if the sets have different sizes. Otherwise, for each value in this set, it checks to make sure the value also exists in the other set. If any value doesn't exist, the method returns false; otherwise, it returns true.

Parameters
object Object: the reference object with which to compare.

Returns
boolean true if this object is the same as the obj argument; false otherwise.

hashCode

added in API level 23
int hashCode ()

Returns a hash code value for the object. This method is supported for the benefit of hash tables such as those provided by HashMap.

The general contract of hashCode is:

  • Whenever it is invoked on the same object more than once during an execution of a Java application, the hashCode method must consistently return the same integer, provided no information used in equals comparisons on the object is modified. This integer need not remain consistent from one execution of an application to another execution of the same application.
  • If two objects are equal according to the equals(Object) method, then calling the hashCode method on each of the two objects must produce the same integer result.
  • It is not required that if two objects are unequal according to the equals(java.lang.Object) method, then calling the hashCode method on each of the two objects must produce distinct integer results. However, the programmer should be aware that producing distinct integer results for unequal objects may improve the performance of hash tables.

As much as is reasonably practical, the hashCode method defined by class Object does return distinct integers for distinct objects. (This is typically implemented by converting the internal address of the object into an integer, but this implementation technique is not required by the Java™ programming language.)

Returns
int a hash code value for this object.

indexOf

added in API level 23
int indexOf (Object key)

Returns the index of a value in the set.

Parameters
key Object: The value to search for.

Returns
int Returns the index of the value if it exists, else a negative integer.

isEmpty

added in API level 23
boolean isEmpty ()

Return true if the array map contains no items.

Returns
boolean true if this collection contains no elements

iterator

added in API level 23
Iterator<E> iterator ()

Return an Iterator over all values in the set.

Note: this is a fairly inefficient way to access the array contents, it requires generating a number of temporary objects and allocates additional state information associated with the container that will remain for the life of the container.

Returns
Iterator<E> an Iterator over the elements in this collection

remove

added in API level 23
boolean remove (Object object)

Removes the specified object from this set.

Parameters
object Object: the object to remove.

Returns
boolean true if this set was modified, false otherwise.

removeAll

added in API level 23
boolean removeAll (Collection<?> collection)

Remove all values in the array set that exist in the given collection.

Parameters
collection Collection: The collection whose contents are to be used to remove values.

Returns
boolean Returns true if any values were removed from the array set, else false.

removeAll

added in API level 23
boolean removeAll (ArraySet<? extends E> array)

Perform a remove(Object) of all values in array

Parameters
array ArraySet: The array whose contents are to be removed.

Returns
boolean

removeAt

added in API level 23
E removeAt (int index)

Remove the key/value mapping at the given index.

Parameters
index int: The desired index, must be between 0 and size()-1.

Returns
E Returns the value that was stored at this index.

retainAll

added in API level 23
boolean retainAll (Collection<?> collection)

Remove all values in the array set that do not exist in the given collection.

Parameters
collection Collection: The collection whose contents are to be used to determine which values to keep.

Returns
boolean Returns true if any values were removed from the array set, else false.

size

added in API level 23
int size ()

Return the number of items in this array map.

Returns
int the number of elements in this collection

toArray

added in API level 23
T[] toArray (T[] array)

Returns an array containing all of the elements in this collection; the runtime type of the returned array is that of the specified array. If the collection fits in the specified array, it is returned therein. Otherwise, a new array is allocated with the runtime type of the specified array and the size of this collection.

If this collection fits in the specified array with room to spare (i.e., the array has more elements than this collection), the element in the array immediately following the end of the collection is set to null. (This is useful in determining the length of this collection only if the caller knows that this collection does not contain any null elements.)

If this collection makes any guarantees as to what order its elements are returned by its iterator, this method must return the elements in the same order.

Like the toArray() method, this method acts as bridge between array-based and collection-based APIs. Further, this method allows precise control over the runtime type of the output array, and may, under certain circumstances, be used to save allocation costs.

Suppose x is a collection known to contain only strings. The following code can be used to dump the collection into a newly allocated array of String:

     String[] y = x.toArray(new String[0]);
Note that toArray(new Object[0]) is identical in function to toArray().

Parameters
array T: the array into which the elements of this collection are to be stored, if it is big enough; otherwise, a new array of the same runtime type is allocated for this purpose.

Returns
T[] an array containing all of the elements in this collection

toArray

added in API level 23
Object[] toArray ()

Returns an array containing all of the elements in this collection. If this collection makes any guarantees as to what order its elements are returned by its iterator, this method must return the elements in the same order.

The returned array will be "safe" in that no references to it are maintained by this collection. (In other words, this method must allocate a new array even if this collection is backed by an array). The caller is thus free to modify the returned array.

This method acts as bridge between array-based and collection-based APIs.

Returns
Object[] an array containing all of the elements in this collection

toString

added in API level 23
String toString ()

Returns a string representation of the object. In general, the toString method returns a string that "textually represents" this object. The result should be a concise but informative representation that is easy for a person to read. It is recommended that all subclasses override this method.

The toString method for class Object returns a string consisting of the name of the class of which the object is an instance, the at-sign character `@', and the unsigned hexadecimal representation of the hash code of the object. In other words, this method returns a string equal to the value of:

 getClass().getName() + '@' + Integer.toHexString(hashCode())
 

This implementation composes a string by iterating over its values. If this set contains itself as a value, the string "(this Set)" will appear in its place.

Returns
String a string representation of the object.

valueAt

added in API level 23
E valueAt (int index)

Return the value at the given index in the array.

Parameters
index int: The desired index, must be between 0 and size()-1.

Returns
E Returns the value stored at the given index.

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