# CopyOnWriteArraySet

Added in[API level 1](https://developer.android.com/guide/topics/manifest/uses-sdk-element.html#ApiLevels)

# CopyOnWriteArraySet

```
open class CopyOnWriteArraySet<E : Any!> : AbstractSet<E>, Serializable
```

|---|---|---|--------------------------------------------------------------------------------------------------------------------------------------|
| [kotlin.Any](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-any/index.html)                                                             ||||
| ↳ | [java.util.AbstractCollection](https://developer.android.com/reference/kotlin/java/util/AbstractCollection.html#)\<E\>                     |||
|   | ↳ | [java.util.AbstractSet](https://developer.android.com/reference/kotlin/java/util/AbstractSet.html#)\<E\>                                ||
|   |   | ↳ | [java.util.concurrent.CopyOnWriteArraySet](https://developer.android.com/reference/kotlin/java/util/concurrent/CopyOnWriteArraySet#) |

A[Set](https://developer.android.com/reference/kotlin/java/util/Set.html#)that uses an internal[CopyOnWriteArrayList](https://developer.android.com/reference/kotlin/java/util/concurrent/CopyOnWriteArrayList)for all of its operations. Thus, it shares the same basic properties:

- It is best suited for applications in which set sizes generally stay small, read-only operations vastly outnumber mutative operations, and you need to prevent interference among threads during traversal.
- It is thread-safe.
- Mutative operations (`add`,`set`,`remove`, etc.) are expensive since they usually entail copying the entire underlying array.
- Iterators do not support the mutative`remove`operation.
- Traversal via iterators is fast and cannot encounter interference from other threads. Iterators rely on unchanging snapshots of the array at the time the iterators were constructed.

<br />

**Sample Usage.**The following code sketch uses a copy-on-write set to maintain a set of Handler objects that perform some action upon state updates.  

```kotlin
<code>class Handler { void handle() { ... } }
 
  class X {
    private final CopyOnWriteArraySet&lt;Handler&gt; handlers
      = new CopyOnWriteArraySet&lt;&gt;();
    public void addHandler(Handler h) { handlers.add(h); }
 
    private long internalState;
    private synchronized void changeState() { internalState = ...; }
 
    public void update() {
      changeState();
      for (Handler handler : handlers)
        handler.handle();
    }
  }</code>
```

This class is a member of the[Java Collections Framework](https://developer.android.com//reference/java/util/package-summary.html#CollectionsFramework).

## Summary

|                                                                                                                                                                                      Public constructors                                                                                                                                                                                      ||
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|---|
| [CopyOnWriteArraySet](https://developer.android.com/reference/kotlin/java/util/concurrent/CopyOnWriteArraySet#CopyOnWriteArraySet())`()` Creates an empty set.                                                                                                                                                                                                                             |
| [CopyOnWriteArraySet](https://developer.android.com/reference/kotlin/java/util/concurrent/CopyOnWriteArraySet#CopyOnWriteArraySet(kotlin.collections.MutableCollection))`(`c:` `[MutableCollection](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/-mutable-collection/index.html)<out` `E>!`)` Creates a set containing all of the elements of the specified collection. |

|                                                                                                                                                                                                                                                            Public methods                                                                                                                                                                                                                                                            ||
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| open[Boolean](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-boolean/index.html)                                                                            | [add](https://developer.android.com/reference/kotlin/java/util/concurrent/CopyOnWriteArraySet#add(java.util.concurrent.CopyOnWriteArraySet.E))`(`element:` `E`)` Adds the specified element to this set if it is not already present.                                                                                                                             |
| open[Boolean](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-boolean/index.html)                                                                            | [addAll](https://developer.android.com/reference/kotlin/java/util/concurrent/CopyOnWriteArraySet#addAll(kotlin.collections.Collection))`(`elements:` `[Collection](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/-collection/index.html)<E>`)` Adds all of the elements in the specified collection to this set if they're not already present. |
| open[Unit](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-unit/index.html)                                                                                  | [clear](https://developer.android.com/reference/kotlin/java/util/concurrent/CopyOnWriteArraySet#clear())`()` Removes all of the elements from this set.                                                                                                                                                                                                           |
| open[Boolean](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-boolean/index.html)                                                                            | [contains](https://developer.android.com/reference/kotlin/java/util/concurrent/CopyOnWriteArraySet#contains(java.util.concurrent.CopyOnWriteArraySet.E))`(`element:` `E?`)` Returns`true`if this set contains the specified element.                                                                                                                              |
| open[Boolean](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-boolean/index.html)                                                                            | [containsAll](https://developer.android.com/reference/kotlin/java/util/concurrent/CopyOnWriteArraySet#containsAll(kotlin.collections.Collection))`(`elements:` `[Collection](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/-collection/index.html)<E>`)` Returns`true`if this set contains all of the elements of the specified collection.     |
| open[Boolean](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-boolean/index.html)                                                                            | [equals](https://developer.android.com/reference/kotlin/java/util/concurrent/CopyOnWriteArraySet#equals(kotlin.Any))`(`other:` `[Any](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-any/index.html)?`)` Compares the specified object with this set for equality.                                                                                          |
| open[Unit](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-unit/index.html)                                                                                  | [forEach](https://developer.android.com/reference/kotlin/java/util/concurrent/CopyOnWriteArraySet#forEach(java.util.function.Consumer))`(`action:` `[Consumer](https://developer.android.com/reference/kotlin/java/util/function/Consumer.html#)<in` `E>`)` <br />                                                                                                |
| open[Boolean](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-boolean/index.html)                                                                            | [isEmpty](https://developer.android.com/reference/kotlin/java/util/concurrent/CopyOnWriteArraySet#isEmpty())`()` Returns`true`if this set contains no elements.                                                                                                                                                                                                   |
| open[MutableIterator](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/-mutable-iterator/index.html)\<E\>                                          | [iterator](https://developer.android.com/reference/kotlin/java/util/concurrent/CopyOnWriteArraySet#iterator())`()` Returns an iterator over the elements contained in this set in the order in which these elements were added.                                                                                                                                   |
| open[Boolean](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-boolean/index.html)                                                                            | [remove](https://developer.android.com/reference/kotlin/java/util/concurrent/CopyOnWriteArraySet#remove(java.util.concurrent.CopyOnWriteArraySet.E))`(`element:` `E?`)` Removes the specified element from this set if it is present.                                                                                                                             |
| open[Boolean](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-boolean/index.html)                                                                            | [removeAll](https://developer.android.com/reference/kotlin/java/util/concurrent/CopyOnWriteArraySet#removeAll(kotlin.collections.Collection))`(`elements:` `[Collection](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/-collection/index.html)<E>`)` Removes from this set all of its elements that are contained in the specified collection.  |
| open[Boolean](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-boolean/index.html)                                                                            | [removeIf](https://developer.android.com/reference/kotlin/java/util/concurrent/CopyOnWriteArraySet#removeIf(java.util.function.Predicate))`(`filter:` `[Predicate](https://developer.android.com/reference/kotlin/java/util/function/Predicate.html#)<in` `E>`)` <br />                                                                                           |
| open[Boolean](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-boolean/index.html)                                                                            | [retainAll](https://developer.android.com/reference/kotlin/java/util/concurrent/CopyOnWriteArraySet#retainAll(kotlin.collections.Collection))`(`elements:` `[Collection](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/-collection/index.html)<E>`)` Retains only the elements in this set that are contained in the specified collection.      |
| open[Spliterator](https://developer.android.com/reference/kotlin/java/util/Spliterator.html#)\<E\>                                                                | [spliterator](https://developer.android.com/reference/kotlin/java/util/concurrent/CopyOnWriteArraySet#spliterator())`()` Returns a[Spliterator](https://developer.android.com/reference/kotlin/java/util/Spliterator.html#)over the elements in this set in the order in which these elements were added.                                                         |
| open[Array](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-array/index.html)\<[Any](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-any/index.html)!\> | [toArray](https://developer.android.com/reference/kotlin/java/util/concurrent/CopyOnWriteArraySet#toArray())`()` Returns an array containing all of the elements in this set.                                                                                                                                                                                     |
| open[Array](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-array/index.html)\<T\>                                                                           | [toArray](https://developer.android.com/reference/kotlin/java/util/concurrent/CopyOnWriteArraySet#toArray(kotlin.Array))`(`a:` `[Array](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-array/index.html)<T>`)` Returns an array containing all of the elements in this set; the runtime type of the returned array is that of the specified array.          |

| Inherited functions ||
|---|---|
| From class[AbstractCollection](https://developer.android.com/reference/kotlin/java/util/AbstractCollection.html#) |-------------------------------------------------------------------------------------------------------------------------------------------------------------|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | [Boolean](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-boolean/index.html)                                                                          | [add](https://developer.android.com/reference/kotlin/java/util/AbstractCollection.html#add(java.util.AbstractCollection.E))`(`element:` `E`)` <br /> Ensures that this collection contains the specified element (optional operation). Returns`true`if this collection changed as a result of the call. (Returns`false`if this collection does not permit duplicates and already contains the specified element.) Collections that support this operation may place limitations on what elements may be added to this collection. In particular, some collections will refuse to add`null`elements, and others will impose restrictions on the type of elements that may be added. Collection classes should clearly specify in their documentation any restrictions on what elements may be added. If a collection refuses to add a particular element for any reason other than that it already contains the element, it*must* throw an exception (rather than returning`false`). This preserves the invariant that a collection always contains the specified element after this call returns. <br /> | | [Boolean](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-boolean/index.html)                                                                          | [addAll](https://developer.android.com/reference/kotlin/java/util/AbstractCollection.html#addAll(kotlin.collections.Collection))`(`elements:` `[Collection](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/-collection/index.html)<E>`)` <br /> Adds all of the elements in the specified collection to this collection (optional operation). The behavior of this operation is undefined if the specified collection is modified while the operation is in progress. (This implies that the behavior of this call is undefined if the specified collection is this collection, and this collection is nonempty.) If the specified collection has a defined[encounter order](https://developer.android.com/reference/kotlin/java/util/SequencedCollection.html#encounter), processing of its elements generally occurs in that order. <br />                                                                                                                                                                                                                                            | | [Unit](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-unit/index.html)                                                                                | [clear](https://developer.android.com/reference/kotlin/java/util/AbstractCollection.html#clear())`()` <br /> Removes all of the elements from this collection (optional operation). The collection will be empty after this method returns. <br />                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       | | [Boolean](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-boolean/index.html)                                                                          | [contains](https://developer.android.com/reference/kotlin/java/util/AbstractCollection.html#contains(java.util.AbstractCollection.E))`(`element:` `E?`)` <br /> Returns`true`if this collection contains the specified element. More formally, returns`true`if and only if this collection contains at least one element`e`such that`Objects.equals(o, e)`. <br />                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       | | [Boolean](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-boolean/index.html)                                                                          | [containsAll](https://developer.android.com/reference/kotlin/java/util/AbstractCollection.html#containsAll(kotlin.collections.Collection))`(`elements:` `[Collection](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/-collection/index.html)<E>`)` <br /> Returns`true`if this collection contains all of the elements in the specified collection. <br />                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              | | [Boolean](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-boolean/index.html)                                                                          | [isEmpty](https://developer.android.com/reference/kotlin/java/util/AbstractCollection.html#isEmpty())`()` <br /> Returns`true`if this collection contains no elements. <br />                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            | | [MutableIterator](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/-mutable-iterator/index.html)<E>                                          | [iterator](https://developer.android.com/reference/kotlin/java/util/AbstractCollection.html#iterator())`()` <br /> Returns an iterator over the elements contained in this collection. <br />                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            | | [Boolean](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-boolean/index.html)                                                                          | [remove](https://developer.android.com/reference/kotlin/java/util/AbstractCollection.html#remove(java.util.AbstractCollection.E))`(`element:` `E?`)` <br /> Removes a single instance of the specified element from this collection, if it is present (optional operation). More formally, removes an element`e`such that`Objects.equals(o, e)`, if this collection contains one or more such elements. Returns`true`if this collection contained the specified element (or equivalently, if this collection changed as a result of the call). <br />                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    | | [Boolean](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-boolean/index.html)                                                                          | [retainAll](https://developer.android.com/reference/kotlin/java/util/AbstractCollection.html#retainAll(kotlin.collections.Collection))`(`elements:` `[Collection](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/-collection/index.html)<E>`)` <br /> Retains only the elements in this collection that are contained in the specified collection (optional operation). In other words, removes from this collection all of its elements that are not contained in the specified collection. <br />                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     | | [Array](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-array/index.html)<[Any](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-any/index.html)!> | [toArray](https://developer.android.com/reference/kotlin/java/util/AbstractCollection.html#toArray())`()` <br /> 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's[runtime component type](https://developer.android.com/reference/kotlin/java/lang/Class.html#getComponentType())is`Object`. 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. <br />                                                                                                                                                                                                                                                                                                                                    | | [Array](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-array/index.html)<T>                                                                           | [toArray](https://developer.android.com/reference/kotlin/java/util/AbstractCollection.html#toArray(kotlin.Array))`(`a:` `[Array](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-array/index.html)<T>`)` <br /> 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. <br />       | | [String](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-string/index.html)                                                                            | [toString](https://developer.android.com/reference/kotlin/java/util/AbstractCollection.html#toString())`()` <br /> Returns a string representation of this collection. The string representation consists of a list of the collection's elements in the order they are returned by its iterator, enclosed in square brackets (`"[]"`). Adjacent elements are separated by the characters`", "`(comma and space). Elements are converted to strings as by[String.valueOf(Object)](https://developer.android.com/reference/kotlin/java/lang/String.html#valueOf(kotlin.Any)). <br />                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       | ||
| From class[AbstractSet](https://developer.android.com/reference/kotlin/java/util/AbstractSet.html#) |----------------------------------------------------------------------------|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | [Int](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-int/index.html) | [hashCode](https://developer.android.com/reference/kotlin/java/util/AbstractSet.html#hashCode())`()` <br /> Returns the hash code value for this set. The hash code of a set is defined to be the sum of the hash codes of the elements in the set, where the hash code of a`null`element is defined to be zero. This ensures that`s1.equals(s2)`implies that`s1.hashCode()==s2.hashCode()`for any two sets`s1`and`s2`, as required by the general contract of[Object.hashCode](https://developer.android.com/reference/kotlin/java/lang/Object.html#hashCode()). This implementation iterates over the set, calling the`hashCode`method on each element in the set, and adding up the results. <br /> | ||

|                                                                                                                 Properties                                                                                                                  ||
|--------------------------------------------------------------------------------|-------------------------------------------------------------------------------------------------------------------------------------------------------------|
| open[Int](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-int/index.html) | [size](https://developer.android.com/reference/kotlin/java/util/concurrent/CopyOnWriteArraySet#size:kotlin.Int) Returns the number of elements in this set. |

| Inherited properties ||
|---|---|
| From class[AbstractCollection](https://developer.android.com/reference/kotlin/java/util/AbstractCollection.html#) |----------------------------------------------------------------------------|-----------------------------------------------------------------------------------------------------------------| | [Int](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-int/index.html) | [size](https://developer.android.com/reference/kotlin/java/util/AbstractCollection.html#size:kotlin.Int) <br /> | ||

## Public constructors

### CopyOnWriteArraySet

Added in[API level 1](https://developer.android.com/guide/topics/manifest/uses-sdk-element.html#ApiLevels)  

```
CopyOnWriteArraySet()
```

Creates an empty set.  

### CopyOnWriteArraySet

Added in[API level 1](https://developer.android.com/guide/topics/manifest/uses-sdk-element.html#ApiLevels)  

```
CopyOnWriteArraySet(c: MutableCollection<out E>!)
```

Creates a set containing all of the elements of the specified collection.

|                                                                                    Parameters                                                                                     ||
|-----|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| `c` | [MutableCollection](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/-mutable-collection/index.html)\<outE\>!:the collection of elements to initially contain |

|                              Exceptions                               ||
|----------------------------------|-------------------------------------|
| `java.lang.NullPointerException` | if the specified collection is null |

## Public methods

### add

Added in[API level 1](https://developer.android.com/guide/topics/manifest/uses-sdk-element.html#ApiLevels)  

```
open fun add(element: E): Boolean
```

Adds the specified element to this set if it is not already present. More formally, adds the specified element`e`to this set if the set contains no element`e2`such that`Objects.equals(e, e2)`. If this set already contains the element, the call leaves the set unchanged and returns`false`.

|              Parameters              ||
|-----|---------------------------------|
| `e` | element to be added to this set |

|                                                                       Return                                                                        ||
|------------------------------------------------------------------------------------|-----------------------------------------------------------------|
| [Boolean](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-boolean/index.html) | `true`if this set did not already contain the specified element |

|                                                          Exceptions                                                           ||
|-------------------------------------------|------------------------------------------------------------------------------------|
| `java.lang.UnsupportedOperationException` | if the`add`operation is not supported by this set                                  |
| `java.lang.ClassCastException`            | if the class of the specified element prevents it from being added to this set     |
| `java.lang.NullPointerException`          | if the specified element is null and this set does not permit null elements        |
| `java.lang.IllegalArgumentException`      | if some property of the specified element prevents it from being added to this set |
| `java.lang.IllegalStateException`         | if the element cannot be added at this time due to insertion restrictions          |

### addAll

Added in[API level 1](https://developer.android.com/guide/topics/manifest/uses-sdk-element.html#ApiLevels)  

```
open fun addAll(elements: Collection<E>): Boolean
```

Adds all of the elements in the specified collection to this set if they're not already present. If the specified collection is also a set, the`addAll`operation effectively modifies this set so that its value is the*union*of the two sets. The behavior of this operation is undefined if the specified collection is modified while the operation is in progress.

|                         Parameters                          ||
|-----|--------------------------------------------------------|
| `c` | collection containing elements to be added to this set |

|                                                                Return                                                                 ||
|------------------------------------------------------------------------------------|---------------------------------------------------|
| [Boolean](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-boolean/index.html) | `true`if this set changed as a result of the call |

|                                                                   Exceptions                                                                   ||
|-------------------------------------------|-----------------------------------------------------------------------------------------------------|
| `java.lang.UnsupportedOperationException` | if the`addAll`operation is not supported by this set                                                |
| `java.lang.ClassCastException`            | if the class of an element of the specified collection prevents it from being added to this set     |
| `java.lang.NullPointerException`          | if the specified collection is null                                                                 |
| `java.lang.IllegalArgumentException`      | if some property of an element of the specified collection prevents it from being added to this set |
| `java.lang.IllegalStateException`         | if not all the elements can be added at this time due to insertion restrictions                     |

**See Also**

### clear

Added in[API level 1](https://developer.android.com/guide/topics/manifest/uses-sdk-element.html#ApiLevels)  

```
open fun clear(): Unit
```

Removes all of the elements from this set. The set will be empty after this call returns.

|                                         Exceptions                                          ||
|-------------------------------------------|--------------------------------------------------|
| `java.lang.UnsupportedOperationException` | if the`clear`method is not supported by this set |

### contains

Added in[API level 1](https://developer.android.com/guide/topics/manifest/uses-sdk-element.html#ApiLevels)  

```
open fun contains(element: E?): Boolean
```

Returns`true`if this set contains the specified element. More formally, returns`true`if and only if this set contains an element`e`such that`Objects.equals(o, e)`.

|                       Parameters                        ||
|-----|----------------------------------------------------|
| `o` | element whose presence in this set is to be tested |

|                                                                Return                                                                ||
|------------------------------------------------------------------------------------|--------------------------------------------------|
| [Boolean](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-boolean/index.html) | `true`if this set contains the specified element |

|                                                                                                            Exceptions                                                                                                            ||
|----------------------------------|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| `java.lang.ClassCastException`   | if the type of the specified element is incompatible with this set ([optional](https://developer.android.com/reference/kotlin/java/util/concurrent/Collection#optional-restrictions))          |
| `java.lang.NullPointerException` | if the specified element is null and this set does not permit null elements ([optional](https://developer.android.com/reference/kotlin/java/util/concurrent/Collection#optional-restrictions)) |

### containsAll

Added in[API level 1](https://developer.android.com/guide/topics/manifest/uses-sdk-element.html#ApiLevels)  

```
open fun containsAll(elements: Collection<E>): Boolean
```

Returns`true`if this set contains all of the elements of the specified collection. If the specified collection is also a set, this method returns`true`if it is a*subset*of this set.

|                        Parameters                         ||
|-----|------------------------------------------------------|
| `c` | collection to be checked for containment in this set |

|                                                                             Return                                                                             ||
|------------------------------------------------------------------------------------|----------------------------------------------------------------------------|
| [Boolean](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-boolean/index.html) | `true`if this set contains all of the elements of the specified collection |

|                                                                                                                      Exceptions                                                                                                                      ||
|----------------------------------|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| `java.lang.ClassCastException`   | if the types of one or more elements in the specified collection are incompatible with this set ([optional](https://developer.android.com/reference/kotlin/java/util/concurrent/Collection#optional-restrictions)) |
| `java.lang.NullPointerException` | if the specified collection is null                                                                                                                                                                                |

**See Also**

### equals

Added in[API level 1](https://developer.android.com/guide/topics/manifest/uses-sdk-element.html#ApiLevels)  

```
open fun equals(other: Any?): Boolean
```

Compares the specified object with this set for equality. Returns`true`if the specified object is the same object as this object, or if it is also a[Set](https://developer.android.com/reference/kotlin/java/util/Set.html#)and the elements returned by an java.util.Set#iterator() over the specified set are the same as the elements returned by an iterator over this set. More formally, the two iterators are considered to return the same elements if they return the same number of elements and for every element`e1`returned by the iterator over the specified set, there is an element`e2`returned by the iterator over this set such that`Objects.equals(e1, e2)`.

|                       Parameters                        ||
|-------|--------------------------------------------------|
| `obj` | the reference object with which to compare.      |
| `o`   | object to be compared for equality with this set |

|                                                                 Return                                                                 ||
|------------------------------------------------------------------------------------|----------------------------------------------------|
| [Boolean](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-boolean/index.html) | `true`if the specified object is equal to this set |

### forEach

Added in[API level 24](https://developer.android.com/guide/topics/manifest/uses-sdk-element.html#ApiLevels)  

```
open fun forEach(action: Consumer<in E>): Unit
```

|                                                                        Parameters                                                                         ||
|----------|-------------------------------------------------------------------------------------------------------------------------------------------------|
| `action` | [Consumer](https://developer.android.com/reference/kotlin/java/util/function/Consumer.html#)\<inE\>:The action to be performed for each element |

|                            Exceptions                             ||
|----------------------------------|---------------------------------|
| `java.lang.NullPointerException` | if the specified action is null |

### isEmpty

Added in[API level 1](https://developer.android.com/guide/topics/manifest/uses-sdk-element.html#ApiLevels)  

```
open fun isEmpty(): Boolean
```

Returns`true`if this set contains no elements.

|                                                           Return                                                           ||
|------------------------------------------------------------------------------------|----------------------------------------|
| [Boolean](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-boolean/index.html) | `true`if this set contains no elements |

### iterator

Added in[API level 1](https://developer.android.com/guide/topics/manifest/uses-sdk-element.html#ApiLevels)  

```
open fun iterator(): MutableIterator<E>
```

Returns an iterator over the elements contained in this set in the order in which these elements were added.

The returned iterator provides a snapshot of the state of the set when the iterator was constructed. No synchronization is needed while traversing the iterator. The iterator does*NOT* support the`remove`method.

|                                                                            Return                                                                             ||
|--------------------------------------------------------------------------------------------------------------------|-------------------------------------------|
| [MutableIterator](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/-mutable-iterator/index.html)<E> | an iterator over the elements in this set |

### remove

Added in[API level 1](https://developer.android.com/guide/topics/manifest/uses-sdk-element.html#ApiLevels)  

```
open fun remove(element: E?): Boolean
```

Removes the specified element from this set if it is present. More formally, removes an element`e`such that`Objects.equals(o, e)`, if this set contains such an element. Returns`true`if this set contained the element (or equivalently, if this set changed as a result of the call). (This set will not contain the element once the call returns.)

|                     Parameters                      ||
|-----|------------------------------------------------|
| `o` | object to be removed from this set, if present |

|                                                                Return                                                                 ||
|------------------------------------------------------------------------------------|---------------------------------------------------|
| [Boolean](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-boolean/index.html) | `true`if this set contained the specified element |

|                                                                                                                Exceptions                                                                                                                 ||
|-------------------------------------------|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| `java.lang.ClassCastException`            | if the type of the specified element is incompatible with this set ([optional](https://developer.android.com/reference/kotlin/java/util/concurrent/Collection#optional-restrictions))          |
| `java.lang.NullPointerException`          | if the specified element is null and this set does not permit null elements ([optional](https://developer.android.com/reference/kotlin/java/util/concurrent/Collection#optional-restrictions)) |
| `java.lang.UnsupportedOperationException` | if the`remove`operation is not supported by this set                                                                                                                                           |

### removeAll

Added in[API level 1](https://developer.android.com/guide/topics/manifest/uses-sdk-element.html#ApiLevels)  

```
open fun removeAll(elements: Collection<E>): Boolean
```

Removes from this set all of its elements that are contained in the specified collection. If the specified collection is also a set, this operation effectively modifies this set so that its value is the*asymmetric set difference*of the two sets.

|                           Parameters                            ||
|-----|------------------------------------------------------------|
| `c` | collection containing elements to be removed from this set |

|                                                                Return                                                                 ||
|------------------------------------------------------------------------------------|---------------------------------------------------|
| [Boolean](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-boolean/index.html) | `true`if this set changed as a result of the call |

|                                                                                                                                        Exceptions                                                                                                                                        ||
|-------------------------------------------|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| `java.lang.UnsupportedOperationException` | if the`removeAll`operation is not supported by this set                                                                                                                                                                                       |
| `java.lang.ClassCastException`            | if the class of an element of this set is incompatible with the specified collection ([optional](https://developer.android.com//reference/java/util/Collection.html#optional-restrictions))                                                   |
| `java.lang.NullPointerException`          | if this set contains a null element and the specified collection does not permit null elements ([optional](https://developer.android.com//reference/java/util/Collection.html#optional-restrictions)), or if the specified collection is null |

**See Also**

### removeIf

Added in[API level 24](https://developer.android.com/guide/topics/manifest/uses-sdk-element.html#ApiLevels)  

```
open fun removeIf(filter: Predicate<in E>): Boolean
```

|                                                                                Parameters                                                                                 ||
|----------|-----------------------------------------------------------------------------------------------------------------------------------------------------------------|
| `filter` | [Predicate](https://developer.android.com/reference/kotlin/java/util/function/Predicate.html#)\<inE\>:a predicate which returns`true`for elements to be removed |

|                                                         Return                                                         ||
|------------------------------------------------------------------------------------|------------------------------------|
| [Boolean](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-boolean/index.html) | `true`if any elements were removed |

|                                                                                                          Exceptions                                                                                                          ||
|-------------------------------------------|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| `java.lang.NullPointerException`          | if the specified filter is null                                                                                                                                                   |
| `java.lang.UnsupportedOperationException` | if elements cannot be removed from this collection. Implementations may throw this exception if a matching element cannot be removed or if, in general, removal is not supported. |

### retainAll

Added in[API level 1](https://developer.android.com/guide/topics/manifest/uses-sdk-element.html#ApiLevels)  

```
open fun retainAll(elements: Collection<E>): Boolean
```

Retains only the elements in this set that are contained in the specified collection. In other words, removes from this set all of its elements that are not contained in the specified collection. If the specified collection is also a set, this operation effectively modifies this set so that its value is the*intersection*of the two sets.

|                           Parameters                           ||
|-----|-----------------------------------------------------------|
| `c` | collection containing elements to be retained in this set |

|                                                                Return                                                                 ||
|------------------------------------------------------------------------------------|---------------------------------------------------|
| [Boolean](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-boolean/index.html) | `true`if this set changed as a result of the call |

|                                                                                                                                        Exceptions                                                                                                                                        ||
|-------------------------------------------|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| `java.lang.UnsupportedOperationException` | if the`retainAll`operation is not supported by this set                                                                                                                                                                                       |
| `java.lang.ClassCastException`            | if the class of an element of this set is incompatible with the specified collection ([optional](https://developer.android.com//reference/java/util/Collection.html#optional-restrictions))                                                   |
| `java.lang.NullPointerException`          | if this set contains a null element and the specified collection does not permit null elements ([optional](https://developer.android.com//reference/java/util/Collection.html#optional-restrictions)), or if the specified collection is null |

**See Also**

### spliterator

Added in[API level 24](https://developer.android.com/guide/topics/manifest/uses-sdk-element.html#ApiLevels)  

```
open fun spliterator(): Spliterator<E>
```

Returns a[Spliterator](https://developer.android.com/reference/kotlin/java/util/Spliterator.html#)over the elements in this set in the order in which these elements were added.

The`Spliterator`reports[Spliterator.IMMUTABLE](https://developer.android.com/reference/kotlin/java/util/Spliterator.html#IMMUTABLE:kotlin.Int),[Spliterator.DISTINCT](https://developer.android.com/reference/kotlin/java/util/Spliterator.html#DISTINCT:kotlin.Int),[Spliterator.SIZED](https://developer.android.com/reference/kotlin/java/util/Spliterator.html#SIZED:kotlin.Int), and[Spliterator.SUBSIZED](https://developer.android.com/reference/kotlin/java/util/Spliterator.html#SUBSIZED:kotlin.Int).

The spliterator provides a snapshot of the state of the set when the spliterator was constructed. No synchronization is needed while operating on the spliterator.

|                                                                  Return                                                                   ||
|----------------------------------------------------------------------------------------------|---------------------------------------------|
| [Spliterator](https://developer.android.com/reference/kotlin/java/util/Spliterator.html#)<E> | a`Spliterator`over the elements in this set |

### toArray

Added in[API level 1](https://developer.android.com/guide/topics/manifest/uses-sdk-element.html#ApiLevels)  

```
open fun toArray(): Array<Any!>
```

Returns an array containing all of the elements in this set. If this set 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 set. (In other words, this method must allocate a new array even if this set 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.

|                                                                                                    Return                                                                                                     ||
|-------------------------------------------------------------------------------------------------------------------------------------------------------------|--------------------------------------------------|
| [Array](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-array/index.html)<[Any](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-any/index.html)!> | an array containing all the elements in this set |

### toArray

Added in[API level 1](https://developer.android.com/guide/topics/manifest/uses-sdk-element.html#ApiLevels)  

```
open fun <T : Any!> toArray(a: Array<T>): Array<T>
```

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

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

If this set 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 set known to contain only strings. The following code can be used to dump the set into a newly allocated array of`String`:  

```kotlin
<code>String[] y = x.toArray(new String[0]);</code>
```
Note that`toArray(new Object[0])`is identical in function to`toArray()`.

|                                                                                                                           Parameters                                                                                                                           ||
|-------|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| `<T>` | the component type of the array to contain the collection                                                                                                                                                                                               |
| `a`   | [Array](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-array/index.html)\<T\>:the array into which the elements of this set are to be stored, if it is big enough; otherwise, a new array of the same runtime type is allocated for this purpose. |

|                                                               Return                                                                ||
|-----------------------------------------------------------------------------------|--------------------------------------------------|
| [Array](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-array/index.html)<T> | an array containing all the elements in this set |

|                                                                    Exceptions                                                                    ||
|----------------------------------|----------------------------------------------------------------------------------------------------------------|
| `java.lang.ArrayStoreException`  | if the runtime type of the specified array is not a supertype of the runtime type of every element in this set |
| `java.lang.NullPointerException` | if the specified array is null                                                                                 |

## Properties

### size

Added in[API level 1](https://developer.android.com/guide/topics/manifest/uses-sdk-element.html#ApiLevels)  

```
open val size: Int
```

Returns the number of elements in this set.

|                                                     Return                                                     ||
|----------------------------------------------------------------------------|------------------------------------|
| [Int](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-int/index.html) | the number of elements in this set |