org.apache.commons.collections4.collection

## Class IndexedCollection<K,C>

- java.lang.Object
- - org.apache.commons.collections4.collection.AbstractCollectionDecorator<C>
  - - org.apache.commons.collections4.collection.IndexedCollection<K,C>

- Type Parameters:
    `K` - the type of object in the index.
    `C` - the type of object in the collection.
  All Implemented Interfaces:
    Serializable, Iterable<C>, Collection<C>

      public class IndexedCollection<K,C>
      extends AbstractCollectionDecorator<C>

  An IndexedCollection is a Map-like view onto a Collection. It accepts a keyTransformer to define how the keys are converted from the values.

  Modifications made to this decorator modify the index as well as the decorated `Collection`. However, modifications to the underlying `Collection` will not update the index and it will get out of sync.

  If modification of the decorated `Collection` is unavoidable, then a call to `reindex()` will update the index to the current contents of the `Collection`.

  Since:
    4.0
  See Also:
    Serialized Form

- - ### Constructor Summary

    Constructors
    Constructor and Description
    `IndexedCollection(Collection<C> coll, Transformer<C,K> keyTransformer, MultiMap<K,C> map, boolean uniqueIndex)` Create a `IndexedCollection`.

  - ### Method Summary

    All Methods Static Methods Instance Methods Concrete Methods
    Modifier and Type | Method and Description
    --- | ---
    `boolean` | `add(C object)`
    `boolean` | `addAll(Collection<? extends C> coll)`
    `void` | `clear()`
    `boolean` | `contains(Object object)`
    `boolean` | `containsAll(Collection<?> coll)`
    `C` | `get(K key)` Get the element associated with the given key.
    `static <K,C> IndexedCollection<K,C>` | `nonUniqueIndexedCollection(Collection<C> coll, Transformer<C,K> keyTransformer)` Create an `IndexedCollection` for a non-unique index.
    `void` | `reindex()` Clears the index and re-indexes the entire decorated `Collection`.
    `boolean` | `remove(Object object)`
    `boolean` | `removeAll(Collection<?> coll)`
    `boolean` | `retainAll(Collection<?> coll)`
    `static <K,C> IndexedCollection<K,C>` | `uniqueIndexedCollection(Collection<C> coll, Transformer<C,K> keyTransformer)` Create an `IndexedCollection` for a unique index.
    `Collection<C>` | `values(K key)` Get all elements associated with the given key.

    - ### Methods inherited from class org.apache.commons.collections4.collection.AbstractCollectionDecorator

      `decorated, isEmpty, iterator, setCollection, size, toArray, toArray, toString`

    - ### Methods inherited from class java.lang.Object

      `clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait`

    - ### Methods inherited from interface java.util.Collection

      `equals, hashCode, parallelStream, removeIf, spliterator, stream`

    - ### Methods inherited from interface java.lang.Iterable

      `forEach`

- - ### Constructor Detail

    - #### IndexedCollection

          public IndexedCollection(Collection<C> coll,
                                   Transformer<C,K> keyTransformer,
                                   MultiMap<K,C> map,
                                   boolean uniqueIndex)

      Create a `IndexedCollection`.
      Parameters:
        `coll` - decorated `Collection`
        `keyTransformer` - `Transformer` for generating index keys
        `map` - map to use as index
        `uniqueIndex` - if the index shall enforce uniqueness of index keys

  - ### Method Detail

    - #### uniqueIndexedCollection

          public static <K,C> IndexedCollection<K,C> uniqueIndexedCollection(Collection<C> coll,
                                                                             Transformer<C,K> keyTransformer)

      Create an `IndexedCollection` for a unique index.

      If an element is added, which maps to an existing key, an `IllegalArgumentException` will be thrown.

      Type Parameters:
        `K` - the index object type.
        `C` - the collection type.
      Parameters:
        `coll` - the decorated `Collection`.
        `keyTransformer` - the `Transformer` for generating index keys.
      Returns:
        the created `IndexedCollection`.

    - #### nonUniqueIndexedCollection

          public static <K,C> IndexedCollection<K,C> nonUniqueIndexedCollection(Collection<C> coll,
                                                                                Transformer<C,K> keyTransformer)

      Create an `IndexedCollection` for a non-unique index.
      Type Parameters:
        `K` - the index object type.
        `C` - the collection type.
      Parameters:
        `coll` - the decorated `Collection`.
        `keyTransformer` - the `Transformer` for generating index keys.
      Returns:
        the created `IndexedCollection`.

    - #### add

          public boolean add(C object)

      Specified by:
        `add` in interface `Collection<C>`
      Overrides:
        `add` in class `AbstractCollectionDecorator<C>`
      Throws:
        `IllegalArgumentException` - if the object maps to an existing key and the index enforces a uniqueness constraint

    - #### addAll

          public boolean addAll(Collection<? extends C> coll)

      Specified by:
        `addAll` in interface `Collection<C>`
      Overrides:
        `addAll` in class `AbstractCollectionDecorator<C>`

    - #### clear

          public void clear()

      Specified by:
        `clear` in interface `Collection<C>`
      Overrides:
        `clear` in class `AbstractCollectionDecorator<C>`

    - #### contains

          public boolean contains(Object object)

      Note: uses the index for fast lookup

      Specified by:
        `contains` in interface `Collection<C>`
      Overrides:
        `contains` in class `AbstractCollectionDecorator<C>`

    - #### containsAll

          public boolean containsAll(Collection<?> coll)

      Note: uses the index for fast lookup

      Specified by:
        `containsAll` in interface `Collection<C>`
      Overrides:
        `containsAll` in class `AbstractCollectionDecorator<C>`

    - #### get

          public C get(K key)

      Get the element associated with the given key.

      In case of a non-unique index, this method will return the first value associated with the given key. To retrieve all elements associated with a key, use `values(Object)`.

      Parameters:
        `key` - key to look up
      Returns:
        element found
      See Also:
        `values(Object)`

    - #### values

          public Collection<C> values(K key)

      Get all elements associated with the given key.
      Parameters:
        `key` - key to look up
      Returns:
        a collection of elements found, or null if `contains(key) == false`

    - #### reindex

          public void reindex()

      Clears the index and re-indexes the entire decorated `Collection`.

    - #### remove

          public boolean remove(Object object)

      Specified by:
        `remove` in interface `Collection<C>`
      Overrides:
        `remove` in class `AbstractCollectionDecorator<C>`

    - #### removeAll

          public boolean removeAll(Collection<?> coll)

      Specified by:
        `removeAll` in interface `Collection<C>`
      Overrides:
        `removeAll` in class `AbstractCollectionDecorator<C>`

    - #### retainAll

          public boolean retainAll(Collection<?> coll)

      Specified by:
        `retainAll` in interface `Collection<C>`
      Overrides:
        `retainAll` in class `AbstractCollectionDecorator<C>`

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