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====AbstractMap class====
{{java|AbstractMap }} is an example of a ''skeletal implementation''.{{sfn|Bloch|2018|loc=Chapter §4 Item 20: Prefer interfaces to abstract classes|pp=99-103}}
The direct subclasses of {{java|ConcurrentHashMapAbstractMap}} class include {{java|ConcurrentSkipListMap}}, {{java|EnumMap}}, {{java|HashMap}}, {{java|IdentityHashMap}}, {{java|TreeMap}} and {{java|WeakHashMap}}.
=====HashMap=====
<code>{{java|HashMap</code>}} uses a [[hash table]]. The hashes of the keys are used to find the elements in various buckets. The <code>{{java|HashMap</code>}} is a hash-based collection. {{sfn|Goetz|Peierls|Bloch|Bowbeer|2006|loc=§5.2.1 ConcurrentHashMap|pp=85-86}}
======LinkedHashMap======
<code>{{java|LinkedHashMap</code>}} extends <code>{{java|HashMap</code>}} by creating a [[doubly linked list]] between the elements, allowing them to be accessed in the order in which they were inserted into the map. <code>{{java|LinkedHashMap</code>}} contains a <code>protected</code> <code>removeEldestEntry</code> method which is called by the <code>put</code> method whenever a new key is added to the <code>Map</code>.{{sfn|Bloch|2018|loc=Chapter §44 Favor the use of standard functional interfaces|pp=199-202}} The <code>Map</code> removes its eldest entry whenever <code>removeEldestEntry</code> returns true.{{sfn|Bloch|2018|loc=Chapter §44 Favor the use of standard functional interfaces|pp=199-202}} The <code>removeEldestEntry</code> method can be overridden.{{sfn|Bloch|2018|loc=Chapter §44 Favor the use of standard functional interfaces|pp=199-202}}
=====TreeMap=====
<code>{{java|TreeMap</code>}}, in contrast to <code>{{java|HashMap</code>}} and <code>{{java|LinkedHashMap</code>}}, uses a red–black tree. The keys are used as the values for the nodes in the tree, and the nodes point to the elements in the <code>Map</code>.<ref>{{cite web|url=http://docs.oracle.com/javase/7/docs/api/java/util/Map.html |title=Map (Java Platform SE 7 ) |publisher=Docs.oracle.com |date=2013-06-06 |access-date=2013-08-16}}</ref>
=====ConcurrentHashMap=====
{{java|ConcurrentHashMap}} is similar to <code>{{java|HashMap</code>}} and is also a hash-based collection. {{sfn|Goetz|Peierls|Bloch|Bowbeer|2006|loc=§5.2.1 ConcurrentHashMap|pp=85-86}} However, there are a number of differences, such as the differences in the locking strategy they use.
The {{java|ConcurrentHashMap}} uses a completely different locking strategy to provide improved scalability and concurrency.{{sfn|Goetz|Peierls|Bloch|Bowbeer|2006|loc=§5.2.1 ConcurrentHashMap|pp=85-86}} {{java|ConcurrentHashMap}} does not synchronize every method using the same lock.{{sfn|Goetz|Peierls|Bloch|Bowbeer|2006|loc=§5.2.1 ConcurrentHashMap|pp=85-86}} Instead, {{java|ConcurrentHashMap}} use a mechanism known as ''lock striping''.{{sfn|Goetz|Peierls|Bloch|Bowbeer|2006|loc=§5.2.1 ConcurrentHashMap|pp=85-86}} This mechanism provides a finer-grained locking mechanism.{{sfn|Goetz|Peierls|Bloch|Bowbeer|2006|loc=§5.2.1 ConcurrentHashMap|pp=85-86}} It also permits a higher degree of shared access.{{sfn|Goetz|Peierls|Bloch|Bowbeer|2006|loc=§5.2.1 ConcurrentHashMap|pp=85-86}}
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