Class LinkedHashMap<K,V>
- Type Parameters:
K- the type of keys maintained by this mapV- the type of mapped values
- All Implemented Interfaces:
Serializable,Cloneable,Map<K,,V> SequencedMap<K,V>
Hash table and linked list implementation of the Map interface,
with well-defined encounter order. This implementation differs from
HashMap in that it maintains a doubly-linked list running through all of
its entries. This linked list defines the encounter order (the order of iteration),
which is normally the order in which keys were inserted into the map
(insertion-order). The least recently inserted entry (the eldest) is
first, and the youngest entry is last. Note that encounter order is not affected
if a key is re-inserted into the map with the put method. (A key
k is reinserted into a map m if m.put(k, v) is invoked when
m.containsKey(k) would return true immediately prior to
the invocation.) The reverse-ordered view of this map is in the opposite order, with
the youngest entry appearing first and the eldest entry appearing last.
The encounter order of entries already in the map can be changed by using
the putFirst and putLast methods.
This implementation spares its clients from the unspecified, generally
chaotic ordering provided by HashMap (and Hashtable),
without incurring the increased cost associated with TreeMap. It
can be used to produce a copy of a map that has the same order as the
original, regardless of the original map's implementation:
void foo(Map<String, Integer> m) {
Map<String, Integer> copy = new LinkedHashMap<>(m);
...
}
This technique is particularly useful if a module takes a map on input,
copies it, and later returns results whose order is determined by that of
the copy. (Clients generally appreciate having things returned in the same
order they were presented.)
A special constructor is
provided to create a linked hash map whose encounter order is the order
in which its entries were last accessed, from least-recently accessed to
most-recently (access-order). This kind of map is well-suited to
building LRU caches. Invoking the put, putIfAbsent,
get, getOrDefault, compute, computeIfAbsent,
computeIfPresent, or merge methods results
in an access to the corresponding entry (assuming it exists after the
invocation completes). The replace methods only result in an access
of the entry if the value is replaced. The putAll method generates one
entry access for each mapping in the specified map, in the order that
key-value mappings are provided by the specified map's entry set iterator.
No other methods generate entry accesses. Invoking these methods on the
reversed view generates accesses to entries on the backing map. Note that in the
reversed view, an access to an entry moves it first in encounter order.
Explicit-positioning methods such as putFirst or lastEntry, whether on
the map or on its reverse-ordered view, perform the positioning operation and
do not generate entry accesses. Operations on the keySet, values,
and entrySet views or on their sequenced counterparts do not affect
the encounter order of the backing map.
The removeEldestEntry(Map.Entry) method may be overridden to
impose a policy for removing stale mappings automatically when new mappings
are added to the map. Alternatively, since the "eldest" entry is the first
entry in encounter order, programs can inspect and remove stale mappings through
use of the firstEntry and pollFirstEntry
methods.
This class provides all of the optional Map and SequencedMap operations,
and it permits null elements. Like HashMap, it provides constant-time
performance for the basic operations (add, contains and
remove), assuming the hash function disperses elements
properly among the buckets. Performance is likely to be just slightly
below that of HashMap, due to the added expense of maintaining the
linked list, with one exception: Iteration over the collection-views
of a LinkedHashMap requires time proportional to the size
of the map, regardless of its capacity. Iteration over a HashMap
is likely to be more expensive, requiring time proportional to its
capacity.
A linked hash map has two parameters that affect its performance:
initial capacity and load factor. They are defined precisely
as for HashMap. Note, however, that the penalty for choosing an
excessively high value for initial capacity is less severe for this class
than for HashMap, as iteration times for this class are unaffected
by capacity.
Note that this implementation is not synchronized.
If multiple threads access a linked hash map concurrently, and at least
one of the threads modifies the map structurally, it must be
synchronized externally. This is typically accomplished by
synchronizing on some object that naturally encapsulates the map.
If no such object exists, the map should be "wrapped" using the
Collections.synchronizedMap
method. This is best done at creation time, to prevent accidental
unsynchronized access to the map:
Map m = Collections.synchronizedMap(new LinkedHashMap(...));A structural modification is any operation that adds or deletes one or more mappings or, in the case of access-ordered linked hash maps, affects iteration order. In insertion-ordered linked hash maps, merely changing the value associated with a key that is already contained in the map is not a structural modification. In access-ordered linked hash maps, merely querying the map with
get is a structural modification.
)
The iterators returned by the iterator method of the collections
returned by all of this class's collection view methods are
fail-fast: if the map is structurally modified at any time after
the iterator is created, in any way except through the iterator's own
remove method, the iterator will throw a ConcurrentModificationException. Thus, in the face of concurrent
modification, the iterator fails quickly and cleanly, rather than risking
arbitrary, non-deterministic behavior at an undetermined time in the future.
Note that the fail-fast behavior of an iterator cannot be guaranteed
as it is, generally speaking, impossible to make any hard guarantees in the
presence of unsynchronized concurrent modification. Fail-fast iterators
throw ConcurrentModificationException on a best-effort basis.
Therefore, it would be wrong to write a program that depended on this
exception for its correctness: the fail-fast behavior of iterators
should be used only to detect bugs.
The spliterators returned by the spliterator method of the collections
returned by all of this class's collection view methods are
late-binding,
fail-fast, and additionally report Spliterator.ORDERED.
This class is a member of the Java Collections Framework.
- Implementation Note:
- The spliterators returned by the spliterator method of the collections returned by all of this class's collection view methods are created from the iterators of the corresponding collections.
- Since:
- 1.4
- See Also:
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Nested Class Summary
Nested classes/interfaces declared in class java.util.AbstractMap
AbstractMap.SimpleEntry<K,V>, AbstractMap.SimpleImmutableEntry<K, V> -
Constructor Summary
ConstructorsConstructorDescriptionConstructs an empty insertion-orderedLinkedHashMapinstance with the default initial capacity (16) and load factor (0.75).LinkedHashMap(int initialCapacity) Constructs an empty insertion-orderedLinkedHashMapinstance with the specified initial capacity and a default load factor (0.75).LinkedHashMap(int initialCapacity, float loadFactor) Constructs an empty insertion-orderedLinkedHashMapinstance with the specified initial capacity and load factor.LinkedHashMap(int initialCapacity, float loadFactor, boolean accessOrder) Constructs an emptyLinkedHashMapinstance with the specified initial capacity, load factor and ordering mode.LinkedHashMap(Map<? extends K, ? extends V> m) Constructs an insertion-orderedLinkedHashMapinstance with the same mappings as the specified map. -
Method Summary
Modifier and TypeMethodDescriptionvoidclear()Removes all of the mappings from this map.booleancontainsValue(Object value) Returnstrueif this map maps one or more keys to the specified value.entrySet()Returns aSetview of the mappings contained in this map.Returns the value to which the specified key is mapped, ornullif this map contains no mapping for the key.keySet()Returns aSetview of the keys contained in this map.static <K,V> LinkedHashMap <K, V> newLinkedHashMap(int numMappings) Creates a new, empty, insertion-ordered LinkedHashMap suitable for the expected number of mappings.Inserts the given mapping into the map if it is not already present, or replaces the value of a mapping if it is already present (optional operation).Inserts the given mapping into the map if it is not already present, or replaces the value of a mapping if it is already present (optional operation).protected booleanremoveEldestEntry(Map.Entry<K, V> eldest) Returnstrueif this map should remove its eldest entry.reversed()Returns a reverse-ordered view of this map.Returns aSequencedSetview of this map'sentrySet.Returns aSequencedSetview of this map'skeySet.Returns aSequencedCollectionview of this map'svaluescollection.values()Returns aCollectionview of the values contained in this map.Methods declared in class java.util.HashMap
clear, clone, compute, computeIfAbsent, computeIfPresent, containsKey, isEmpty, merge, newHashMap, put, putAll, remove, sizeMethods declared in class java.util.AbstractMap
equals, hashCode, toStringMethods declared in interface java.util.Map
compute, computeIfAbsent, computeIfPresent, containsKey, equals, forEach, getOrDefault, hashCode, isEmpty, merge, put, putAll, putIfAbsent, remove, remove, replace, replace, replaceAll, sizeMethods declared in interface java.util.SequencedMap
firstEntry, lastEntry, pollFirstEntry, pollLastEntry
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Constructor Details
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LinkedHashMap
public LinkedHashMap(int initialCapacity, float loadFactor) Constructs an empty insertion-orderedLinkedHashMapinstance with the specified initial capacity and load factor.- API Note:
- To create a
LinkedHashMapwith an initial capacity that accommodates an expected number of mappings, usenewLinkedHashMap. - Parameters:
initialCapacity- the initial capacityloadFactor- the load factor- Throws:
IllegalArgumentException- if the initial capacity is negative or the load factor is nonpositive
-
LinkedHashMap
public LinkedHashMap(int initialCapacity) Constructs an empty insertion-orderedLinkedHashMapinstance with the specified initial capacity and a default load factor (0.75).- API Note:
- To create a
LinkedHashMapwith an initial capacity that accommodates an expected number of mappings, usenewLinkedHashMap. - Parameters:
initialCapacity- the initial capacity- Throws:
IllegalArgumentException- if the initial capacity is negative
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LinkedHashMap
public LinkedHashMap()Constructs an empty insertion-orderedLinkedHashMapinstance with the default initial capacity (16) and load factor (0.75). -
LinkedHashMap
Constructs an insertion-orderedLinkedHashMapinstance with the same mappings as the specified map. TheLinkedHashMapinstance is created with a default load factor (0.75) and an initial capacity sufficient to hold the mappings in the specified map.- Parameters:
m- the map whose mappings are to be placed in this map- Throws:
NullPointerException- if the specified map is null
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LinkedHashMap
public LinkedHashMap(int initialCapacity, float loadFactor, boolean accessOrder) Constructs an emptyLinkedHashMapinstance with the specified initial capacity, load factor and ordering mode.- Parameters:
initialCapacity- the initial capacityloadFactor- the load factoraccessOrder- the ordering mode -truefor access-order,falsefor insertion-order- Throws:
IllegalArgumentException- if the initial capacity is negative or the load factor is nonpositive
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-
Method Details
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putFirst
Inserts the given mapping into the map if it is not already present, or replaces the value of a mapping if it is already present (optional operation). After this operation completes normally, the given mapping will be present in this map, and it will be the first mapping in this map's encounter order.If this map already contains a mapping for this key, the mapping is relocated if necessary so that it is first in encounter order.
- Specified by:
putFirstin interfaceSequencedMap<K,V> - Parameters:
k- the keyv- the value- Returns:
- the value previously associated with k, or null if none
- Since:
- 21
-
putLast
Inserts the given mapping into the map if it is not already present, or replaces the value of a mapping if it is already present (optional operation). After this operation completes normally, the given mapping will be present in this map, and it will be the last mapping in this map's encounter order.If this map already contains a mapping for this key, the mapping is relocated if necessary so that it is last in encounter order.
- Specified by:
putLastin interfaceSequencedMap<K,V> - Parameters:
k- the keyv- the value- Returns:
- the value previously associated with k, or null if none
- Since:
- 21
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containsValue
Returnstrueif this map maps one or more keys to the specified value.- Specified by:
containsValuein interfaceMap<K,V> - Overrides:
containsValuein classHashMap<K,V> - Parameters:
value- value whose presence in this map is to be tested- Returns:
trueif this map maps one or more keys to the specified value
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get
Returns the value to which the specified key is mapped, ornullif this map contains no mapping for the key.More formally, if this map contains a mapping from a key
kto a valuevsuch that(key==null ? k==null : key.equals(k)), then this method returnsv; otherwise it returnsnull. (There can be at most one such mapping.)A return value of
nulldoes not necessarily indicate that the map contains no mapping for the key; it's also possible that the map explicitly maps the key tonull. ThecontainsKeyoperation may be used to distinguish these two cases. -
clear
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removeEldestEntry
Returnstrueif this map should remove its eldest entry. This method is invoked byputandputAllafter inserting a new entry into the map. It provides the implementor with the opportunity to remove the eldest entry each time a new one is added. This is useful if the map represents a cache: it allows the map to reduce memory consumption by deleting stale entries.Sample use: this override will allow the map to grow up to 100 entries and then delete the eldest entry each time a new entry is added, maintaining a steady state of 100 entries.
private static final int MAX_ENTRIES = 100; protected boolean removeEldestEntry(Map.Entry eldest) { return size() > MAX_ENTRIES; }This method typically does not modify the map in any way, instead allowing the map to modify itself as directed by its return value. It is permitted for this method to modify the map directly, but if it does so, it must return
false(indicating that the map should not attempt any further modification). The effects of returningtrueafter modifying the map from within this method are unspecified.This implementation merely returns
false(so that this map acts like a normal map - the eldest element is never removed).- Parameters:
eldest- The least recently inserted entry in the map, or if this is an access-ordered map, the least recently accessed entry. This is the entry that will be removed if this method returnstrue. If the map was empty prior to theputorputAllinvocation resulting in this invocation, this will be the entry that was just inserted; in other words, if the map contains a single entry, the eldest entry is also the newest.- Returns:
trueif the eldest entry should be removed from the map;falseif it should be retained.
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keySet
Returns aSetview of the keys contained in this map. The encounter order of the keys in the view matches the encounter order of mappings of this map. The set is backed by the map, so changes to the map are reflected in the set, and vice-versa. If the map is modified while an iteration over the set is in progress (except through the iterator's ownremoveoperation), the results of the iteration are undefined. The set supports element removal, which removes the corresponding mapping from the map, via theIterator.remove,Set.remove,removeAll,retainAll, andclearoperations. It does not support theaddoraddAlloperations. ItsSpliteratortypically provides faster sequential performance but much poorer parallel performance than that ofHashMap. -
sequencedKeySet
Returns aSequencedSetview of this map'skeySet.The returned view has the same characteristics as specified for the view returned by the
keySetmethod.- Specified by:
sequencedKeySetin interfaceSequencedMap<K,V> - Returns:
- a
SequencedSetview of this map'skeySet - Since:
- 21
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values
Returns aCollectionview of the values contained in this map. The encounter order of values in the view matches the encounter order of entries in this map. The collection is backed by the map, so changes to the map are reflected in the collection, and vice-versa. If the map is modified while an iteration over the collection is in progress (except through the iterator's ownremoveoperation), the results of the iteration are undefined. The collection supports element removal, which removes the corresponding mapping from the map, via theIterator.remove,Collection.remove,removeAll,retainAllandclearoperations. It does not support theaddoraddAlloperations. ItsSpliteratortypically provides faster sequential performance but much poorer parallel performance than that ofHashMap. -
sequencedValues
Returns aSequencedCollectionview of this map'svaluescollection.The returned view has the same characteristics as specified for the view returned by the
valuesmethod.- Specified by:
sequencedValuesin interfaceSequencedMap<K,V> - Returns:
- a
SequencedCollectionview of this map'svaluescollection - Since:
- 21
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entrySet
Returns aSetview of the mappings contained in this map. The encounter order of the view matches the encounter order of entries of this map. The set is backed by the map, so changes to the map are reflected in the set, and vice-versa. If the map is modified while an iteration over the set is in progress (except through the iterator's ownremoveoperation, or through thesetValueoperation on a map entry returned by the iterator) the results of the iteration are undefined. The set supports element removal, which removes the corresponding mapping from the map, via theIterator.remove,Set.remove,removeAll,retainAllandclearoperations. It does not support theaddoraddAlloperations. ItsSpliteratortypically provides faster sequential performance but much poorer parallel performance than that ofHashMap. -
sequencedEntrySet
Returns aSequencedSetview of this map'sentrySet.The returned view has the same characteristics as specified for the view returned by the
entrySetmethod.- Specified by:
sequencedEntrySetin interfaceSequencedMap<K,V> - Returns:
- a
SequencedSetview of this map'sentrySet - Since:
- 21
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newLinkedHashMap
Creates a new, empty, insertion-ordered LinkedHashMap suitable for the expected number of mappings. The returned map uses the default load factor of 0.75, and its initial capacity is generally large enough so that the expected number of mappings can be added without resizing the map.- Type Parameters:
K- the type of keys maintained by the new mapV- the type of mapped values- Parameters:
numMappings- the expected number of mappings- Returns:
- the newly created map
- Throws:
IllegalArgumentException- if numMappings is negative- Since:
- 19
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reversed
Returns a reverse-ordered view of this map. The encounter order of mappings in the returned view is the inverse of the encounter order of mappings in this map. The reverse ordering affects all order-sensitive operations, including those on the view collections of the returned view. If the implementation permits modifications to this view, the modifications "write through" to the underlying map. Changes to the underlying map might or might not be visible in this reversed view, depending upon the implementation.Modifications to the reversed view and its map views are permitted and will be propagated to this map. In addition, modifications to this map will be visible in the reversed view and its map views.
- Specified by:
reversedin interfaceSequencedMap<K,V> - Returns:
- a reverse-ordered view of this map
- Since:
- 21
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