LinkedHashMap Class
Definition
Important
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Hash table and linked list implementation of the Map interface,
with predictable iteration order.
[Android.Runtime.Register("java/util/LinkedHashMap", DoNotGenerateAcw=true)]
[Java.Interop.JavaTypeParameters(new System.String[] { "K", "V" })]
public class LinkedHashMap : Java.Util.HashMap, IDisposable, Java.Interop.IJavaPeerable[<Android.Runtime.Register("java/util/LinkedHashMap", DoNotGenerateAcw=true)>]
[<Java.Interop.JavaTypeParameters(new System.String[] { "K", "V" })>]
type LinkedHashMap = class
inherit HashMap
interface IMap
interface IJavaObject
interface IDisposable
interface IJavaPeerable- Inheritance
- Attributes
- Implements
Remarks
Hash table and linked list implementation of the Map interface, with predictable iteration 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 iteration ordering, which is normally the order in which keys were inserted into the map (insertion-order). Note that insertion order is not affected if a key is re-inserted into the map. (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.)
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 m) {
Map 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 #LinkedHashMap(int,float,boolean) constructor is provided to create a linked hash map whose order of iteration 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. In particular, operations on collection-views do not affect the order of iteration 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.
This class provides all of the optional Map operations, and 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.
<strong>Note that this implementation is not synchronized.</strong> If multiple threads access a linked hash map concurrently, and at least one of the threads modifies the map structurally, it <em>must</em> 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 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. <strong>In access-ordered linked hash maps, merely querying the map with get is a structural modification. </strong>)
The iterators returned by the iterator method of the collections returned by all of this class's collection view methods are <em>fail-fast</em>: 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 <em>late-binding</em>, <em>fail-fast</em>, and additionally report Spliterator#ORDERED. <em>Note</em>: The implementation of these spliterators in Android Nougat (API levels 24 and 25) uses the wrong order (inconsistent with the iterators, which use the correct order), despite reporting Spliterator#ORDERED. You may use the following code fragments to obtain a correctly ordered Spliterator on API level 24 and 25: <ul> <li>For a Collection view c = lhm.keySet(), c = lhm.entrySet() or c = lhm.values(), use java.util.Spliterators.spliterator(c, c.spliterator().characteristics()) instead of c.spliterator(). <li>Instead of c.stream() or c.parallelStream(), use java.util.stream.StreamSupport.stream(spliterator, false) to construct a (nonparallel) java.util.stream.Stream from such a Spliterator. </ul> Note that these workarounds are only suggested where lhm is a LinkedHashMap.
This class is a member of the Java Collections Framework.
Added in 1.4.
Java documentation for java.util.LinkedHashMap.
Portions of this page are modifications based on work created and shared by the Android Open Source Project and used according to terms described in the Creative Commons 2.5 Attribution License.
Constructors
| LinkedHashMap() |
Constructs an empty insertion-ordered |
| LinkedHashMap(IDictionary) |
Constructs an insertion-ordered |
| LinkedHashMap(Int32) |
Constructs an empty insertion-ordered |
| LinkedHashMap(Int32, Single) |
Constructs an empty insertion-ordered |
| LinkedHashMap(Int32, Single, Boolean) |
Constructs an empty |
| LinkedHashMap(IntPtr, JniHandleOwnership) |
A constructor used when creating managed representations of JNI objects; called by the runtime. |
Properties
| Class |
Returns the runtime class of this |
| Handle |
The handle to the underlying Android instance. (Inherited from Object) |
| IsEmpty |
To be added (Inherited from AbstractMap) |
| JniIdentityHashCode | (Inherited from Object) |
| JniPeerMembers | |
| PeerReference | (Inherited from Object) |
| ThresholdClass |
This API supports the Mono for Android infrastructure and is not intended to be used directly from your code. |
| ThresholdType |
This API supports the Mono for Android infrastructure and is not intended to be used directly from your code. |
Methods
| Clear() |
To be added (Inherited from AbstractMap) |
| Clone() |
Returns a shallow copy of this |
| Compute(Object, IBiFunction) | (Inherited from HashMap) |
| ComputeIfAbsent(Object, IFunction) | (Inherited from HashMap) |
| ComputeIfPresent(Object, IBiFunction) | (Inherited from HashMap) |
| ContainsKey(Object) |
To be added (Inherited from AbstractMap) |
| ContainsValue(Object) |
To be added (Inherited from AbstractMap) |
| Dispose() | (Inherited from Object) |
| Dispose(Boolean) | (Inherited from Object) |
| EntrySet() |
Returns a |
| Equals(Object) |
Indicates whether some other object is "equal to" this one. (Inherited from Object) |
| ForEach(IBiConsumer) | (Inherited from HashMap) |
| Get(Object) |
To be added (Inherited from AbstractMap) |
| GetHashCode() |
Returns a hash code value for the object. (Inherited from Object) |
| GetOrDefault(Object, Object) | (Inherited from HashMap) |
| JavaFinalize() |
Called by the garbage collector on an object when garbage collection determines that there are no more references to the object. (Inherited from Object) |
| KeySet() |
To be added (Inherited from AbstractMap) |
| Merge(Object, Object, IBiFunction) | (Inherited from HashMap) |
| Notify() |
Wakes up a single thread that is waiting on this object's monitor. (Inherited from Object) |
| NotifyAll() |
Wakes up all threads that are waiting on this object's monitor. (Inherited from Object) |
| Put(Object, Object) |
To be added (Inherited from AbstractMap) |
| PutAll(IDictionary) |
To be added (Inherited from AbstractMap) |
| PutIfAbsent(Object, Object) | (Inherited from HashMap) |
| Remove(Object) |
To be added (Inherited from AbstractMap) |
| Remove(Object, Object) |
Removes the mapping for the specified key from this map if present. (Inherited from HashMap) |
| RemoveEldestEntry(IMapEntry) |
Returns |
| Replace(Object, Object) | (Inherited from HashMap) |
| Replace(Object, Object, Object) | (Inherited from HashMap) |
| ReplaceAll(IBiFunction) | (Inherited from HashMap) |
| SetHandle(IntPtr, JniHandleOwnership) |
Sets the Handle property. (Inherited from Object) |
| Size() |
To be added (Inherited from AbstractMap) |
| ToArray<T>() | (Inherited from Object) |
| ToString() |
Returns a string representation of the object. (Inherited from Object) |
| UnregisterFromRuntime() | (Inherited from Object) |
| Values() |
To be added (Inherited from AbstractMap) |
| Wait() |
Causes the current thread to wait until it is awakened, typically by being <em>notified</em> or <em>interrupted</em>. (Inherited from Object) |
| Wait(Int64) |
Causes the current thread to wait until it is awakened, typically by being <em>notified</em> or <em>interrupted</em>, or until a certain amount of real time has elapsed. (Inherited from Object) |
| Wait(Int64, Int32) |
Causes the current thread to wait until it is awakened, typically by being <em>notified</em> or <em>interrupted</em>, or until a certain amount of real time has elapsed. (Inherited from Object) |
Explicit Interface Implementations
| IJavaPeerable.Disposed() | (Inherited from Object) |
| IJavaPeerable.DisposeUnlessReferenced() | (Inherited from Object) |
| IJavaPeerable.Finalized() | (Inherited from Object) |
| IJavaPeerable.JniManagedPeerState | (Inherited from Object) |
| IJavaPeerable.SetJniIdentityHashCode(Int32) | (Inherited from Object) |
| IJavaPeerable.SetJniManagedPeerState(JniManagedPeerStates) | (Inherited from Object) |
| IJavaPeerable.SetPeerReference(JniObjectReference) | (Inherited from Object) |
Extension Methods
| JavaCast<TResult>(IJavaObject) |
Performs an Android runtime-checked type conversion. |
| JavaCast<TResult>(IJavaObject) | |
| GetJniTypeName(IJavaPeerable) | |