LinkedHashMappublic class LinkedHashMap extends HashMap implements MapHash 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 {@link HashMap} (and {@link Hashtable}),
without incurring the increased cost associated with {@link 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 {@link #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 or get method
results in an access to the corresponding entry (assuming it exists after
the invocation completes). 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 {@link #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.
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
{@link 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. 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 {@link
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.
This class is a member of the
Java Collections Framework. |
| Fields Summary |
|---|
| private static final long | serialVersionUID | | private transient Entry | headerThe head of the doubly linked list. | | private final boolean | accessOrderThe iteration ordering method for this linked hash map: true
for access-order, false for insertion-order. |
| Constructors Summary |
|---|
public LinkedHashMap(int initialCapacity, float loadFactor)Constructs an empty insertion-ordered LinkedHashMap instance
with the specified initial capacity and load factor.
super(initialCapacity, loadFactor);
accessOrder = false;
| public LinkedHashMap(int initialCapacity)Constructs an empty insertion-ordered LinkedHashMap instance
with the specified initial capacity and a default load factor (0.75).
super(initialCapacity);
accessOrder = false;
| public LinkedHashMap()Constructs an empty insertion-ordered LinkedHashMap instance
with the default initial capacity (16) and load factor (0.75).
super();
accessOrder = false;
| public LinkedHashMap(Map m)Constructs an insertion-ordered LinkedHashMap instance with
the same mappings as the specified map. The LinkedHashMap
instance is created with a default load factor (0.75) and an initial
capacity sufficient to hold the mappings in the specified map.
super(m);
accessOrder = false;
| public LinkedHashMap(int initialCapacity, float loadFactor, boolean accessOrder)Constructs an empty LinkedHashMap instance with the
specified initial capacity, load factor and ordering mode.
super(initialCapacity, loadFactor);
this.accessOrder = accessOrder;
|
| Methods Summary |
|---|
| void | addEntry(int hash, K key, V value, int bucketIndex)This override alters behavior of superclass put method. It causes newly
allocated entry to get inserted at the end of the linked list and
removes the eldest entry if appropriate.
createEntry(hash, key, value, bucketIndex);
// Remove eldest entry if instructed, else grow capacity if appropriate
Entry<K,V> eldest = header.after;
if (removeEldestEntry(eldest)) {
removeEntryForKey(eldest.key);
} else {
if (size >= threshold)
resize(2 * table.length);
}
| | public void | clear()Removes all of the mappings from this map.
The map will be empty after this call returns.
super.clear();
header.before = header.after = header;
| | public boolean | containsValue(java.lang.Object value)Returns true if this map maps one or more keys to the
specified value.
// Overridden to take advantage of faster iterator
if (value==null) {
for (Entry e = header.after; e != header; e = e.after)
if (e.value==null)
return true;
} else {
for (Entry e = header.after; e != header; e = e.after)
if (value.equals(e.value))
return true;
}
return false;
| | void | createEntry(int hash, K key, V value, int bucketIndex)This override differs from addEntry in that it doesn't resize the
table or remove the eldest entry.
HashMap.Entry<K,V> old = table[bucketIndex];
Entry<K,V> e = new Entry<K,V>(hash, key, value, old);
table[bucketIndex] = e;
e.addBefore(header);
size++;
| | public V | get(java.lang.Object key)Returns the value to which the specified key is mapped,
or {@code null} if this map contains no mapping for the key.
More formally, if this map contains a mapping from a key
{@code k} to a value {@code v} such that {@code (key==null ? k==null :
key.equals(k))}, then this method returns {@code v}; otherwise
it returns {@code null}. (There can be at most one such mapping.)
A return value of {@code null} does not necessarily
indicate that the map contains no mapping for the key; it's also
possible that the map explicitly maps the key to {@code null}.
The {@link #containsKey containsKey} operation may be used to
distinguish these two cases.
Entry<K,V> e = (Entry<K,V>)getEntry(key);
if (e == null)
return null;
e.recordAccess(this);
return e.value;
| | void | init()Called by superclass constructors and pseudoconstructors (clone,
readObject) before any entries are inserted into the map. Initializes
the chain.
header = new Entry<K,V>(-1, null, null, null);
header.before = header.after = header;
| | java.util.Iterator | newEntryIterator() return new EntryIterator();
| | java.util.Iterator | newKeyIterator() return new KeyIterator();
| | java.util.Iterator | newValueIterator() return new ValueIterator();
| | protected boolean | removeEldestEntry(java.util.Map$Entry eldest)Returns true if this map should remove its eldest entry.
This method is invoked by put and putAll after
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 returning true
after 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).
return false;
| | void | transfer(HashMap.Entry[] newTable)Transfers all entries to new table array. This method is called
by superclass resize. It is overridden for performance, as it is
faster to iterate using our linked list.
int newCapacity = newTable.length;
for (Entry<K,V> e = header.after; e != header; e = e.after) {
int index = indexFor(e.hash, newCapacity);
e.next = newTable[index];
newTable[index] = e;
}
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