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This implementation provides constant-time performance for the basic operations (get and put), assuming the hash function disperses the elements properly among the buckets. Iteration over collection views requires time proportional to the "capacity" of the HashMap instance (the number of buckets) plus its size (the number of key-value mappings). Thus, it's very important not to set the initial capacity too high (or the load factor too low) if iteration performance is important.
An instance of HashMap has two parameters that affect its performance: initial capacity and load factor. The capacity is the number of buckets in the hash table, and the initial capacity is simply the capacity at the time the hash table is created. The load factor is a measure of how full the hash table is allowed to get before its capacity is automatically increased. When the number of entries in the hash table exceeds the product of the load factor and the current capacity, the hash table is rehashed (that is, internal data structures are rebuilt) so that the hash table has approximately twice the number of buckets.
As a general rule, the default load factor (.75) offers a good tradeoff between time and space costs. Higher values decrease the space overhead but increase the lookup cost (reflected in most of the operations of the HashMap class, including get and put). The expected number of entries in the map and its load factor should be taken into account when setting its initial capacity, so as to minimize the number of rehash operations. If the initial capacity is greater than the maximum number of entries divided by the load factor, no rehash operations will ever occur.
If many mappings are to be stored in a HashMap instance, creating it with a sufficiently large capacity will allow the mappings to be stored more efficiently than letting it perform automatic rehashing as needed to grow the table. Note that using many keys with the same hashCode() is a sure way to slow down performance of any hash table. To ameliorate impact, when keys are Comparable, this class may use comparison order among keys to help break ties.
Note that this implementation is not synchronized. If multiple threads access a hash map concurrently, and at least one of the threads modifies the map structurally, it must be synchronized externally. (A structural modification is any operation that adds or deletes one or more mappings; merely changing the value associated with a key that an instance already contains is not a structural modification.) 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 HashMap(...));
The iterators 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.
This class is a member of the Java Collections Framework.
Nested Class Summary |
Nested classes/interfaces inherited from class java.util.AbstractMap |
Nested classes/interfaces inherited from interface java.util.Map |
Constructor Summary |
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HashMap()
Constructs an empty HashMap with the default initial capacity
(16) and the default load factor (0.75).
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HashMap(int initialCapacity)
Constructs an empty HashMap with the specified initial
capacity and the default load factor (0.75).
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HashMap(int initialCapacity, float loadFactor)
Constructs an empty HashMap with the specified initial
capacity and load factor.
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Constructs a new HashMap with the same mappings as the
specified Map.
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Method Summary |
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void |
clear()
Removes all of the mappings from this map.
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clone()
Returns a shallow copy of this HashMap instance: the keys and
values themselves are not cloned.
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Attempts to compute a mapping for the specified key and its current
mapped value (or null if there is no current mapping).
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If the specified key is not already associated with a value (or is mapped
to null), attempts to compute its value using the given mapping
function and enters it into this map unless null.
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If the value for the specified key is present and non-null, attempts to
compute a new mapping given the key and its current mapped value.
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boolean |
containsKey(Object key)
Returns true if this map contains a mapping for the
specified key.
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boolean |
containsValue(Object value)
Returns true if this map maps one or more keys to the
specified value.
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entrySet()
Returns a Set view of the mappings contained in this map.
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void |
Performs the given action for each entry in this map until all entries
have been processed or the action throws an exception.
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Returns the value to which the specified key is mapped,
or null if this map contains no mapping for the key.
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Returns the value to which the specified key is mapped, or
defaultValue if this map contains no mapping for the key.
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boolean |
isEmpty()
Returns true if this map contains no key-value mappings.
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keySet()
Returns a Set view of the keys contained in this map.
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If the specified key is not already associated with a value or is
associated with null, associates it with the given non-null value.
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Associates the specified value with the specified key in this map.
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void |
Copies all of the mappings from the specified map to this map.
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If the specified key is not already associated with a value (or is mapped
to null) associates it with the given value and returns
null, else returns the current value.
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Removes the mapping for the specified key from this map if present.
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boolean |
Removes the entry for the specified key only if it is currently
mapped to the specified value.
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Replaces the entry for the specified key only if it is
currently mapped to some value.
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boolean |
Replaces the entry for the specified key only if currently
mapped to the specified value.
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void |
Replaces each entry's value with the result of invoking the given
function on that entry until all entries have been processed or the
function throws an exception.
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int |
size()
Returns the number of key-value mappings in this map.
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values()
Returns a Collection view of the values contained in this map.
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Methods inherited from class java.util.AbstractMap |
Methods inherited from class java.lang.Object |
Methods inherited from interface java.util.Map |
public HashMap |
(int initialCapacity, float loadFactor) |
public HashMap |
(int initialCapacity) |
public HashMap |
() |
public HashMap |
public int size |
() |
public boolean isEmpty |
() |
public V get |
(Object key) |
More formally, if this map contains a mapping from a key k to a value v such that (key==null ? k==null : key.equals(k)), then this method returns v; otherwise it returns null. (There can be at most one such mapping.)
A return value of 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 null. The containsKey operation may be used to distinguish these two cases.
public boolean containsKey |
(Object key) |
public V put |
public void putAll |
public V remove |
(Object key) |
public void clear |
() |
public boolean containsValue |
(Object value) |
() |
() |
() |
public V getOrDefault |
public V putIfAbsent |
public boolean remove |
public boolean replace |
public V replace |
public V computeIfAbsent |
If the mapping function returns null, no mapping is recorded. If the mapping function itself throws an (unchecked) exception, the exception is rethrown, and no mapping is recorded. The most common usage is to construct a new object serving as an initial mapped value or memoized result, as in:
map.computeIfAbsent(key, k -> new Value(f(k)));
Or to implement a multi-value map, Map<K,Collection<V>>, supporting multiple values per key:
map.computeIfAbsent(key, k -> new HashSet<V>()).add(v);
The mapping function should not modify this map during computation.
This method will, on a best-effort basis, throw a ConcurrentModificationException if it is detected that the mapping function modifies this map during computation.
public V computeIfPresent |
If the remapping function returns null, the mapping is removed. If the remapping function itself throws an (unchecked) exception, the exception is rethrown, and the current mapping is left unchanged.
The remapping function should not modify this map during computation.
This method will, on a best-effort basis, throw a ConcurrentModificationException if it is detected that the remapping function modifies this map during computation.
public V compute |
map.compute(key, (k, v) -> (v == null) ? msg : v.concat(msg))
(Method merge() is often simpler to use for such purposes.)
If the remapping function returns null, the mapping is removed (or remains absent if initially absent). If the remapping function itself throws an (unchecked) exception, the exception is rethrown, and the current mapping is left unchanged.
The remapping function should not modify this map during computation.
This method will, on a best-effort basis, throw a ConcurrentModificationException if it is detected that the remapping function modifies this map during computation.
public V merge |
map.merge(key, msg, String::concat)
If the remapping function returns null, the mapping is removed. If the remapping function itself throws an (unchecked) exception, the exception is rethrown, and the current mapping is left unchanged.
The remapping function should not modify this map during computation.
This method will, on a best-effort basis, throw a ConcurrentModificationException if it is detected that the remapping function modifies this map during computation.
public void forEach |
public void replaceAll |
public Object clone |
() |
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FlexDoc/Javadoc 2.0 Demo Java Doc |