Хеш таблица java примеры
This class implements a hash table, which maps keys to values. Any non- null object can be used as a key or as a value. To successfully store and retrieve objects from a hashtable, the objects used as keys must implement the hashCode method and the equals method. An instance of Hashtable 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. Note that the hash table is open: in the case of a «hash collision», a single bucket stores multiple entries, which must be searched sequentially. The load factor is a measure of how full the hash table is allowed to get before its capacity is automatically increased. The initial capacity and load factor parameters are merely hints to the implementation. The exact details as to when and whether the rehash method is invoked are implementation-dependent. Generally, the default load factor (.75) offers a good tradeoff between time and space costs. Higher values decrease the space overhead but increase the time cost to look up an entry (which is reflected in most Hashtable operations, including get and put). The initial capacity controls a tradeoff between wasted space and the need for rehash operations, which are time-consuming. No rehash operations will ever occur if the initial capacity is greater than the maximum number of entries the Hashtable will contain divided by its load factor. However, setting the initial capacity too high can waste space. If many entries are to be made into a Hashtable , creating it with a sufficiently large capacity may allow the entries to be inserted more efficiently than letting it perform automatic rehashing as needed to grow the table. This example creates a hashtable of numbers. It uses the names of the numbers as keys:
Hashtable numbers = new Hashtable(); numbers.put("one", 1); numbers.put("two", 2); numbers.put("three", 3); Hashtable
public Hashtable(int initialCapacity)
Constructs a new, empty hashtable with the specified initial capacity and default load factor (0.75).
Hashtable
Hashtable
Constructs a new hashtable with the same mappings as the given Map. The hashtable is created with an initial capacity sufficient to hold the mappings in the given Map and a default load factor (0.75).
Method Detail
size
isEmpty
keys
elements
Returns an enumeration of the values in this hashtable. Use the Enumeration methods on the returned object to fetch the elements sequentially.
contains
Tests if some key maps into the specified value in this hashtable. This operation is more expensive than the containsKey method. Note that this method is identical in functionality to containsValue , (which is part of the Map interface in the collections framework).
containsValue
Returns true if this hashtable maps one or more keys to this value. Note that this method is identical in functionality to contains (which predates the Map interface).
containsKey
get
Returns the value to which the specified key is mapped, or null if this map contains no mapping for the key. More formally, if this map contains a mapping from a key k to a value v such that (key.equals(k)) , then this method returns v ; otherwise it returns null . (There can be at most one such mapping.)
rehash
Increases the capacity of and internally reorganizes this hashtable, in order to accommodate and access its entries more efficiently. This method is called automatically when the number of keys in the hashtable exceeds this hashtable’s capacity and load factor.
put
Maps the specified key to the specified value in this hashtable. Neither the key nor the value can be null . The value can be retrieved by calling the get method with a key that is equal to the original key.
remove
Removes the key (and its corresponding value) from this hashtable. This method does nothing if the key is not in the hashtable.
putAll
Copies all of the mappings from the specified map to this hashtable. These mappings will replace any mappings that this hashtable had for any of the keys currently in the specified map.
clear
clone
Creates a shallow copy of this hashtable. All the structure of the hashtable itself is copied, but the keys and values are not cloned. This is a relatively expensive operation.
toString
Returns a string representation of this Hashtable object in the form of a set of entries, enclosed in braces and separated by the ASCII characters «, » (comma and space). Each entry is rendered as the key, an equals sign =, and the associated element, where the toString method is used to convert the key and element to strings.
keySet
Returns a Set view of the keys contained in 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 own remove operation), the results of the iteration are undefined. The set supports element removal, which removes the corresponding mapping from the map, via the Iterator.remove, Set.remove, removeAll, retainAll, and clear operations. It does not support the add or addAll operations.
entrySet
Returns a Set view of the mappings contained in 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 own remove operation, or through the setValue operation 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 the Iterator.remove, Set.remove, removeAll, retainAll and clear operations. It does not support the add or addAll operations.
values
Returns a Collection view of the values contained 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 own remove operation), the results of the iteration are undefined. The collection supports element removal, which removes the corresponding mapping from the map, via the Iterator.remove, Collection.remove, removeAll, retainAll and clear operations. It does not support the add or addAll operations.
equals
Compares the specified Object with this Map for equality, as per the definition in the Map interface.
hashCode
getOrDefault
public V getOrDefault(Object key, V defaultValue)
Returns the value to which the specified key is mapped, or defaultValue if this map contains no mapping for the key.
forEach
Performs the given action for each entry in this map until all entries have been processed or the action throws an exception. Unless otherwise specified by the implementing class, actions are performed in the order of entry set iteration (if an iteration order is specified.) Exceptions thrown by the action are relayed to the caller.
replaceAll
public void replaceAll(BiFunctionK,? super V,? extends V> function)
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. Exceptions thrown by the function are relayed to the caller.
putIfAbsent
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.
remove
replace
public boolean replace(K key, V oldValue, V newValue)
replace
computeIfAbsent
public V computeIfAbsent(K key, Function mappingFunction)
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 . If the function returns null no mapping is recorded. If the 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))); map.computeIfAbsent(key, k -> new HashSet()).add(v); computeIfPresent
public V computeIfPresent(K key, BiFunction remappingFunction)
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. If the function returns null , the mapping is removed. If the function itself throws an (unchecked) exception, the exception is rethrown, and the current mapping is left unchanged.
compute
public V compute(K key, BiFunction remappingFunction)
Attempts to compute a mapping for the specified key and its current mapped value (or null if there is no current mapping). For example, to either create or append a String msg to a value mapping:
map.compute(key, (k, v) -> (v == null) ? msg : v.concat(msg)) (Method merge() is often simpler to use for such purposes.) If the function returns null , the mapping is removed (or remains absent if initially absent). If the function itself throws an (unchecked) exception, the exception is rethrown, and the current mapping is left unchanged.
merge
public V merge(K key, V value, BiFunction remappingFunction)
If the specified key is not already associated with a value or is associated with null, associates it with the given non-null value. Otherwise, replaces the associated value with the results of the given remapping function, or removes if the result is null . This method may be of use when combining multiple mapped values for a key. For example, to either create or append a String msg to a value mapping:
map.merge(key, msg, String::concat) If the function returns null the mapping is removed. If the function itself throws an (unchecked) exception, the exception is rethrown, and the current mapping is left unchanged.
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