java.util.Hashtable Maven / Gradle / Ivy
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This work corresponds to the API signatures of JSR 219: Foundation
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package java.util;
import java.io.*;
/**
* This class implements a hashtable, 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.
* When the number of entries in the hashtable exceeds the product of the load
* factor and the current capacity, the capacity is increased by calling the
* rehash method.
*
* 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", new Integer(1));
* numbers.put("two", new Integer(2));
* numbers.put("three", new Integer(3));
*
*
* To retrieve a number, use the following code:
*
* Integer n = (Integer)numbers.get("two");
* if (n != null) {
* System.out.println("two = " + n);
* }
*
*
* As of the Java 2 platform v1.2, this class has been retrofitted to
* implement Map, so that it becomes a part of Java's collection framework.
* Unlike the new collection implementations, Hashtable is synchronized.
*
* The Iterators returned by the iterator and listIterator methods
* of the Collections returned by all of Hashtable's "collection view methods"
* are fail-fast: if the Hashtable is structurally modified
* at any time after the Iterator is created, in any way except through the
* Iterator's own remove or add methods, 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.
* The Enumerations returned by Hashtable's keys and values methods are
* not fail-fast.
*
*
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.
*
* @author Arthur van Hoff
* @author Josh Bloch
* @version 1.82, 02/02/00
* @see Object#equals(java.lang.Object)
* @see Object#hashCode()
* @see Hashtable#rehash()
* @see Collection
* @see Map
* @see HashMap
* @see TreeMap
* @since JDK1.0
*/
public class Hashtable extends Dictionary
implements Map, Cloneable, Serializable
{
/**
* The table is rehashed when its size exceeds this threshold. (The
* value of this field is (int)(capacity * loadFactor).)
*
* @serial
*/
private int threshold;
/**
* The load factor for the hashtable.
*
* @serial
*/
private float loadFactor;
/** use serialVersionUID from JDK 1.0.2 for interoperability */
private static final long serialVersionUID = 1421746759512286392L;
/**
* Constructs a new, empty hashtable with the specified initial
* capacity and the specified load factor.
*
* @param initialCapacity the initial capacity of the hashtable.
* @param loadFactor the load factor of the hashtable.
* @exception IllegalArgumentException if the initial capacity is less
* than zero, or if the load factor is nonpositive.
*/
public Hashtable(int initialCapacity, float loadFactor) { }
/**
* Constructs a new, empty hashtable with the specified initial capacity
* and default load factor, which is 0.75.
*
* @param initialCapacity the initial capacity of the hashtable.
* @exception IllegalArgumentException if the initial capacity is less
* than zero.
*/
public Hashtable(int initialCapacity) { }
/**
* Constructs a new, empty hashtable with a default initial capacity (11)
* and load factor, which is 0.75.
*/
public 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, which is
* 0.75.
*
* @param t the map whose mappings are to be placed in this map.
* @throws NullPointerException if the specified map is null.
* @since 1.2
*/
public Hashtable(Map t) { }
/**
* Returns the number of keys in this hashtable.
*
* @return the number of keys in this hashtable.
*/
public synchronized int size() {
return 0;
}
/**
* Tests if this hashtable maps no keys to values.
*
* @return true if this hashtable maps no keys to values;
* false otherwise.
*/
public synchronized boolean isEmpty() {
return false;
}
/**
* Returns an enumeration of the keys in this hashtable.
*
* @return an enumeration of the keys in this hashtable.
* @see Enumeration
* @see #elements()
* @see #keySet()
* @see Map
*/
public synchronized Enumeration keys() {
return null;
}
/**
* Returns an enumeration of the values in this hashtable.
* Use the Enumeration methods on the returned object to fetch the elements
* sequentially.
*
* @return an enumeration of the values in this hashtable.
* @see java.util.Enumeration
* @see #keys()
* @see #values()
* @see Map
*/
public synchronized Enumeration elements() {
return null;
}
/**
* 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).
*
* @param value a value to search for.
* @return true if and only if some key maps to the
* value argument in this hashtable as
* determined by the equals method;
* false otherwise.
* @exception NullPointerException if the value is null.
* @see #containsKey(Object)
* @see #containsValue(Object)
* @see Map
*/
public synchronized boolean contains(Object value) {
return false;
}
/**
* 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).
*
* @param value value whose presence in this Hashtable is to be tested.
* @return true if this map maps one or more keys to the
* specified value.
* @throws NullPointerException if the value is null.
* @see Map
* @since 1.2
*/
public boolean containsValue(Object value) {
return false;
}
/**
* Tests if the specified object is a key in this hashtable.
*
* @param key possible key.
* @return true if and only if the specified object
* is a key in this hashtable, as determined by the
* equals method; false otherwise.
* @throws NullPointerException if the key is null.
* @see #contains(Object)
*/
public synchronized boolean containsKey(Object key) {
return false;
}
/**
* Returns the value to which the specified key is mapped in this hashtable.
*
* @param key a key in the hashtable.
* @return the value to which the key is mapped in this hashtable;
* null if the key is not mapped to any value in
* this hashtable.
* @throws NullPointerException if the key is null.
* @see #put(Object, Object)
*/
public synchronized Object get(Object key) {
return null;
}
/**
* 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.
*/
protected void rehash() { }
/**
* 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.
*
* @param key the hashtable key.
* @param value the value.
* @return the previous value of the specified key in this hashtable,
* or null if it did not have one.
* @exception NullPointerException if the key or value is
* null.
* @see Object#equals(Object)
* @see #get(Object)
*/
public synchronized Object put(Object key, Object value) {
return null;
}
/**
* Removes the key (and its corresponding value) from this
* hashtable. This method does nothing if the key is not in the hashtable.
*
* @param key the key that needs to be removed.
* @return the value to which the key had been mapped in this hashtable,
* or null if the key did not have a mapping.
* @throws NullPointerException if the key is null.
*/
public synchronized Object remove(Object key) {
return null;
}
/**
* 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.
*
* @param t Mappings to be stored in this map.
* @throws NullPointerException if the specified map is null.
* @since 1.2
*/
public synchronized void putAll(Map t) { }
/**
* Clears this hashtable so that it contains no keys.
*/
public synchronized void clear() { }
/**
* 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.
*
* @return a clone of the hashtable.
*/
public synchronized Object clone() {
return null;
}
/**
* 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.
Overrides to
* toString method of Object.
*
* @return a string representation of this hashtable.
*/
public synchronized String toString() {
return null;
}
/**
* Returns a Set view of the keys contained in this Hashtable. The Set
* is backed by the Hashtable, so changes to the Hashtable are reflected
* in the Set, and vice-versa. The Set supports element removal
* (which removes the corresponding entry from the Hashtable), but not
* element addition.
*
* @return a set view of the keys contained in this map.
* @since 1.2
*/
public Set keySet() {
return null;
}
/**
* Returns a Set view of the entries contained in this Hashtable.
* Each element in this collection is a Map.Entry. The Set is
* backed by the Hashtable, so changes to the Hashtable are reflected in
* the Set, and vice-versa. The Set supports element removal
* (which removes the corresponding entry from the Hashtable),
* but not element addition.
*
* @return a set view of the mappings contained in this map.
* @see Map.Entry
* @since 1.2
*/
public Set entrySet() {
return null;
}
/**
* Returns a Collection view of the values contained in this Hashtable.
* The Collection is backed by the Hashtable, so changes to the Hashtable
* are reflected in the Collection, and vice-versa. The Collection
* supports element removal (which removes the corresponding entry from
* the Hashtable), but not element addition.
*
* @return a collection view of the values contained in this map.
* @since 1.2
*/
public Collection values() {
return null;
}
/**
* Compares the specified Object with this Map for equality,
* as per the definition in the Map interface.
*
* @param o object to be compared for equality with this Hashtable
* @return true if the specified Object is equal to this Map.
* @see Map#equals(Object)
* @since 1.2
*/
public synchronized boolean equals(Object o) {
return false;
}
/**
* Returns the hash code value for this Map as per the definition in the
* Map interface.
*
* @see Map#hashCode()
* @since 1.2
*/
public synchronized int hashCode() {
return 0;
}
/**
* Reconstitute the Hashtable from a stream (i.e., deserialize it).
*/
private void readObject(ObjectInputStream s)
throws IOException, ClassNotFoundException
{ }
/**
* Save the state of the Hashtable to a stream (i.e., serialize it).
*
* @serialData The capacity of the Hashtable (the length of the
* bucket array) is emitted (int), followed by the
* size of the Hashtable (the number of key-value
* mappings), followed by the key (Object) and value (Object)
* for each key-value mapping represented by the Hashtable
* The key-value mappings are emitted in no particular order.
*/
private synchronized void writeObject(ObjectOutputStream s)
throws IOException
{ }
static class Entry implements Map.Entry {
public Object getKey() {
return null;
}
public Object getValue() {
return null;
}
public Object setValue(Object newValue) {
return null;
}
public boolean equals(Object o) {
return false;
}
public int hashCode() {
return 0;
}
public String toString() {
return null;
}
}
}