java.util.HashMap Maven / Gradle / Ivy
/*
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stub files, must be accompanied by this notice in its entirety.
This work corresponds to the API signatures of JSR 219: Foundation
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*/
package java.util;
import java.io.*;
/**
* Hash table based implementation of the Map interface. This
* implementation provides all of the optional map operations, and permits
* null values and the null key. (The HashMap
* class is roughly equivalent to Hashtable, except that it is
* unsynchronized and permits nulls.) This class makes no guarantees as to
* the order of the map; in particular, it does not guarantee that the order
* will remain constant over time.
*
*
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 capacity is roughly doubled by calling the
* rehash method.
*
*
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 this implementation is not synchronized. If multiple
* threads access this 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 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.
*
*
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 Doug Lea
* @author Josh Bloch
* @author Arthur van Hoff
* @version 1.38, 02/02/00
* @see Object#hashCode()
* @see Collection
* @see Map
* @see TreeMap
* @see Hashtable
* @since 1.2
*/
public class HashMap extends AbstractMap implements Map, Cloneable, Serializable
{
/**
* The next size value at which to resize (capacity * load factor).
* @serial
*/
int threshold;
/**
* The load factor for the hash table.
*
* @serial
*/
final float loadFactor;
/**
* Constructs an empty HashMap with the specified initial
* capacity and load factor.
*
* @param initialCapacity The initial capacity.
* @param loadFactor The load factor.
* @throws IllegalArgumentException if the initial capacity is negative
* or the load factor is nonpositive.
*/
public HashMap(int initialCapacity, float loadFactor) {
this.loadFactor = loadFactor;
}
/**
* Constructs an empty HashMap with the specified initial
* capacity and the default load factor (0.75).
*
* @param initialCapacity the initial capacity.
* @throws IllegalArgumentException if the initial capacity is negative.
*/
public HashMap(int initialCapacity) {
loadFactor = 0.0f;
}
/**
* Constructs an empty HashMap with the default initial capacity
* (16) and the default load factor (0.75).
*/
public HashMap() {
loadFactor = 0.0f;
}
/**
* Constructs a new HashMap with the same mappings as the
* specified Map. The HashMap is created with
* default load factor (0.75) and an initial capacity sufficient to
* hold the mappings in the specified Map.
*
* @param m the map whose mappings are to be placed in this map.
* @throws NullPointerException if the specified map is null.
*/
public HashMap(Map m) {
loadFactor = 0.0f;
}
/**
* Returns the number of key-value mappings in this map.
*
* @return the number of key-value mappings in this map.
*/
public int size() {
return 0;
}
/**
* Returns true if this map contains no key-value mappings.
*
* @return true if this map contains no key-value mappings.
*/
public boolean isEmpty() {
return false;
}
/**
* Returns the value to which the specified key is mapped in this identity
* hash map, or null if the map contains no mapping for this key.
* A return value of null does not necessarily indicate
* that the map contains no mapping for the key; it is also possible that
* the map explicitly maps the key to null. The
* containsKey method may be used to distinguish these two cases.
*
* @param key the key whose associated value is to be returned.
* @return the value to which this map maps the specified key, or
* null if the map contains no mapping for this key.
* @see #put(Object, Object)
*/
public Object get(Object key) {
return null;
}
/**
* Returns true if this map contains a mapping for the
* specified key.
*
* @param key The key whose presence in this map is to be tested
* @return true if this map contains a mapping for the specified
* key.
*/
public boolean containsKey(Object key) {
return false;
}
/**
* Associates the specified value with the specified key in this map.
* If the map previously contained a mapping for this key, the old
* value is replaced.
*
* @param key key with which the specified value is to be associated.
* @param value value to be associated with the specified key.
* @return previous value associated with specified key, or null
* if there was no mapping for key. A null return can
* also indicate that the HashMap previously associated
* null with the specified key.
*/
public Object put(Object key, Object value) {
return null;
}
/**
* Copies all of the mappings from the specified map to this map
* These mappings will replace any mappings that
* this map had for any of the keys currently in the specified map.
*
* @param m mappings to be stored in this map.
* @throws NullPointerException if the specified map is null.
*/
public void putAll(Map m) { }
/**
* Removes the mapping for this key from this map if present.
*
* @param key key whose mapping is to be removed from the map.
* @return previous value associated with specified key, or null
* if there was no mapping for key. A null return can
* also indicate that the map previously associated null
* with the specified key.
*/
public Object remove(Object key) {
return null;
}
/**
* Removes all mappings from this map.
*/
public void clear() { }
/**
* Returns true if this map maps one or more keys to the
* specified value.
*
* @param value value whose presence in this map is to be tested.
* @return true if this map maps one or more keys to the
* specified value.
*/
public boolean containsValue(Object value) {
return false;
}
/**
* Returns a shallow copy of this HashMap instance: the keys and
* values themselves are not cloned.
*
* @return a shallow copy of this map.
*/
public Object clone() {
return null;
}
/**
* 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. The set supports element removal, which removes the
* corresponding mapping from this map, via the Iterator.remove,
* Set.remove, removeAll, retainAll, and
* clear operations. It does not support the add or
* addAll operations.
*
* @return a set view of the keys contained in this map.
*/
public Set keySet() {
return null;
}
/**
* 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. The collection supports element
* removal, which removes the corresponding mapping from this map, via the
* Iterator.remove, Collection.remove,
* removeAll, retainAll, and clear operations.
* It does not support the add or addAll operations.
*
* @return a collection view of the values contained in this map.
*/
public Collection values() {
return null;
}
/**
* Returns a collection view of the mappings contained in this map. Each
* element in the returned collection is a Map.Entry. The
* collection is backed by the map, so changes to the map are reflected in
* the collection, and vice-versa. 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.
*
* @return a collection view of the mappings contained in this map.
* @see Map.Entry
*/
public Set entrySet() {
return null;
}
/**
* Reconstitute the HashMap instance from a stream (i.e.,
* deserialize it).
*/
private void readObject(ObjectInputStream s)
throws IOException, ClassNotFoundException
{ }
/**
* Save the state of the HashMap instance to a stream (i.e.,
* serialize it).
*
* @serialData The capacity of the HashMap (the length of the
* bucket array) is emitted (int), followed by the
* size of the HashMap (the number of key-value
* mappings), followed by the key (Object) and value (Object)
* for each key-value mapping represented by the HashMap
* The key-value mappings are emitted in the order that they
* are returned by entrySet().iterator().
*
*/
private void writeObject(ObjectOutputStream s) throws IOException { }
private static final long serialVersionUID = 362498820763181265L;
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;
}
}
}