Nedenfor koden fra klassen java.util.HashMap, der er da lidt kommentarer hist og pist. Med mindre du står med en konkret skoleopgave, kan du bare bruge den.
Lidt forklaring: En map indeholder en liste af nøgler, og for hver nøgle er der gemt en værdi. Når du slår op i mappen, angiver du en nøgle, f.eks. en streng, og får værdien return. D.v.s. mappen skal 1. finde ud af om den indeholder en nøgle magen til 2. returnere værdien svarende til nøglen.
Men hvad nu hvis mappen indeholder en million nøgler? Så er den nødt til at checke alle nøgler, for at se, om en af dem skulle være magen til den angivne, og det tager lang tid.
Kunne man mon finde enhurtigere metode til at fremsøge nøglerne. Her er en ide: Vi laver et array, og hver plads kan indeholde en liste af nøgler. HashMappen har så indbygget en smart regel der gør, at hvis den skulle ske at have f.eks. nøglen "KrimsKrams" som en af sine 1mio nøgler, så skal denne befinde sig på plads 487122 i array'et. d.v.s den slår bare op på denne plads, finder en liste af nøgler (som f.eks. kan have to elementer), og undersøger om nøglen er et af dem. Det går MEGET hurtigere.
Eneste problem er at finde ud af, om der skal checkes på plads 487122 eller på plads 12765. Til dette formål udregnes en hashværdi, alle java-objekter har en hashCode()-metode, som returnere en int. Denne hashcode fortæller mere eller mindre hvilken plads i arrayet, nøglen skal placeres i. D.v.s når der puttes noget ind i mappen, udregnes nøglens hashcode, og nøglen placeres på den rigtige plads. Når der spørges efter værdien svarende til en given nøgle, udregnes igen hashcoden af nøglen, og det undersøges om mappen har en tilsvarende nøgle placeret på den tilsvarende plads i arrayet.
Det sidste lille problem er, at hashCode() jo kan returnere uendeligt mange værdier, mens et array jo kun har endeligt mange pladser. Dette løses ved, at hvis f.eks. arrayet har 100000 pladser, så beregnes en nøgles korrekte placering som den rest man får ved at dividere hashcoden med 100000 (det giver jo et tal mellem 0 og 100000).
/*
* @(#)HashMap.java 1.38 00/02/02
*
* Copyright 1997-2000 Sun Microsystems, Inc. All Rights Reserved.
*
* This software is the proprietary information of Sun Microsystems, Inc.
* Use is subject to license terms.
*
*/
package java.util;
import java.io.*;
/**
* Hash table based implementation of the <tt>Map</tt> interface. This
* implementation provides all of the optional map operations, and permits
* <tt>null</tt> values and the <tt>null</tt> key. (The <tt>HashMap</tt>
* class is roughly equivalent to <tt>Hashtable</tt>, 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.<p>
*
* This implementation provides constant-time performance for the basic
* operations (<tt>get</tt> and <tt>put</tt>), assuming the hash function
* disperses the elements properly among the buckets. Iteration over
* collection views requires time proportional to the "capacity" of the
* <tt>HashMap</tt> instance (the number of buckets) plus its size (the number
* of key-value mappings). Thus, it's very important not to set the intial
* capacity too high (or the load factor too low) if iteration performance is
* important.<p>
*
* An instance of <tt>HashMap</tt> has two parameters that affect its
* performance: <i>initial capacity</i> and <i>load factor</i>. The
* <i>capacity</i> 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
* <i>load factor</i> 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
* <tt>rehash</tt> method.<p>
*
* 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
* <tt>HashMap</tt> class, including <tt>get</tt> and <tt>put</tt>). 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 <tt>rehash</tt> operations. If the initial capacity is greater
* than the maximum number of entries divided by the load factor, no
* <tt>rehash</tt> operations will ever occur.<p>
*
* If many mappings are to be stored in a <tt>HashMap</tt> 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.<p>
*
* <b>Note that this implementation is not synchronized.</b> If multiple
* threads access this map concurrently, and at least one of the threads
* modifies the map structurally, it <i>must</i> 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 <tt>Collections.synchronizedMap</tt> method. This is
* best done at creation time, to prevent accidental unsynchronized access to
* the map: <pre> Map m = Collections.synchronizedMap(new HashMap(...));
* </pre><p>
*
* The iterators returned by all of this class's "collection view methods" are
* <i>fail-fast</i>: if the map is structurally modified at any time after the
* iterator is created, in any way except through the iterator's own
* <tt>remove</tt> or <tt>add</tt> methods, the iterator will throw a
* <tt>ConcurrentModificationException</tt>. 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.
*
* @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,
java.io.Serializable {
/**
* The hash table data.
*/
private transient Entry table[];
/**
* The total number of mappings in the hash table.
*/
private transient int count;
/**
* 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;
/**
* The number of times this HashMap has been structurally modified
* Structural modifications are those that change the number of mappings in
* the HashMap or otherwise modify its internal structure (e.g.,
* rehash). This field is used to make iterators on Collection-views of
* the HashMap fail-fast. (See ConcurrentModificationException).
*/
private transient int modCount = 0;
/**
* Constructs a new, empty map with the specified initial
* capacity and the specified load factor.
*
* @param initialCapacity the initial capacity of the HashMap.
* @param loadFactor the load factor of the HashMap
* @throws IllegalArgumentException if the initial capacity is less
* than zero, or if the load factor is nonpositive.
*/
public HashMap(int initialCapacity, float loadFactor) {
if (initialCapacity < 0)
throw new IllegalArgumentException("Illegal Initial Capacity: "+
initialCapacity);
if (loadFactor <= 0 || Float.isNaN(loadFactor))
throw new IllegalArgumentException("Illegal Load factor: "+
loadFactor);
if (initialCapacity==0)
initialCapacity = 1;
this.loadFactor = loadFactor;
table = new Entry[initialCapacity];
threshold = (int)(initialCapacity * loadFactor);
}
/**
* Constructs a new, empty map with the specified initial capacity
* and default load factor, which is <tt>0.75</tt>.
*
* @param initialCapacity the initial capacity of the HashMap.
* @throws IllegalArgumentException if the initial capacity is less
* than zero.
*/
public HashMap(int initialCapacity) {
this(initialCapacity, 0.75f);
}
/**
* Constructs a new, empty map with a default capacity and load
* factor, which is <tt>0.75</tt>.
*/
public HashMap() {
this(11, 0.75f);
}
/**
* Constructs a new map with the same mappings as the given map. The
* map is created with a capacity of twice the number of mappings in
* the given map or 11 (whichever is greater), and a default load factor,
* which is <tt>0.75</tt>.
*
* @param t the map whose mappings are to be placed in this map.
*/
public HashMap(Map t) {
this(Math.max(2*t.size(), 11), 0.75f);
putAll(t);
}
/**
* Returns the number of key-value mappings in this map.
*
* @return the number of key-value mappings in this map.
*/
public int size() {
return count;
}
/**
* Returns <tt>true</tt> if this map contains no key-value mappings.
*
* @return <tt>true</tt> if this map contains no key-value mappings.
*/
public boolean isEmpty() {
return count == 0;
}
/**
* Returns <tt>true</tt> 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 <tt>true</tt> if this map maps one or more keys to the
* specified value.
*/
public boolean containsValue(Object value) {
Entry tab[] = table;
if (value==null) {
for (int i = tab.length ; i-- > 0 ;)
for (Entry e = tab[i] ; e != null ; e = e.next)
if (e.value==null)
return true;
} else {
for (int i = tab.length ; i-- > 0 ;)
for (Entry e = tab[i] ; e != null ; e = e.next)
if (value.equals(e.value))
return true;
}
return false;
}
/**
* Returns <tt>true</tt> if this map contains a mapping for the specified
* key.
*
* @return <tt>true</tt> if this map contains a mapping for the specified
* key.
* @param key key whose presence in this Map is to be tested.
*/
public boolean containsKey(Object key) {
Entry tab[] = table;
if (key != null) {
int hash = key.hashCode();
int index = (hash & 0x7FFFFFFF) % tab.length;
for (Entry e = tab[index]; e != null; e = e.next)
if (e.hash==hash && key.equals(e.key))
return true;
} else {
for (Entry e = tab[0]; e != null; e = e.next)
if (e.key==null)
return true;
}
return false;
}
/**
* Returns the value to which this map maps the specified key. Returns
* <tt>null</tt> if the map contains no mapping for this key. A return
* value of <tt>null</tt> does not <i>necessarily</i> indicate that the
* map contains no mapping for the key; it's also possible that the map
* explicitly maps the key to <tt>null</tt>. The <tt>containsKey</tt>
* operation may be used to distinguish these two cases.
*
* @return the value to which this map maps the specified key.
* @param key key whose associated value is to be returned.
*/
public Object get(Object key) {
Entry tab[] = table;
if (key != null) {
int hash = key.hashCode();
int index = (hash & 0x7FFFFFFF) % tab.length;
for (Entry e = tab[index]; e != null; e = e.next)
if ((e.hash == hash) && key.equals(e.key))
return e.value;
} else {
for (Entry e = tab[0]; e != null; e = e.next)
if (e.key==null)
return e.value;
}
return null;
}
/**
* Rehashes the contents of this map into a new <tt>HashMap</tt> instance
* with a larger capacity. This method is called automatically when the
* number of keys in this map exceeds its capacity and load factor.
*/
private void rehash() {
int oldCapacity = table.length;
Entry oldMap[] = table;
int newCapacity = oldCapacity * 2 + 1;
Entry newMap[] = new Entry[newCapacity];
modCount++;
threshold = (int)(newCapacity * loadFactor);
table = newMap;
for (int i = oldCapacity ; i-- > 0 ;) {
for (Entry old = oldMap[i] ; old != null ; ) {
Entry e = old;
old = old.next;
int index = (e.hash & 0x7FFFFFFF) % newCapacity;
e.next = newMap[index];
newMap[index] = e;
}
}
}
/**
* 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 <tt>null</tt>
* if there was no mapping for key. A <tt>null</tt> return can
* also indicate that the HashMap previously associated
* <tt>null</tt> with the specified key.
*/
public Object put(Object key, Object value) {
// Makes sure the key is not already in the HashMap.
Entry tab[] = table;
int hash = 0;
int index = 0;
if (key != null) {
hash = key.hashCode();
index = (hash & 0x7FFFFFFF) % tab.length;
for (Entry e = tab[index] ; e != null ; e = e.next) {
if ((e.hash == hash) && key.equals(e.key)) {
Object old = e.value;
e.value = value;
return old;
}
}
} else {
for (Entry e = tab[0] ; e != null ; e = e.next) {
if (e.key == null) {
Object old = e.value;
e.value = value;
return old;
}
}
}
modCount++;
if (count >= threshold) {
// Rehash the table if the threshold is exceeded
rehash();
tab = table;
index = (hash & 0x7FFFFFFF) % tab.length;
}
// Creates the new entry.
Entry e = new Entry(hash, key, value, tab[index]);
tab[index] = e;
count++;
return null;
}
/**
* 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 <tt>null</tt>
* if there was no mapping for key. A <tt>null</tt> return can
* also indicate that the map previously associated <tt>null</tt>
* with the specified key.
*/
public Object remove(Object key) {
Entry tab[] = table;
if (key != null) {
int hash = key.hashCode();
int index = (hash & 0x7FFFFFFF) % tab.length;
for (Entry e = tab[index], prev = null; e != null;
prev = e, e = e.next) {
if ((e.hash == hash) && key.equals(e.key)) {
modCount++;
if (prev != null)
prev.next = e.next;
else
tab[index] = e.next;
count--;
Object oldValue = e.value;
e.value = null;
return oldValue;
}
}
} else {
for (Entry e = tab[0], prev = null; e != null;
prev = e, e = e.next) {
if (e.key == null) {
modCount++;
if (prev != null)
prev.next = e.next;
else
tab[0] = e.next;
count--;
Object oldValue = e.value;
e.value = null;
return oldValue;
}
}
}
return null;
}
/**
* Copies all of the mappings from the specified map to this one.
*
* These mappings replace any mappings that this map had for any of the
* keys currently in the specified Map.
*
* @param t Mappings to be stored in this map.
*/
public void putAll(Map t) {
Iterator i = t.entrySet().iterator();
while (i.hasNext()) {
Map.Entry e = (Map.Entry) i.next();
put(e.getKey(), e.getValue());
}
}
/**
* Removes all mappings from this map.
*/
public void clear() {
Entry tab[] = table;
modCount++;
for (int index = tab.length; --index >= 0; )
tab[index] = null;
count = 0;
}
/**
* Returns a shallow copy of this <tt>HashMap</tt> instance: the keys and
* values themselves are not cloned.
*
* @return a shallow copy of this map.
*/
public Object clone() {
try {
HashMap t = (HashMap)super.clone();
t.table = new Entry[table.length];
for (int i = table.length ; i-- > 0 ; ) {
t.table[i] = (table[i] != null)
? (Entry)table[i].clone() : null;
}
t.keySet = null;
t.entrySet = null;
t.values = null;
t.modCount = 0;
return t;
} catch (CloneNotSupportedException e) {
// this shouldn't happen, since we are Cloneable
throw new InternalError();
}
}
// Views
private transient Set keySet = null;
private transient Set entrySet = null;
private transient Collection values = 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 <tt>Iterator.remove</tt>,
* <tt>Set.remove</tt>, <tt>removeAll</tt>, <tt>retainAll</tt>, and
* <tt>clear</tt> operations. It does not support the <tt>add</tt> or
* <tt>addAll</tt> operations.
*
* @return a set view of the keys contained in this map.
*/
public Set keySet() {
if (keySet == null) {
keySet = new AbstractSet() {
public Iterator iterator() {
return getHashIterator(KEYS);
}
public int size() {
return count;
}
public boolean contains(Object o) {
return containsKey(o);
}
public boolean remove(Object o) {
int oldSize = count;
HashMap.this.remove(o);
return count != oldSize;
}
public void clear() {
HashMap.this.clear();
}
};
}
return keySet;
}
/**
* 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
* <tt>Iterator.remove</tt>, <tt>Collection.remove</tt>,
* <tt>removeAll</tt>, <tt>retainAll</tt>, and <tt>clear</tt> operations.
* It does not support the <tt>add</tt> or <tt>addAll</tt> operations.
*
* @return a collection view of the values contained in this map.
*/
public Collection values() {
if (values==null) {
values = new AbstractCollection() {
public Iterator iterator() {
return getHashIterator(VALUES);
}
public int size() {
return count;
}
public boolean contains(Object o) {
return containsValue(o);
}
public void clear() {
HashMap.this.clear();
}
};
}
return values;
}
/**
* Returns a collection view of the mappings contained in this map. Each
* element in the returned collection is a <tt>Map.Entry</tt>. 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
* <tt>Iterator.remove</tt>, <tt>Collection.remove</tt>,
* <tt>removeAll</tt>, <tt>retainAll</tt>, and <tt>clear</tt> operations.
* It does not support the <tt>add</tt> or <tt>addAll</tt> operations.
*
* @return a collection view of the mappings contained in this map.
* @see Map.Entry
*/
public Set entrySet() {
if (entrySet==null) {
entrySet = new AbstractSet() {
public Iterator iterator() {
return getHashIterator(ENTRIES);
}
public boolean contains(Object o) {
if (!(o instanceof Map.Entry))
return false;
Map.Entry entry = (Map.Entry)o;
Object key = entry.getKey();
Entry tab[] = table;
int hash = (key==null ? 0 : key.hashCode());
int index = (hash & 0x7FFFFFFF) % tab.length;
for (Entry e = tab[index]; e != null; e = e.next)
if (e.hash==hash && e.equals(entry))
return true;
return false;
}
public boolean remove(Object o) {
if (!(o instanceof Map.Entry))
return false;
Map.Entry entry = (Map.Entry)o;
Object key = entry.getKey();
Entry tab[] = table;
int hash = (key==null ? 0 : key.hashCode());
int index = (hash & 0x7FFFFFFF) % tab.length;
for (Entry e = tab[index], prev = null; e != null;
prev = e, e = e.next) {
if (e.hash==hash && e.equals(entry)) {
modCount++;
if (prev != null)
prev.next = e.next;
else
tab[index] = e.next;
count--;
e.value = null;
return true;
}
}
return false;
}
public int size() {
return count;
}
public void clear() {
HashMap.this.clear();
}
};
}
return entrySet;
}
private Iterator getHashIterator(int type) {
if (count == 0) {
return emptyHashIterator;
} else {
return new HashIterator(type);
}
}
/**
* HashMap collision list entry.
*/
private static class Entry implements Map.Entry {
int hash;
Object key;
Object value;
Entry next;
Entry(int hash, Object key, Object value, Entry next) {
this.hash = hash;
this.key = key;
this.value = value;
this.next = next;
}
protected Object clone() {
return new Entry(hash, key, value,
(next==null ? null : (Entry)next.clone()));
}
// Map.Entry Ops
public Object getKey() {
return key;
}
public Object getValue() {
return value;
}
public Object setValue(Object value) {
Object oldValue = this.value;
this.value = value;
return oldValue;
}
public boolean equals(Object o) {
if (!(o instanceof Map.Entry))
return false;
Map.Entry e = (Map.Entry)o;
return (key==null ? e.getKey()==null : key.equals(e.getKey())) &&
(value==null ? e.getValue()==null : value.equals(e.getValue()));
}
public int hashCode() {
return hash ^ (value==null ? 0 : value.hashCode());
}
public String toString() {
return key+"="+value;
}
}
// Types of Iterators
private static final int KEYS = 0;
private static final int VALUES = 1;
private static final int ENTRIES = 2;
private static EmptyHashIterator emptyHashIterator
= new EmptyHashIterator();
private static class EmptyHashIterator implements Iterator {
EmptyHashIterator() {
}
public boolean hasNext() {
return false;
}
public Object next() {
throw new NoSuchElementException();
}
public void remove() {
throw new IllegalStateException();
}
}
private class HashIterator implements Iterator {
Entry[] table = HashMap.this.table;
int index = table.length;
Entry entry = null;
Entry lastReturned = null;
int type;
/**
* The modCount value that the iterator believes that the backing
* List should have. If this expectation is violated, the iterator
* has detected concurrent modification.
*/
private int expectedModCount = modCount;
HashIterator(int type) {
this.type = type;
}
public boolean hasNext() {
Entry e = entry;
int i = index;
Entry t[] = table;
/* Use locals for faster loop iteration */
while (e == null && i > 0)
e = t[--i];
entry = e;
index = i;
return e != null;
}
public Object next() {
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
Entry et = entry;
int i = index;
Entry t[] = table;
/* Use locals for faster loop iteration */
while (et == null && i > 0)
et = t[--i];
entry = et;
index = i;
if (et != null) {
Entry e = lastReturned = entry;
entry = e.next;
return type == KEYS ? e.key : (type == VALUES ? e.value : e);
}
throw new NoSuchElementException();
}
public void remove() {
if (lastReturned == null)
throw new IllegalStateException();
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
Entry[] tab = HashMap.this.table;
int index = (lastReturned.hash & 0x7FFFFFFF) % tab.length;
for (Entry e = tab[index], prev = null; e != null;
prev = e, e = e.next) {
if (e == lastReturned) {
modCount++;
expectedModCount++;
if (prev == null)
tab[index] = e.next;
else
prev.next = e.next;
count--;
lastReturned = null;
return;
}
}
throw new ConcurrentModificationException();
}
}
/**
* Save the state of the <tt>HashMap</tt> instance to a stream (i.e.,
* serialize it).
*
* @serialData The <i>capacity</i> of the HashMap (the length of the
* bucket array) is emitted (int), followed by the
* <i>size</i> 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 no particular order.
*/
private void writeObject(java.io.ObjectOutputStream s)
throws IOException
{
// Write out the threshold, loadfactor, and any hidden stuff
s.defaultWriteObject();
// Write out number of buckets
s.writeInt(table.length);
// Write out size (number of Mappings)
s.writeInt(count);
// Write out keys and values (alternating)
for (int index = table.length-1; index >= 0; index--) {
Entry entry = table[index];
while (entry != null) {
s.writeObject(entry.key);
s.writeObject(entry.value);
entry = entry.next;
}
}
}
private static final long serialVersionUID = 362498820763181265L;
/**
* Reconstitute the <tt>HashMap</tt> instance from a stream (i.e.,
* deserialize it).
*/
private void readObject(java.io.ObjectInputStream s)
throws IOException, ClassNotFoundException
{
// Read in the threshold, loadfactor, and any hidden stuff
s.defaultReadObject();
// Read in number of buckets and allocate the bucket array;
int numBuckets = s.readInt();
table = new Entry[numBuckets];
// Read in size (number of Mappings)
int size = s.readInt();
// Read the keys and values, and put the mappings in the HashMap
for (int i=0; i<size; i++) {
Object key = s.readObject();
Object value = s.readObject();
put(key, value);
}
}
int capacity() {
return table.length;
}
float loadFactor() {
return loadFactor;
}
}
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