java.util.IdentityHashMap Maven / Gradle / Ivy
/*
* Licensed to the Apache Software Foundation (ASF) under one or more
* contributor license agreements. See the NOTICE file distributed with
* this work for additional information regarding copyright ownership.
* The ASF licenses this file to You under the Apache License, Version 2.0
* (the "License"); you may not use this file except in compliance with
* the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package java.util;
/**
* IdentityHashMap is a variant on HashMap which tests equality by reference
* instead of equality by value. Basically, keys and values are compared for
* equality by checking if their references are equal rather than by calling the
* "equals" function.
*
* Note: This class intentionally violates the general contract of {@code
* Map}'s on comparing objects by their {@code equals} method.
*
* IdentityHashMap uses open addressing (linear probing in particular) for
* collision resolution. This is different from HashMap which uses Chaining.
*
* Like HashMap, IdentityHashMap is not thread safe, so access by multiple
* threads must be synchronized by an external mechanism such as
* Collections.synchronizedMap.
*
* @since 1.4
*/
public class IdentityHashMap extends AbstractMap implements
Map {
/*
* The internal data structure to hold key value pairs This array holds keys
* and values in an alternating fashion.
*/
transient Object[] elementData;
/* Actual number of key-value pairs. */
int size;
/*
* maximum number of elements that can be put in this map before having to
* rehash.
*/
transient int threshold;
/*
* default threshold value that an IdentityHashMap created using the default
* constructor would have.
*/
private static final int DEFAULT_MAX_SIZE = 21;
/* Default load factor of 0.75; */
private static final int loadFactor = 7500;
/*
* modification count, to keep track of structural modifications between the
* IdentityHashMap and the iterator
*/
transient int modCount = 0;
/*
* Object used to represent null keys and values. This is used to
* differentiate a literal 'null' key value pair from an empty spot in the
* map.
*/
private static final Object NULL_OBJECT = new Object(); //$NON-LOCK-1$
static class IdentityHashMapEntry extends MapEntry {
IdentityHashMapEntry(K theKey, V theValue) {
super(theKey, theValue);
}
@Override
public boolean equals(Object object) {
if (this == object) {
return true;
}
if (object instanceof Map.Entry) {
Map.Entry entry = (Map.Entry) object;
return (key == entry.getKey()) && (value == entry.getValue());
}
return false;
}
@Override
public int hashCode() {
return System.identityHashCode(key)
^ System.identityHashCode(value);
}
@Override
public String toString() {
return key + "=" + value; //$NON-NLS-1$
}
}
static class IdentityHashMapIterator implements Iterator {
private int position = 0; // the current position
// the position of the entry that was last returned from next()
private int lastPosition = 0;
final IdentityHashMap associatedMap;
int expectedModCount;
final MapEntry.Type type;
boolean canRemove = false;
IdentityHashMapIterator(MapEntry.Type value,
IdentityHashMap hm) {
associatedMap = hm;
type = value;
expectedModCount = hm.modCount;
}
public boolean hasNext() {
while (position < associatedMap.elementData.length) {
// if this is an empty spot, go to the next one
if (associatedMap.elementData[position] == null) {
position += 2;
} else {
return true;
}
}
return false;
}
void checkConcurrentMod() throws ConcurrentModificationException {
if (expectedModCount != associatedMap.modCount) {
throw new ConcurrentModificationException();
}
}
public E next() {
checkConcurrentMod();
if (!hasNext()) {
throw new NoSuchElementException();
}
IdentityHashMapEntry result = associatedMap
.getEntry(position);
lastPosition = position;
position += 2;
canRemove = true;
return type.get(result);
}
public void remove() {
checkConcurrentMod();
if (!canRemove) {
throw new IllegalStateException();
}
canRemove = false;
associatedMap.remove(associatedMap.elementData[lastPosition]);
position = lastPosition;
expectedModCount++;
}
}
static class IdentityHashMapEntrySet extends
AbstractSet> {
private final IdentityHashMap associatedMap;
public IdentityHashMapEntrySet(IdentityHashMap hm) {
associatedMap = hm;
}
IdentityHashMap hashMap() {
return associatedMap;
}
@Override
public int size() {
return associatedMap.size;
}
@Override
public void clear() {
associatedMap.clear();
}
@Override
public boolean remove(Object object) {
if (contains(object)) {
associatedMap.remove(((Map.Entry) object).getKey());
return true;
}
return false;
}
@Override
public boolean contains(Object object) {
if (object instanceof Map.Entry) {
IdentityHashMapEntry entry = associatedMap
.getEntry(((Map.Entry) object).getKey());
// we must call equals on the entry obtained from "this"
return entry != null && entry.equals(object);
}
return false;
}
@Override
public Iterator> iterator() {
return new IdentityHashMapIterator, KT, VT>(
new MapEntry.Type, KT, VT>() {
public Map.Entry get(MapEntry entry) {
return entry;
}
}, associatedMap);
}
}
/**
* Creates an IdentityHashMap with default expected maximum size.
*/
public IdentityHashMap() {
this(DEFAULT_MAX_SIZE);
}
/**
* Creates an IdentityHashMap with the specified maximum size parameter.
*
* @param maxSize
* The estimated maximum number of entries that will be put in
* this map.
*/
public IdentityHashMap(int maxSize) {
if (maxSize >= 0) {
this.size = 0;
threshold = getThreshold(maxSize);
elementData = newElementArray(computeElementArraySize());
} else {
throw new IllegalArgumentException();
}
}
private int getThreshold(int maxSize) {
// assign the threshold to maxSize initially, this will change to a
// higher value if rehashing occurs.
return maxSize > 3 ? maxSize : 3;
}
private int computeElementArraySize() {
int arraySize = (int) (((long) threshold * 10000) / loadFactor) * 2;
// ensure arraySize is positive, the above cast from long to int type
// leads to overflow and negative arraySize if threshold is too big
return arraySize < 0 ? -arraySize : arraySize;
}
/**
* Create a new element array
*
* @param s
* the number of elements
* @return Reference to the element array
*/
private Object[] newElementArray(int s) {
return new Object[s];
}
/**
* Creates an IdentityHashMap using the given map as initial values.
*
* @param map
* A map of (key,value) pairs to copy into the IdentityHashMap.
*/
public IdentityHashMap(Map map) {
this(map.size() < 6 ? 11 : map.size() * 2);
putAllImpl(map);
}
@SuppressWarnings("unchecked")
private V massageValue(Object value) {
return (V) ((value == NULL_OBJECT) ? null : value);
}
/**
* Removes all elements from this map, leaving it empty.
*
* @see #isEmpty()
* @see #size()
*/
@Override
public void clear() {
size = 0;
for (int i = 0; i < elementData.length; i++) {
elementData[i] = null;
}
modCount++;
}
/**
* Returns whether this map contains the specified key.
*
* @param key
* the key to search for.
* @return {@code true} if this map contains the specified key,
* {@code false} otherwise.
*/
@Override
public boolean containsKey(Object key) {
if (key == null) {
key = NULL_OBJECT;
}
int index = findIndex(key, elementData);
return elementData[index] == key;
}
/**
* Returns whether this map contains the specified value.
*
* @param value
* the value to search for.
* @return {@code true} if this map contains the specified value,
* {@code false} otherwise.
*/
@Override
public boolean containsValue(Object value) {
if (value == null) {
value = NULL_OBJECT;
}
for (int i = 1; i < elementData.length; i = i + 2) {
if (elementData[i] == value) {
return true;
}
}
return false;
}
/**
* Returns the value of the mapping with the specified key.
*
* @param key
* the key.
* @return the value of the mapping with the specified key.
*/
@Override
public V get(Object key) {
if (key == null) {
key = NULL_OBJECT;
}
int index = findIndex(key, elementData);
if (elementData[index] == key) {
Object result = elementData[index + 1];
return massageValue(result);
}
return null;
}
private IdentityHashMapEntry getEntry(Object key) {
if (key == null) {
key = NULL_OBJECT;
}
int index = findIndex(key, elementData);
if (elementData[index] == key) {
return getEntry(index);
}
return null;
}
/**
* Convenience method for getting the IdentityHashMapEntry without the
* NULL_OBJECT elements
*/
@SuppressWarnings("unchecked")
private IdentityHashMapEntry getEntry(int index) {
Object key = elementData[index];
Object value = elementData[index + 1];
if (key == NULL_OBJECT) {
key = null;
}
if (value == NULL_OBJECT) {
value = null;
}
return new IdentityHashMapEntry((K) key, (V) value);
}
/**
* Returns the index where the key is found at, or the index of the next
* empty spot if the key is not found in this table.
*/
private int findIndex(Object key, Object[] array) {
int length = array.length;
int index = getModuloHash(key, length);
int last = (index + length - 2) % length;
while (index != last) {
if (array[index] == key || (array[index] == null)) {
/*
* Found the key, or the next empty spot (which means key is not
* in the table)
*/
break;
}
index = (index + 2) % length;
}
return index;
}
private int getModuloHash(Object key, int length) {
return ((System.identityHashCode(key) & 0x7FFFFFFF) % (length / 2)) * 2;
}
/**
* Maps the specified key to the specified value.
*
* @param key
* the key.
* @param value
* the value.
* @return the value of any previous mapping with the specified key or
* {@code null} if there was no such mapping.
*/
@Override
public V put(K key, V value) {
Object _key = key;
Object _value = value;
if (_key == null) {
_key = NULL_OBJECT;
}
if (_value == null) {
_value = NULL_OBJECT;
}
int index = findIndex(_key, elementData);
// if the key doesn't exist in the table
if (elementData[index] != _key) {
modCount++;
if (++size > threshold) {
rehash();
index = findIndex(_key, elementData);
}
// insert the key and assign the value to null initially
elementData[index] = _key;
elementData[index + 1] = null;
}
// insert value to where it needs to go, return the old value
Object result = elementData[index + 1];
elementData[index + 1] = _value;
return massageValue(result);
}
/**
* Copies all the mappings in the specified map to this map. These mappings
* will replace all mappings that this map had for any of the keys currently
* in the given map.
*
* @param map
* the map to copy mappings from.
* @throws NullPointerException
* if {@code map} is {@code null}.
*/
@Override
public void putAll(Map map) {
putAllImpl(map);
}
private void rehash() {
int newlength = elementData.length << 1;
if (newlength == 0) {
newlength = 1;
}
Object[] newData = newElementArray(newlength);
for (int i = 0; i < elementData.length; i = i + 2) {
Object key = elementData[i];
if (key != null) {
// if not empty
int index = findIndex(key, newData);
newData[index] = key;
newData[index + 1] = elementData[i + 1];
}
}
elementData = newData;
computeMaxSize();
}
private void computeMaxSize() {
threshold = (int) ((long) (elementData.length / 2) * loadFactor / 10000);
}
/**
* Removes the mapping with the specified key from this map.
*
* @param key
* the key of the mapping to remove.
* @return the value of the removed mapping, or {@code null} if no mapping
* for the specified key was found.
*/
@Override
public V remove(Object key) {
if (key == null) {
key = NULL_OBJECT;
}
boolean hashedOk;
int index, next, hash;
Object result, object;
index = next = findIndex(key, elementData);
if (elementData[index] != key) {
return null;
}
// store the value for this key
result = elementData[index + 1];
// shift the following elements up if needed
// until we reach an empty spot
int length = elementData.length;
while (true) {
next = (next + 2) % length;
object = elementData[next];
if (object == null) {
break;
}
hash = getModuloHash(object, length);
hashedOk = hash > index;
if (next < index) {
hashedOk = hashedOk || (hash <= next);
} else {
hashedOk = hashedOk && (hash <= next);
}
if (!hashedOk) {
elementData[index] = object;
elementData[index + 1] = elementData[next + 1];
index = next;
}
}
size--;
modCount++;
// clear both the key and the value
elementData[index] = null;
elementData[index + 1] = null;
return massageValue(result);
}
/**
* Returns a set containing all of the mappings in this map. Each mapping is
* an instance of {@link Map.Entry}. As the set is backed by this map,
* changes in one will be reflected in the other.
*
* @return a set of the mappings.
*/
@Override
public Set> entrySet() {
return new IdentityHashMapEntrySet(this);
}
/**
* Returns a set of the keys contained in this map. The set is backed by
* this map so changes to one are reflected by the other. The set does not
* support adding.
*
* @return a set of the keys.
*/
@Override
public Set keySet() {
if (keySet == null) {
keySet = new AbstractSet() {
@Override
public boolean contains(Object object) {
return containsKey(object);
}
@Override
public int size() {
return IdentityHashMap.this.size();
}
@Override
public void clear() {
IdentityHashMap.this.clear();
}
@Override
public boolean remove(Object key) {
if (containsKey(key)) {
IdentityHashMap.this.remove(key);
return true;
}
return false;
}
@Override
public Iterator iterator() {
return new IdentityHashMapIterator(
new MapEntry.Type() {
public K get(MapEntry entry) {
return entry.key;
}
}, IdentityHashMap.this);
}
};
}
return keySet;
}
/**
* Returns a collection of the values contained in this map. The collection
* is backed by this map so changes to one are reflected by the other. The
* collection supports remove, removeAll, retainAll and clear operations,
* and it does not support add or addAll operations.
*
* This method returns a collection which is the subclass of
* AbstractCollection. The iterator method of this subclass returns a
* "wrapper object" over the iterator of map's entrySet(). The {@code size}
* method wraps the map's size method and the {@code contains} method wraps
* the map's containsValue method.
*
* The collection is created when this method is called for the first time
* and returned in response to all subsequent calls. This method may return
* different collections when multiple concurrent calls occur, since no
* synchronization is performed.
*
* @return a collection of the values contained in this map.
*/
@Override
public Collection values() {
if (valuesCollection == null) {
valuesCollection = new AbstractCollection() {
@Override
public boolean contains(Object object) {
return containsValue(object);
}
@Override
public int size() {
return IdentityHashMap.this.size();
}
@Override
public void clear() {
IdentityHashMap.this.clear();
}
@Override
public Iterator iterator() {
return new IdentityHashMapIterator(
new MapEntry.Type() {
public V get(MapEntry entry) {
return entry.value;
}
}, IdentityHashMap.this);
}
@Override
public boolean remove(Object object) {
Iterator it = iterator();
while (it.hasNext()) {
if (object == it.next()) {
it.remove();
return true;
}
}
return false;
}
};
}
return valuesCollection;
}
/**
* Compares this map with other objects. This map is equal to another map is
* it represents the same set of mappings. With this map, two mappings are
* the same if both the key and the value are equal by reference. When
* compared with a map that is not an IdentityHashMap, the equals method is
* neither necessarily symmetric (a.equals(b) implies b.equals(a)) nor
* transitive (a.equals(b) and b.equals(c) implies a.equals(c)).
*
* @param object
* the object to compare to.
* @return whether the argument object is equal to this object.
*/
@Override
public boolean equals(Object object) {
/*
* We need to override the equals method in AbstractMap because
* AbstractMap.equals will call ((Map) object).entrySet().contains() to
* determine equality of the entries, so it will defer to the argument
* for comparison, meaning that reference-based comparison will not take
* place. We must ensure that all comparison is implemented by methods
* in this class (or in one of our inner classes) for reference-based
* comparison to take place.
*/
if (this == object) {
return true;
}
if (object instanceof Map) {
Map map = (Map) object;
if (size() != map.size()) {
return false;
}
Set> set = entrySet();
// ensure we use the equals method of the set created by "this"
return set.equals(map.entrySet());
}
return false;
}
/**
* Returns whether this IdentityHashMap has no elements.
*
* @return {@code true} if this IdentityHashMap has no elements,
* {@code false} otherwise.
* @see #size()
*/
@Override
public boolean isEmpty() {
return size == 0;
}
/**
* Returns the number of mappings in this IdentityHashMap.
*
* @return the number of mappings in this IdentityHashMap.
*/
@Override
public int size() {
return size;
}
private void putAllImpl(Map map) {
if (map.entrySet() != null) {
super.putAll(map);
}
}
}