java.util.LinkedHashMap 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;
/**
* LinkedHashMap is an implementation of {@link Map} that guarantees iteration order.
* All optional operations are supported.
*
* All elements are permitted as keys or values, including null.
*
*
Entries are kept in a doubly-linked list. The iteration order is, by default, the
* order in which keys were inserted. Reinserting an already-present key doesn't change the
* order. If the three argument constructor is used, and {@code accessOrder} is specified as
* {@code true}, the iteration will be in the order that entries were accessed.
* The access order is affected by {@code put}, {@code get}, and {@code putAll} operations,
* but not by operations on the collection views.
*
*
Note: the implementation of {@code LinkedHashMap} is not synchronized.
* If one thread of several threads accessing an instance modifies the map
* structurally, access to the map needs to be synchronized. For
* insertion-ordered instances a structural modification is an operation that
* removes or adds an entry. Access-ordered instances also are structurally
* modified by {@code put}, {@code get}, and {@code putAll} since these methods
* change the order of the entries. Changes in the value of an entry are not structural changes.
*
*
The {@code Iterator} created by calling the {@code iterator} method
* may throw a {@code ConcurrentModificationException} if the map is structurally
* changed while an iterator is used to iterate over the elements. Only the
* {@code remove} method that is provided by the iterator allows for removal of
* elements during iteration. It is not possible to guarantee that this
* mechanism works in all cases of unsynchronized concurrent modification. It
* should only be used for debugging purposes.
*/
public class LinkedHashMap extends HashMap {
/**
* A dummy entry in the circular linked list of entries in the map.
* The first real entry is header.nxt, and the last is header.prv.
* If the map is empty, header.nxt == header && header.prv == header.
*/
transient LinkedEntry header;
/**
* True if access ordered, false if insertion ordered.
*/
private final boolean accessOrder;
/**
* Constructs a new empty {@code LinkedHashMap} instance.
*/
public LinkedHashMap() {
init();
accessOrder = false;
}
/**
* Constructs a new {@code LinkedHashMap} instance with the specified
* capacity.
*
* @param initialCapacity
* the initial capacity of this map.
* @exception IllegalArgumentException
* when the capacity is less than zero.
*/
public LinkedHashMap(int initialCapacity) {
this(initialCapacity, DEFAULT_LOAD_FACTOR);
}
/**
* Constructs a new {@code LinkedHashMap} instance with the specified
* capacity and load factor.
*
* @param initialCapacity
* the initial capacity of this map.
* @param loadFactor
* the initial load factor.
* @throws IllegalArgumentException
* when the capacity is less than zero or the load factor is
* less or equal to zero.
*/
public LinkedHashMap(int initialCapacity, float loadFactor) {
this(initialCapacity, loadFactor, false);
}
/**
* Constructs a new {@code LinkedHashMap} instance with the specified
* capacity, load factor and a flag specifying the ordering behavior.
*
* @param initialCapacity
* the initial capacity of this hash map.
* @param loadFactor
* the initial load factor.
* @param accessOrder
* {@code true} if the ordering should be done based on the last
* access (from least-recently accessed to most-recently
* accessed), and {@code false} if the ordering should be the
* order in which the entries were inserted.
* @throws IllegalArgumentException
* when the capacity is less than zero or the load factor is
* less or equal to zero.
*/
public LinkedHashMap(
int initialCapacity, float loadFactor, boolean accessOrder) {
super(initialCapacity, loadFactor);
init();
this.accessOrder = accessOrder;
}
/**
* Constructs a new {@code LinkedHashMap} instance containing the mappings
* from the specified map. The order of the elements is preserved.
*
* @param map
* the mappings to add.
*/
public LinkedHashMap(Map map) {
this(capacityForInitSize(map.size()));
constructorPutAll(map);
}
@Override void init() {
header = new LinkedEntry();
}
/**
* LinkedEntry adds nxt/prv double-links to plain HashMapEntry.
*/
static class LinkedEntry extends HashMapEntry {
LinkedEntry nxt;
LinkedEntry prv;
/** Create the header entry */
LinkedEntry() {
super(null, null, 0, null);
nxt = prv = this;
}
/** Create a normal entry */
LinkedEntry(K key, V value, int hash, HashMapEntry next,
LinkedEntry nxt, LinkedEntry prv) {
super(key, value, hash, next);
this.nxt = nxt;
this.prv = prv;
}
}
/**
* Returns the eldest entry in the map, or {@code null} if the map is empty.
* @hide
*/
public Map.Entry eldest() {
LinkedEntry eldest = header.nxt;
return eldest != header ? eldest : null;
}
/**
* Evicts eldest entry if instructed, creates a new entry and links it in
* as head of linked list. This method should call constructorNewEntry
* (instead of duplicating code) if the performance of your VM permits.
*
* It may seem strange that this method is tasked with adding the entry
* to the hash table (which is properly the province of our superclass).
* The alternative of passing the "next" link in to this method and
* returning the newly created element does not work! If we remove an
* (eldest) entry that happens to be the first entry in the same bucket
* as the newly created entry, the "next" link would become invalid, and
* the resulting hash table corrupt.
*/
@Override void addNewEntry(K key, V value, int hash, int index) {
LinkedEntry header = this.header;
// Remove eldest entry if instructed to do so.
LinkedEntry eldest = header.nxt;
if (eldest != header && removeEldestEntry(eldest)) {
remove(eldest.key);
}
// Create new entry, link it on to list, and put it into table
LinkedEntry oldTail = header.prv;
LinkedEntry newTail = new LinkedEntry(
key, value, hash, table[index], header, oldTail);
table[index] = oldTail.nxt = header.prv = newTail;
}
@Override void addNewEntryForNullKey(V value) {
LinkedEntry header = this.header;
// Remove eldest entry if instructed to do so.
LinkedEntry eldest = header.nxt;
if (eldest != header && removeEldestEntry(eldest)) {
remove(eldest.key);
}
// Create new entry, link it on to list, and put it into table
LinkedEntry oldTail = header.prv;
LinkedEntry newTail = new LinkedEntry(
null, value, 0, null, header, oldTail);
entryForNullKey = oldTail.nxt = header.prv = newTail;
}
/**
* As above, but without eviction.
*/
@Override HashMapEntry constructorNewEntry(
K key, V value, int hash, HashMapEntry next) {
LinkedEntry header = this.header;
LinkedEntry oldTail = header.prv;
LinkedEntry newTail
= new LinkedEntry(key, value, hash, next, header, oldTail);
return oldTail.nxt = header.prv = newTail;
}
/**
* Returns the value of the mapping with the specified key.
*
* @param key
* the key.
* @return the value of the mapping with the specified key, or {@code null}
* if no mapping for the specified key is found.
*/
@Override public V get(Object key) {
/*
* This method is overridden to eliminate the need for a polymorphic
* invocation in superclass at the expense of code duplication.
*/
if (key == null) {
HashMapEntry e = entryForNullKey;
if (e == null)
return null;
if (accessOrder)
makeTail((LinkedEntry) e);
return e.value;
}
// Replace with Collections.secondaryHash when the VM is fast enough (http://b/8290590).
int hash = secondaryHash(key);
HashMapEntry[] tab = table;
for (HashMapEntry e = tab[hash & (tab.length - 1)];
e != null; e = e.next) {
K eKey = e.key;
if (eKey == key || (e.hash == hash && key.equals(eKey))) {
if (accessOrder)
makeTail((LinkedEntry) e);
return e.value;
}
}
return null;
}
/**
* Relinks the given entry to the tail of the list. Under access ordering,
* this method is invoked whenever the value of a pre-existing entry is
* read by Map.get or modified by Map.put.
*/
private void makeTail(LinkedEntry e) {
// Unlink e
e.prv.nxt = e.nxt;
e.nxt.prv = e.prv;
// Relink e as tail
LinkedEntry header = this.header;
LinkedEntry oldTail = header.prv;
e.nxt = header;
e.prv = oldTail;
oldTail.nxt = header.prv = e;
modCount++;
}
@Override void preModify(HashMapEntry e) {
if (accessOrder) {
makeTail((LinkedEntry) e);
}
}
@Override void postRemove(HashMapEntry e) {
LinkedEntry le = (LinkedEntry) e;
le.prv.nxt = le.nxt;
le.nxt.prv = le.prv;
le.nxt = le.prv = null; // Help the GC (for performance)
}
/**
* This override is done for LinkedHashMap performance: iteration is cheaper
* via LinkedHashMap nxt links.
*/
@Override public boolean containsValue(Object value) {
if (value == null) {
for (LinkedEntry header = this.header, e = header.nxt;
e != header; e = e.nxt) {
if (e.value == null) {
return true;
}
}
return false;
}
// value is non-null
for (LinkedEntry header = this.header, e = header.nxt;
e != header; e = e.nxt) {
if (value.equals(e.value)) {
return true;
}
}
return false;
}
public void clear() {
super.clear();
// Clear all links to help GC
LinkedEntry header = this.header;
for (LinkedEntry e = header.nxt; e != header; ) {
LinkedEntry nxt = e.nxt;
e.nxt = e.prv = null;
e = nxt;
}
header.nxt = header.prv = header;
}
private abstract class LinkedHashIterator implements Iterator {
LinkedEntry next = header.nxt;
LinkedEntry lastReturned = null;
int expectedModCount = modCount;
public final boolean hasNext() {
return next != header;
}
final LinkedEntry nextEntry() {
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
LinkedEntry e = next;
if (e == header)
throw new NoSuchElementException();
next = e.nxt;
return lastReturned = e;
}
public final void remove() {
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
if (lastReturned == null)
throw new IllegalStateException();
LinkedHashMap.this.remove(lastReturned.key);
lastReturned = null;
expectedModCount = modCount;
}
}
private final class KeyIterator extends LinkedHashIterator {
public final K next() { return nextEntry().key; }
}
private final class ValueIterator extends LinkedHashIterator {
public final V next() { return nextEntry().value; }
}
private final class EntryIterator
extends LinkedHashIterator> {
public final Map.Entry next() { return nextEntry(); }
}
// Override view iterator methods to generate correct iteration order
@Override Iterator newKeyIterator() {
return new KeyIterator();
}
@Override Iterator newValueIterator() {
return new ValueIterator();
}
@Override Iterator> newEntryIterator() {
return new EntryIterator();
}
protected boolean removeEldestEntry(Map.Entry eldest) {
return false;
}
}