HashMap、Hashtable、LinkedHashMap和TreeMap 比较
一、相同点与不同点
共同点：
HashMap,LinkedHashMap,TreeMap都属于Map；Map 主要用于存储键(key)值(value)对，根据键得到值，因此键不允许键重复,但允许值重复。不同点:
1.HashMap里面存入的键值对在取出的时候是随机的,也是我们最常用的一个Map.它根据键的HashCode值存储数据,根据键可以直接获取它的值，具有很快的访问速度。在Map 中插入、删除和定位元素，HashMap 是最好的选择。
2.TreeMap取出来的是排序后的键值对。但如果您要按自然顺序或自定义顺序遍历键，那么TreeMap会更好。
3. LinkedHashMap 是HashMap的一个子类，如果需要输出的顺序和输入的相同,那么用LinkedHashMap可以实现.
二、HashMap源码分析
HashMap主要是用数组来存储数据的，其对key进行哈希运算，哈希运算会有重复的哈希值，对于哈希值的冲突，HashMap采用链表来解决的。
1、几个关键的属性
transient Entry[] table; //存储数据的数组static final int DEFAULT_INITIAL_CAPACITY = 16;//默认容量
static final int MAXIMUM_CAPACITY = 1 << 30;//最大容量
static final float DEFAULT_LOAD_FACTOR = 0.75f;//默认加载因子，加载因子是一个比例，当HashMap的数据大小>=容量*加载因子时，HashMap会将容量扩容
int threshold;//当实际数据大小超过threshold时，HashMap会将容量扩容，threshold＝容量*加载因子
final float loadFactor;//加载因子
2、HashMap初始化
/**
* Constructs an empty <tt>HashMap</tt> with the default initial capacity
* (16) and the default load factor (0.75).
*/
public HashMap() {
this.loadFactor = DEFAULT_LOAD_FACTOR;//默认加载因子，加载因子是一个比例，当HashMap的数据大小>=容量*加载因子时，HashMap会将容量扩容
threshold = (int)(DEFAULT_INITIAL_CAPACITY * DEFAULT_LOAD_FACTOR);
table = new Entry[DEFAULT_INITIAL_CAPACITY];//数组来存储数据
init();
}
3、放入HashMap
/**
* Associates the specified value with the specified key in this map.
* If the map previously contained a mapping for the 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 the previous value associated with <tt>key</tt>, or
* <tt>null</tt> if there was no mapping for <tt>key</tt>.
* (A <tt>null</tt> return can also indicate that the map
* previously associated <tt>null</tt> with <tt>key</tt>.)
*/
public V put(K key, V value) {
if (key == null)
return putForNullKey(value);
int hash = hash(key.hashCode());
int i = indexFor(hash, table.length);
for (Entry<K,V> e = table[i]; e != null; e = e.next) {
Object k;
if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {
V oldValue = e.value;
e.value = value;
e.recordAccess(this);
return oldValue;
}
}
modCount++;
addEntry(hash, key, value, i);
return null;
}看addEntry
/**
* Adds a new entry with the specified key, value and hash code to
* the specified bucket. It is the responsibility of this
* method to resize the table if appropriate.
*
* Subclass overrides this to alter the behavior of put method.
*/
void addEntry(int hash, K key, V value, int bucketIndex) {
Entry<K,V> e = table[bucketIndex];
table[bucketIndex] = new Entry<K,V>(hash, key, value, e);
if (size++ >= threshold)
resize(2 * table.length);
}看resize
/**
* Rehashes the contents of this map into a new array with a
* larger capacity. This method is called automatically when the
* number of keys in this map reaches its threshold.
*
* If current capacity is MAXIMUM_CAPACITY, this method does not
* resize the map, but sets threshold to Integer.MAX_VALUE.
* This has the effect of preventing future calls.
*
* @param newCapacity the new capacity, MUST be a power of two;
* must be greater than current capacity unless current
* capacity is MAXIMUM_CAPACITY (in which case value
* is irrelevant).
*/
void resize(int newCapacity) {
Entry[] oldTable = table;
int oldCapacity = oldTable.length;
if (oldCapacity == MAXIMUM_CAPACITY) {
threshold = Integer.MAX_VALUE;
return;
}
Entry[] newTable = new Entry[newCapacity];
transfer(newTable);
table = newTable;
threshold = (int)(newCapacity * loadFactor);
}
看transfer
/**
* Transfers all entries from current table to newTable.
*/
void transfer(Entry[] newTable) {
Entry[] src = table;
int newCapacity = newTable.length;
for (int j = 0; j < src.length; j++) {
Entry<K,V> e = src[j];
if (e != null) {
src[j] = null;
do {
Entry<K,V> next = e.next;
int i = indexFor(e.hash, newCapacity);
e.next = newTable[i];
newTable[i] = e;
e = next;
} while (e != null);
}
}
}
4、图示
三、Hashtable分析
Hashtable与HashMap的实现机制是一致的，只是其实线程安全的，几乎每个public方法都是
synchronized的。
如向表中放入数据：
/**
* Maps the specified <code>key</code> to the specified
* <code>value</code> in this hashtable. Neither the key nor the
* value can be <code>null</code>. <p>
*
* The value can be retrieved by calling the <code>get</code> 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 <code>null</code> if it did not have one
* @exception NullPointerException if the key or value is
* <code>null</code>
* @see Object#equals(Object)
* @see #get(Object)
*/
public synchronized V put(K key, V value) {
// Make sure the value is not null
if (value == null) {
throw new NullPointerException();
}
// Makes sure the key is not already in the hashtable.
Entry tab[] = table;
int hash = key.hashCode();
int index = (hash & 0x7FFFFFFF) % tab.length;
for (Entry<K,V> e = tab[index] ; e != null ; e = e.next) {
if ((e.hash == hash) && e.key.equals(key)) {
V 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<K,V> e = tab[index];
tab[index] = new Entry<K,V>(hash, key, value, e);
count++;
return null;
}
四、LinkedHashMap分析
HashMap与LinkedHashMap最大的不同在于，后者维护者一个运行于所有条目的双向链表。有了这个双向链表，就可以在迭代的时候按照插入的顺序迭代出元素。
继承关系
public class LinkedHashMap<K,V>
extends HashMap<K,V>
implements Map<K,V>
1、初始化
/**
* Called by superclass constructors and pseudoconstructors (clone,
* readObject) before any entries are inserted into the map. Initializes
* the chain.
*/
void init() {
header = new Entry<K,V>(-1, null, null, null);
header.before = header.after = header;
}
2、新增before、after
/**
* LinkedHashMap entry.
*/
private static class Entry<K,V> extends HashMap.Entry<K,V> {
// These fields comprise the doubly linked list used for iteration.
Entry<K,V> before, after;
Entry(int hash, K key, V value, HashMap.Entry<K,V> next) {
super(hash, key, value, next);
}
3、覆写了HashMap的方法
/**
* This override alters behavior of superclass put method. It causes newly
* allocated entry to get inserted at the end of the linked list and
* removes the eldest entry if appropriate.
*/
void addEntry(int hash, K key, V value, int bucketIndex)
{
createEntry(hash, key, value, bucketIndex);
// Remove eldest entry if instructed, else grow capacity if appropriate
Entry<K, V> eldest = header.after;
if (removeEldestEntry(eldest))
{
removeEntryForKey(eldest.key);
}
else
{
if (size >= threshold)
resize(2 * table.length);
}
}
/**
* This override differs from addEntry in that it doesn't resize the
* table or remove the eldest entry.
*/
void createEntry(int hash, K key, V value, int bucketIndex)
{
HashMap.Entry<K, V> old = table[bucketIndex];
Entry<K, V> e = new Entry<K, V>(hash, key, value, old);
table[bucketIndex] = e;
e.addBefore(header);
size++;
}
/**
* Inserts this entry before the specified existing entry in the list.
*/
private void addBefore(Entry<K, V> existingEntry)
{
after = existingEntry;
before = existingEntry.before;
before.after = this;
after.before = this;
}
五、代码示例
/**
* Map用于存储键值对，不允许键重复，值可以重复。
* （1）HashMap是一个最常用的Map，它根据键的hashCode值存储数据，根据键可以直接获取它的值，具有很快的访问速度。
* HashMap最多只允许一条记录的键为null，允许多条记录的值为null。
* HashMap不支持线程的同步，即任一时刻可以有多个线程同时写HashMap，可能会导致数据的不一致。
* 如果需要同步，可以用Collections.synchronizedMap(HashMap map)方法使HashMap具有同步的能力。
* （2）Hashtable与HashMap类似，不同的是：它不允许记录的键或者值为空；
* 它支持线程的同步，即任一时刻只有一个线程能写Hashtable，然而，这也导致了Hashtable在写入时会比较慢。
* （3）LinkedHashMap保存了记录的插入顺序，在用Iteraor遍历LinkedHashMap时，先得到的记录肯定是先插入的。
* 在遍历的时候会比HashMap慢。
* （4）TreeMap能够把它保存的记录根据键排序，默认是按升序排序，也可以指定排序的比较器。当用Iteraor遍历TreeMap时，
* 得到的记录是排过序的。
*/
import java.util.HashMap;
import java.util.Hashtable;
import java.util.Iterator;
import java.util.LinkedHashMap;
import java.util.Map;
import java.util.TreeMap;
/**
* 演示各个Map的实现类
*/
public class TestMap {
/**
* 初始化一个Map
* @param map
*/
public static void init(Map map){
if (map != null){
String key = null;
for (int i=5; i>0; i--){
key = new Integer(i).toString() + ".0";
map.put(key, key.toString());
//Map中的键是不重复的，如果插入两个键值一样的记录，
//那么后插入的记录会覆盖先插入的记录
map.put(key, key.toString() + "0"); }
}
}
/**
* 输出一个Map
* @param map
*/
public static void output(Map map){
if (map != null){
Object key = null;
Object value = null;
//使用迭代器遍历Map的键，根据键取值
Iterator it = map.keySet().iterator();
while (it.hasNext()){
key = it.next();
value = map.get(key);
System.out.println("key: " + key + "; value: " + value );
}
//或者使用迭代器遍历Map的记录Map.Entry
Map.Entry entry = null;
it = map.entrySet().iterator();
while (it.hasNext()){
//一个Map.Entry代表一条记录
entry = (Map.Entry)it.next();
//通过entry可以获得记录的键和值
//System.out.println("key: " + entry.getKey() + "; value: " + entry.getValue());
}
}
}
/**
* 判断map是否包含某个键
* @param map
* @param key
* @return
*/
public static boolean containsKey(Map map, Object key){
if (map != null){
return map.containsKey(key);
}
return false;
}
/**
* 判断map是否包含某个值
* @param map
* @param value
* @return
*/
public static boolean containsValue(Map map, Object value){
if (map != null){
return map.containsValue(value);
}
return false;
}
/**
* 演示HashMap
*/
public static void testHashMap(){
Map myMap = new HashMap();
init(myMap);
//HashMap的键可以为null
myMap.put(null,"ddd");
//HashMap的值可以为null
myMap.put("aaa", null);
output(myMap);
}
/**
* 演示Hashtable
*/
public static void testHashtable(){
Map myMap = new Hashtable();
init(myMap);
//Hashtable的键不能为null
//myMap.put(null,"ddd");
//Hashtable的值不能为null
//myMap.put("aaa", null);
output(myMap);
}
/**
* 演示LinkedHashMap
*/
public static void testLinkedHashMap(){
Map myMap = new LinkedHashMap();
init(myMap);
//LinkedHashMap的键可以为null
myMap.put(null,"ddd");
//LinkedHashMap的值可以为null
myMap.put("aaa", null);
output(myMap);
}
/**
* 演示TreeMap
*/
public static void testTreeMap(){
Map myMap = new TreeMap();
init(myMap);
//TreeMap的键不能为null
//myMap.put(null,"ddd");
//TreeMap的值不能为null
//myMap.put("aaa", null);
output(myMap);
}
public static void main(String[] args) {
System.out.println("采用HashMap");
TestMap.testHashMap();
System.out.println("采用Hashtable");
TestMap.testHashtable();
System.out.println("采用LinkedHashMap");
TestMap.testLinkedHashMap();
System.out.println("采用TreeMap");
TestMap.testTreeMap();
Map myMap = new HashMap();
TestMap.init(myMap);
System.out.println("新初始化一个Map: myMap");
TestMap.output(myMap);
//清空Map
myMap.clear();
System.out.println("将myMap clear后，myMap空了么? " + myMap.isEmpty());
TestMap.output(myMap);
myMap.put("aaa", "aaaa");
myMap.put("bbb", "bbbb");
//判断Map是否包含某键或者某值
System.out.println("myMap包含键aaa? "+ TestMap.containsKey(myMap, "aaa"));
System.out.println("myMap包含值aaaa? "+ TestMap.containsValue(myMap, "aaaa"));
//根据键删除Map中的记录
myMap.remove("aaa");
System.out.println("删除键aaa后，myMap包含键aaa? "+ TestMap.containsKey(myMap, "aaa"));
//获取Map的记录数
System.out.println("myMap包含的记录数: " + myMap.size());
}
}