本文主要介绍Map接口以及其主要实现类：HashMap、LinkedHashMap、TreeMap、Hashtable、Properties，其中包括HashMap、TreeMap的底层实现原理。
Map的遍历方式
方式一,这是最常见的并且在大多数情况下也是最可取的遍历方式。在键值都需要时使用:
Map<Integer, Integer> map = new HashMap<Integer, Integer>();
for (Map.Entry<Integer, Integer> entry : map.entrySet()) {
System.out.println("Key = " + entry.getKey() + ", Value = " + entry.getValue());
}
方式二，在for-each循环中遍历keys或values(据说该方法比entrySet遍历在性能上稍好（快了10%），我还没有测试过)：
Map<Integer, Integer> map = new HashMap<Integer, Integer>();
//遍历map中的键
for (Integer key : map.keySet()) {
System.out.println("Key = " + key);
}
//遍历map中的值
for (Integer value : map.values()) {
System.out.println("Value = " + value);
}
方式三，使用迭代器遍历（该种方式看起来冗余却有其优点所在。首先，在老版本java中这是惟一遍历map的方式。另一个好处是，你可以在遍历时调用iterator.remove()来删除entries，另两个方法则不能。）：
// 不使用泛型
Map map = new HashMap();
Iterator entries = map.entrySet().iterator();
while (entries.hasNext()) {
Map.Entry entry = (Map.Entry) entries.next();
Integer key = (Integer)entry.getKey();
Integer value = (Integer)entry.getValue();
System.out.println("Key = " + key + ", Value = " + value);
}
// 使用泛型
Map<Integer, Integer> map = new HashMap<>();
Iterator<Map.Entry<Integer, Integer>> iterator = map.entrySet().iterator();
while (iterator.hasNext()){
Map.Entry<Integer, Integer> next = iterator.next();
System.out.println(next);
}
方式四，通过键找值遍历（循环中通过key来拿到value效率低）：
Map<Integer, Integer> map = new HashMap<Integer, Integer>();
for (Integer key : map.keySet()) {
Integer value = map.get(key);
System.out.println("Key = " + key + ", Value = " + value);
}
JDK HashMap源码分析
HashMap初始化：
static final int MAXIMUM_CAPACITY = 1 << 30;
static final float DEFAULT_LOAD_FACTOR = 0.75f;
public HashMap(int initialCapacity, float loadFactor) {
if (initialCapacity < 0)
throw new IllegalArgumentException("Illegal initial capacity: " +
initialCapacity);
if (initialCapacity > MAXIMUM_CAPACITY)
initialCapacity = MAXIMUM_CAPACITY;
if (loadFactor <= 0 || Float.isNaN(loadFactor))
throw new IllegalArgumentException("Illegal load factor: " +
loadFactor);
this.loadFactor = loadFactor;
this.threshold = tableSizeFor(initialCapacity);
}
public HashMap(int initialCapacity) {
this(initialCapacity, DEFAULT_LOAD_FACTOR);
}
public HashMap() {
this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
}
public HashMap(Map<? extends K, ? extends V> m) {
this.loadFactor = DEFAULT_LOAD_FACTOR;
putMapEntries(m, false);
}
static final int tableSizeFor(int cap) {
int n = cap - 1;
n |= n >>> 1;
n |= n >>> 2;
n |= n >>> 4;
n |= n >>> 8;
n |= n >>> 16;
return (n < 0) ? 1 : (n >= MAXIMUM_CAPACITY) ? MAXIMUM_CAPACITY : n + 1;
}
显然，在初始化时如果没有传入Map的大小，则只是改了下了一个加载因子；而在传入initialCapacity时则会计算出threshold（临界值），用于判断是否进行扩容。然后在使用put()的时候才会真正创建hashTable：
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16
/**
* 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) {
return putVal(hash(key), key, value, false, true);
}
/**
* Implements Map.put and related methods
*
* @param hash hash for key
* @param key the key
* @param value the value to put
* @param onlyIfAbsent if true, don't change existing value
* @param evict if false, the table is in creation mode.
* @return previous value, or null if none
*/
final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
boolean evict) {
Node<K,V>[] tab; Node<K,V> p; int n, i;
if ((tab = table) == null || (n = tab.length) == 0)
n = (tab = resize()).length;
if ((p = tab[i = (n - 1) & hash]) == null)
tab[i] = newNode(hash, key, value, null);
else {
Node<K,V> e; K k;
// 此时p == tab[i = (n - 1) & hash]
if (p.hash == hash &&
((k = p.key) == key || (key != null && key.equals(k))))
e = p;
else if (p instanceof TreeNode)
// 向红黑树中插入结点
e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
else {
for (int binCount = 0; ; ++binCount) { //没有终止条件的循环用于查看该索引上所有的node结点
if ((e = p.next) == null) { //下一个为空时新建一个结点
p.next = newNode(hash, key, value, null);
if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
treeifyBin(tab, hash);
break;
}
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
// 此时需要进行替换value
break;
p = e;
}
}
if (e != null) { // existing mapping for key，key的hash值发生冲突
V oldValue = e.value;
// key的hash发生冲突后必然会进行一次替换（onlyIfAbsent默认为false）
if (!onlyIfAbsent || oldValue == null)
e.value = value;
afterNodeAccess(e);
return oldValue;
}
}
++modCount;
if (++size > threshold)
resize();
afterNodeInsertion(evict);
return null;
}
/**
* Initializes or doubles table size. If null, allocates in
* accord with initial capacity target held in field threshold.
* Otherwise, because we are using power-of-two expansion, the
* elements from each bin must either stay at same index, or move
* with a power of two offset in the new table.
*
* @return the table
*/
final Node<K,V>[] resize() {
Node<K,V>[] oldTab = table;
int oldCap = (oldTab == null) ? 0 : oldTab.length;
int oldThr = threshold;
int newCap, newThr = 0;
if (oldCap > 0) {
if (oldCap >= MAXIMUM_CAPACITY) {
threshold = Integer.MAX_VALUE;
return oldTab;
}
else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
oldCap >= DEFAULT_INITIAL_CAPACITY)
newThr = oldThr << 1; // double threshold
}
else if (oldThr > 0) // initial capacity was placed in threshold
newCap = oldThr;
else { // zero initial threshold signifies using defaults
newCap = DEFAULT_INITIAL_CAPACITY; //默认16
newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY); // 默认0.75*16 = 12
}
if (newThr == 0) {
float ft = (float)newCap * loadFactor;
newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ?
(int)ft : Integer.MAX_VALUE);
}
threshold = newThr;
@SuppressWarnings({"rawtypes","unchecked"})
Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
table = newTab;
if (oldTab != null) {
for (int j = 0; j < oldCap; ++j) {
Node<K,V> e;
if ((e = oldTab[j]) != null) {
oldTab[j] = null;
if (e.next == null)
newTab[e.hash & (newCap - 1)] = e;
else if (e instanceof TreeNode)
((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
else { // preserve order
Node<K,V> loHead = null, loTail = null;
Node<K,V> hiHead = null, hiTail = null;
Node<K,V> next;
do {
next = e.next;
if ((e.hash & oldCap) == 0) {
if (loTail == null)
loHead = e;
else
loTail.next = e;
loTail = e;
}
else {
if (hiTail == null)
hiHead = e;
else
hiTail.next = e;
hiTail = e;
}
} while ((e = next) != null);
if (loTail != null) {
loTail.next = null;
newTab[j] = loHead;
}
if (hiTail != null) {
hiTail.next = null;
newTab[j + oldCap] = hiHead;
}
}
}
}
}
return newTab;
}
从上面的代码以及部分我自己标注的中文注释可以看到：默认是创建了一个长度为16的表(Node<K,V>[])new Node[newCap]；其临界值threshold为12。在后面每次调用put操作时也是调用上面三个方法。当然，上面的代码也是可以看出，put首先调用key1所在类的hashCode()计算key1的哈希值，过程如下：
static final int hash(Object key) {
int h;
return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
}
此哈希值经过i = ((tab = resize()).length - 1) & hash后获得插入到哈希数组中的index，在put过程中会有如下情况：
- 如果此位置的数据为空，此时的key1-value1添加成功。
- 如果此位置的数据不为空，（意味这此位置有一个或者多个数据存在），比较key1和已经存在数据的哈希值：
- 如果key1的哈希值与已经存在的数据的哈希值都不相同，此时key1-value1添加到第一个位置，后续为原来的链表数据。
- 如果key1的哈希值与已经存在的的某个数据(key2-value2)的hash值相同，继续比较；调用key对象的equals()方法，
- 如果equals()返回false：，此时key1-value1添加到第一个位置，后续为原来的链表数据。
- 如果equals()返回true:此时使用value1替换value2并返回value2具体比较代码如下：
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
当然，在这里还是得说下jdk1.8相较与jdk1.7在底层实现方面还是的一些不同：
- 初始化时jdk1.7时直接创建了一个长度为16的数组，而1.8没有。
- jdk1.8底层的数组时Node[]而非Entry[],当他们实际是差不多的。
- jdk1.8是在首次调用put()方法的时候创建数组。
- jdk1.7的底层结构只有数组+链表；jdk1.8的底层结构为数组+链表+红黑树当数组的某一个索引位置上的元素以链表的形式存在的数据个数>8且当前数组的长度>64时，此时此索引位置上的所有数据改为使用红黑树存储。
JDK LinkedHashMap源码分析
LinkedHashMap初始化：
public class LinkedHashMap<K,V>
extends HashMap<K,V>
implements Map<K,V>
{
public LinkedHashMap(int initialCapacity, float loadFactor) {
super(initialCapacity, loadFactor);
accessOrder = false;
}
public LinkedHashMap(int initialCapacity) {
super(initialCapacity);
accessOrder = false;
}
public LinkedHashMap() {
super();
accessOrder = false;
}
public LinkedHashMap(Map<? extends K, ? extends V> m) {
super();
accessOrder = false;
putMapEntries(m, false);
}
public LinkedHashMap(int initialCapacity,
float loadFactor,
boolean accessOrder) {
super(initialCapacity, loadFactor);
this.accessOrder = accessOrder;
}
}
在初始化的过程中，我们发现LinkedHashMap就是HashMap，那么它和HashMap有什么区别呢，我们还是得看它的put()方法，然后在LinkedHashMap中却没有找到put()方法，这个时候怎么办呢？看它的父类，查看父类中的put()方法：
public V put(K key, V value) {
return putVal(hash(key), key, value, false, true);
}
发现还是得找putVal方法，于是接着看：
final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
boolean evict) {
Node<K,V>[] tab; Node<K,V> p; int n, i;
if ((tab = table) == null || (n = tab.length) == 0)
n = (tab = resize()).length;
if ((p = tab[i = (n - 1) & hash]) == null)
tab[i] = newNode(hash, key, value, null);
else {
Node<K,V> e; K k;
if (p.hash == hash &&
((k = p.key) == key || (key != null && key.equals(k))))
e = p;
else if (p instanceof TreeNode)
e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
else {
for (int binCount = 0; ; ++binCount) {
if ((e = p.next) == null) {
p.next = newNode(hash, key, value, null);
if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
treeifyBin(tab, hash);
break;
}
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
break;
p = e;
}
}
if (e != null) { // existing mapping for key
V oldValue = e.value;
if (!onlyIfAbsent || oldValue == null)
e.value = value;
afterNodeAccess(e);
return oldValue;
}
}
++modCount;
if (++size > threshold)
resize();
afterNodeInsertion(evict);
return null;
}
初始化的过程中我们必定java必定会执行：tab[i] = newNode(hash, key, value, null);这句代码，于是去看newNode,点进去，发现HashMap中的newNode方法是这样子的：
Node<K,V> newNode(int hash, K key, V value, Node<K,V> next) {
return new Node<>(hash, key, value, next);
}
那LinkedHashMap怎么和HashMap一样啊，这个时候就该思考，会不会是子类中还有什么是我们没有看到的，接着看子类LinkedHashMap，发现LinkedHashMap中也有newNode方法，原来是重写了：
Node<K,V> newNode(int hash, K key, V value, Node<K,V> e) {
LinkedHashMap.Entry<K,V> p =
new LinkedHashMap.Entry<K,V>(hash, key, value, e);
linkNodeLast(p);
return p;
}
再接着点进Entry看看是什么东西：
static class Entry<K,V> extends HashMap.Node<K,V> {
Entry<K,V> before, after;
Entry(int hash, K key, V value, Node<K,V> next) {
super(hash, key, value, next);
}
}
哦，原来它生成了一个和HashMap一样的结点，还在Entry<K,V> before, after;这里使用了before和after去记录生成的结点的前后顺序。
JDK TreeMap源码分析
A Red-Black tree based NavigableMap implementation. The map is sorted according to the Comparable natural ordering of its keys, or by a Comparator provided at map creation time, depending on which constructor is used.
上面这段话是java官方给出的说明，显然，TreeMap底层就是使用红黑树来实现的，并且是一个可以排序的Map，其中既可以使用自然排序，也可以是使用Comparator比较器来进行排序。自然排序：
@Test
public void test9(){
Map<Integer, Object> map = new TreeMap<>();
map.put(11, "dasd");
map.put(-1, "dasda");
map.put(22, "dasd");
map.put(33, "dasd");
map.put(44, "dasd");
map.put(44, "dad");
Iterator<Map.Entry<Integer, Object>> iterator = map.entrySet().iterator();
while (iterator.hasNext()){
Map.Entry<Integer, Object> next = iterator.next();
System.out.println(next);
}
/**
* 打印结果：
* -1=dasda
* 11=dasd
* 22=dasd
* 33=dasd
* 44=dad
*/
}
Comparetor比较器排序：
@Test
public void test10(){
Map<Employee, Object> map = new TreeMap<>();
for(int i = 5; i > 0; i--){
map.put(new Employee("test"+i, i, i), "dasd");
}
Iterator<Map.Entry<Employee, Object>> iterator = map.entrySet().iterator();
while (iterator.hasNext()){
Map.Entry<Employee, Object> next = iterator.next();
System.out.println(next);
}
/**
* 打印结果：
* Employee{name='test1', age=1, salary=1}=dasd
* Employee{name='test2', age=2, salary=2}=dasd
* Employee{name='test3', age=3, salary=3}=dasd
* Employee{name='test4', age=4, salary=4}=dasd
* Employee{name='test5', age=5, salary=5}=dasd
*/
}
Java Properties使用实例——HashMap应用
Properties是HashMap的重要应用，利用了HashMap通过key查找迅速的特点，将这一特点用在了读取配置文件上，具体的使用如下：在项目的资源文件夹下新建test.properties文件：
name=1baidu
测试代码：
@Test
public void test1() throws Exception{
Properties properties = new Properties();
FileInputStream fileInputStream= new FileInputStream("test.properties");
properties.load(fileInputStream);
System.out.println("name="+properties.getProperty("name"));
/**
* 打印结果：
* name=1baidu
*/
}
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