Java Map以及HashMap、TreeMap、HashTable

Map

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);
}

HashMap

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 1sttreeifyBin(tab, hash);break;}if (e.hash == hash &&((k = e.key) == key || (key != null && key.equals(k))))// 此时需要进行替换valuebreak;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 thresholdnewCap = oldThr;else {               // zero initial threshold signifies using defaultsnewCap = DEFAULT_INITIAL_CAPACITY; //默认16newThr = (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 orderNode<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;elseloTail.next = e;loTail = e;}else {if (hiTail == null)hiHead = e;elsehiTail.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：
1. 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。
2. 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，
  1. 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。
  2. 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。

LinkedHashMap

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 1sttreeifyBin(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 keyV 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。

TreeMap

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*/
}

Properties

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*/
}