IdentityHashMap简介

  IdentityHashMap利用哈希表实现Map接口，不同的是，其比较键（或值）时，使用引用相等性代替对象相等性。换句话说，在IdentityHashMap中，当且仅当k1==k2时，才认为两个键相等；而在正常的Map实现中（比如HashMap），当且仅当两个键k1和K2满足:k1==null?k2==null：k1.equals(k2)时，两个键才相等。

  此类不是通用的Map实现，实现Map接口的常规协定是比较对象时使用equals方法。而IdentityHashMap不是，此类设计仅用于需要引用相等性语义的情形。此类提供所有的可选映射操作，并且允许 null 值和 null 键。此类对映射的顺序不提供任何保证；特别是不保证顺序随时间的推移保持不变。

主要看下put和get方法的实现：

public V put(K key, V value) {
    //判断key是否为空
    final Object k = maskNull(key);

    retryAfterResize: for (;;) {
        final Object[] tab = table;
        final int len = tab.length;
        //根据key和哈希表长度计算哈希值
        int i = hash(k, len);

        for (Object item; (item = tab[i]) != null;
             i = nextKeyIndex(i, len)) {
             //引用比较key的值
            if (item == k) {
                @SuppressWarnings("unchecked")
                    V oldValue = (V) tab[i + 1];
                tab[i + 1] = value;
                //当该key存在时，返回旧值
                return oldValue;
            }
        }
        
        final int s = size + 1;
        // Use optimized form of 3 * s.
        // Next capacity is len, 2 * current capacity.
        if (s + (s << 1) > len && resize(len))
            continue retryAfterResize;

        modCount++;
        tab[i] = k;
        tab[i + 1] = value;
        size = s;
        //当该key不存在时，返回null
        return null;
    }
}


public V get(Object key) {
    Object k = maskNull(key);
    Object[] tab = table;
    int len = tab.length;
    int i = hash(k, len);
    while (true) {
        Object item = tab[i];
        //引用比较key的值
        if (item == k)
            return (V) tab[i + 1];
        if (item == null)
            return null;
        i = nextKeyIndex(i, len);
    }
}

对比HashMap的put和get实现：

//put方法实现
public V put(K key, V value) {
    return putVal(hash(key), key, value, false, true);
}
final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
               boolean evict) {
    Node<K,V>[] tab; Node<K,V> p; int n, i;
    //判断哈希表是否为空，若为空，进行初始化
    if ((tab = table) == null || (n = tab.length) == 0)
        n = (tab = resize()).length;
        //根据key的哈希值计算key在哈希表中的存储位置
    if ((p = tab[i = (n - 1) & hash]) == null)
        //该位置为空，新建一个node，存入哈希表（tab[i]）
        tab[i] = newNode(hash, key, value, null);
    else {
        Node<K,V> e; K k;
        //比较key
        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;
}

//get方法实现
public V get(Object key) {
    Node<K,V> e;
    return (e = getNode(hash(key), key)) == null ? null : e.value;
}
final Node<K,V> getNode(int hash, Object key) {
    Node<K,V>[] tab; Node<K,V> first, e; int n; K k;
    if ((tab = table) != null && (n = tab.length) > 0 &&
        (first = tab[(n - 1) & hash]) != null) {
        if (first.hash == hash && // always check first node
            ((k = first.key) == key || (key != null && key.equals(k))))
            return first;
        if ((e = first.next) != null) {
            if (first instanceof TreeNode)
                return ((TreeNode<K,V>)first).getTreeNode(hash, key);
            do {
                if (e.hash == hash &&
                    ((k = e.key) == key || (key != null && key.equals(k))))
                    return e;
            } while ((e = e.next) != null);
        }
    }
    return null;
}

从上面的put和get的实现中可以看出IdentityHashMap和通用Map的区别，IdentityHashMap比较key时使用引用相等性（==），而通用Map比较key还使用equals方法。

举例：

public class Person {
private String id;
private String name;
public String getId() {
    return id;
}
public void setId(String id) {
    this.id=id;
}
public String getName() {
    return name;
}
public void setName(String name) {
    this.name=name;
}
public Person() {}
public Person(String id,String name) {
    super();
    this.id=id;
    this.name=name;
}
public String toString() {
    return "Person[id="+id+",name="+name+"]";
}
//重写equals方法
public boolean equals(Object object) {
    if(object instanceof Person) {
        Person person=(Person)object;
        return Objects.equal(this.id,person.getId());
    }
    return false;
}
public int hashCode() {
    return this.id.hashCode();
    }
}

import java.util.HashMap;
import java.util.IdentityHashMap;

public class MainDemo {
  public static void main(String args[]) {
    IdentityHashMap<Student,String> imap=new IdentityHashMap<>();
    HashMap<Student,String> map=new HashMap<>();
    Student s1=new Student("1","whisper");
    Student s2=new Student("1","whisper");
    map.put(s1, "whisper");
    map.put(s2, "godyan");
    imap.put(s1, "whisper");
    imap.put(s2, "godyan");
    System.out.println("map.size()="+map.size()+",map.get(s1)="+map.get(s1)+",map.get(s2)="+map.get(s2));
    System.out.println("---------------------------------------------");
    System.out.println("imap.size()="+imap.size()+",imap.get(s1)="+imap.get(s1)+",imap.get(s2)="+imap.get(s2));
  }
}

输出：

map.size()=1,map.get(s1)=godyan,map.get(s2)=godyan
---------------------------------------------
imap.size()=2,imap.get(s1)=whisper,imap.get(s2)=godyan

注意：Identity不是线程安全的，当多线程同时修改IdentityHashMap时，必须外部保持同步。