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java集合基本概念:HashMap(JDK1.8)
2018-01-03 11:28:09      个评论    来源:liuyufeihu的专栏  
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一、基本概念

HashMap基于哈希表的 Map 接口的实现。此实现提供所有可选的映射操作,并允许使用 null 值和 null 键。以前JDK中HashMap采用的是位桶+链表的方式,即我们常说的散列链表的方式,而JDK1.8中采用的是位桶+链表/红黑树的方式,也是非线程安全的。当某个位桶的链表的长度达到某个阀值的时候,这个链表就将转换成红黑树。
注意事项:

HashMap 是一个散列表,它存储的内容是键值对(key-value)映射; HashMap 继承于AbstractMap,实现了Map、Cloneable、java.io.Serializable接口; HashMap 的实现不是同步的,这意味着它不是线程安全的。它的key、value都可以为null; HashMap中的映射不是有序的; HashMap 的实例有两个参数影响其性能:“初始容量” 和 “加载因子”;

1:常量

   /**
   map的最大容量
     */
    static final int MAXIMUM_CAPACITY = 1 << 30;

    /**
     * 默认加载因子
     */
    static final float DEFAULT_LOAD_FACTOR = 0.75f;

    /**
     将list链表转为红黑数阀值,即list的size超过时转化
     */
    static final int TREEIFY_THRESHOLD = 8;

    /**
     resize操作中,决定是否untreeify的阈值
     */
    static final int UNTREEIFY_THRESHOLD = 6;

    /**
     决定是否转换成tree的最小容量
     */
    static final int MIN_TREEIFY_CAPACITY = 64;

2:主要字段

/**
存储元素的数组
*/
 transient Node[] table;
 /**
 用于map迭代遍历
 */
 transient Set> entrySet;

    /**
     元素个数
     */
    transient int size;

    /**
     修改次数
     */
    transient int modCount;

    /**
    阀值,用于扩容阀值
     */
    int threshold;

    /**
     加载因子
     */
    final float loadFactor;

3:主要方法
get方法:

public V get(Object key) {
        Node e;
        return (e = getNode(hash(key), key)) == null ? null : e.value;
    }

    /**
     * Implements Map.get and related methods
     *
     * @param hash hash for key
     * @param key the key
     * @return the node, or null if none
     */
    final Node getNode(int hash, Object key) {
        Node[] tab; Node first, e; int n; K k;
        if ((tab = table) != null && (n = tab.length) > 0 &&
            (first = tab[(n - 1) & hash]) != null) {
            if (first.hash == hash && // 总是判断第一个元素是否满足条件
                ((k = first.key) == key || (key != null && key.equals(k))))
                return first;
            if ((e = first.next) != null) {
            //第一个节点为TreeNode,则调用TreeNode.getTreeNode()方法遍历红黑数进行查询
                if (first instanceof TreeNode)
                    return ((TreeNode)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;
    }

put方法

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[] tab; Node p; int n, i;
        if ((tab = table) == null || (n = tab.length) == 0)
        //table为空,n为table的长度
            n = (tab = resize()).length;
            //i位置为空,直接存储
        if ((p = tab[i = (n - 1) & hash]) == null)
            tab[i] = newNode(hash, key, value, null);
        else {
        // 若i位置上的值不为空,判断当前位置上的Node p 是否与要插入的key的hash和key相同
            Node e; K k;
            if (p.hash == hash &&
                ((k = p.key) == key || (key != null && key.equals(k))))
                //相同key时直接覆盖
                e = p;
            else if (p instanceof TreeNode)
            //不相同时,若当前p已经为TreeNode,则插入该树上
                e = ((TreeNode)p).putTreeVal(this, tab, hash, key, value);
            else {
            //在i位置上的链表中找到p.next为null的位置,binCount计算出当前链表的长度,如果继续将冲突的节点插入到该链表中,会使链表的长度大于tree化的阈值,则将链表转换成tree。
                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;
    }

resize(),由于解决冲突的方法可能是list,也可能是红黑数,所以resize()较为复杂点。

/**
     * 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[] resize() {
        Node[] oldTab = table;
        int oldCap = (oldTab == null) ? 0 : oldTab.length;
        int oldThr = threshold;
        int newCap, newThr = 0;
        if (oldCap > 0) {
        //若超过最大容量,则不能扩容
            if (oldCap >= MAXIMUM_CAPACITY) {
                threshold = Integer.MAX_VALUE;
                return oldTab;
            }
            else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
                     oldCap >= DEFAULT_INITIAL_CAPACITY)
                newThr = oldThr << 1; //阀值扩大2倍 
        }
        else if (oldThr > 0) // initial capacity was placed in threshold
            newCap = oldThr;
        else {               // zero initial threshold signifies using defaults
            newCap = DEFAULT_INITIAL_CAPACITY;
            newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);
        }
        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"})
        // 创建容量为newCap的newTab,并将oldTab中的Node迁移过来,这里需要考虑链表和tree两种情况。
            Node[] newTab = (Node[])new Node[newCap];
        table = newTab;
        if (oldTab != null) {
            for (int j = 0; j < oldCap; ++j) {
                Node e;
                if ((e = oldTab[j]) != null) {
                    oldTab[j] = null;
                    if (e.next == null)
                        newTab[e.hash & (newCap - 1)] = e;
                    else if (e instanceof TreeNode)
                    // split方法会将树分割为lower 和upper tree两个树,如果子树的节点数小于了UNTREEIFY_THRESHOLD阈值,则将树untreeify,将节点都存放在newTab中。
                        ((TreeNode)e).split(this, newTab, j, oldCap);
                    else { // preserve order
                        Node loHead = null, loTail = null;
                        Node hiHead = null, hiTail = null;
                        Node 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;
    }

remove()方法

public V remove(Object key) {
        Node e;
        return (e = removeNode(hash(key), key, null, false, true)) == null ?
            null : e.value;
    }

    /**
     * Implements Map.remove and related methods
     *
     * @param hash hash for key
     * @param key the key
     * @param value the value to match if matchValue, else ignored
     * @param matchValue if true only remove if value is equal
     * @param movable if false do not move other nodes while removing
     * @return the node, or null if none
     */
    final Node removeNode(int hash, Object key, Object value,
                               boolean matchValue, boolean movable) {
        Node[] tab; Node p; int n, index;
        if ((tab = table) != null && (n = tab.length) > 0 &&
            (p = tab[index = (n - 1) & hash]) != null) {
            Node node = null, e; K k; V v;
            if (p.hash == hash &&
                ((k = p.key) == key || (key != null && key.equals(k))))
                node = p;
                //若是用红黑数解决冲突则getTreeNode方法查找到节点
            else if ((e = p.next) != null) {
                if (p instanceof TreeNode)
                    node = ((TreeNode)p).getTreeNode(hash, key);
                else {
                    do {
                        if (e.hash == hash &&
                            ((k = e.key) == key ||
                             (key != null && key.equals(k)))) {
                            node = e;
                            break;
                        }
                        p = e;
                    } while ((e = e.next) != null);
                }
            }
            if (node != null && (!matchValue || (v = node.value) == value ||
                                 (value != null && value.equals(v)))) {
                if (node instanceof TreeNode)
                    ((TreeNode)node).removeTreeNode(this, tab, movable);
                else if (node == p)
                    tab[index] = node.next;
                else
                    p.next = node.next;
                ++modCount;
                --size;
                afterNodeRemoval(node);
                return node;
            }
        }
        return null;
    }

HashMap实例

1:Hashmap的遍历方法

package com.csu.collection;

import java.util.HashMap;
import java.util.Iterator;
import java.util.Map.Entry;
import java.util.Set;

public class HashMapTest {

    public static void main(String[]args)
    {
        HashMap map=new HashMap<>();
        for(int i=0;i<10000000;i++)
        {
            map.put(i, i);
        }
        System.out.println("第一种遍历方法:for each map.entrySet()");
        long startTime1=System.currentTimeMillis();
        for(Entry entry:map.entrySet())
        {
            entry.getValue();
            entry.getKey();
        }
        long endTime1=System.currentTimeMillis();
        System.out.println("第一种遍历方法用时:"+(endTime1-startTime1)+"ms");
        System.out.println("第2种遍历方法:map.entrySet()的集合迭代器");
        long startTime2=System.currentTimeMillis();
        Iterator> iterator=map.entrySet().iterator();
        while(iterator.hasNext())
        {
            HashMap.Entry entry=(Entry) iterator.next();
            entry.getValue();
            entry.getKey();
        }
        long endTime2=System.currentTimeMillis();
        System.out.println("第2种遍历方法用时:"+(endTime2-startTime2)+"ms");
        System.out.println("第3种遍历方法: for each map.keySet(),再调用get获取");
        long startTime3=System.currentTimeMillis();
        for (Integer key : map.keySet()) {
            map.get(key);
        }
        long endTime3=System.currentTimeMillis();
        System.out.println("第3种遍历方法用时:"+(endTime3-startTime3)+"ms");
        System.out.println("第4种遍历方法:for each map.entrySet(),用临时变量保存map.entrySet()");
        long startTime4=System.currentTimeMillis();
        Set> entrySet = map.entrySet();
        for (Entry entry : entrySet) {
            entry.getKey();
            entry.getValue();
        }
        long endTime4=System.currentTimeMillis();
        System.out.println("第4种遍历方法用时:"+(endTime4-startTime4)+"ms");
    }
}

运行结果

第一种遍历方法:for each map.entrySet()
第一种遍历方法用时:71ms
第2种遍历方法:map.entrySet()的集合迭代器
第2种遍历方法用时:83ms
第3种遍历方法: for each map.keySet(),再调用get获取
第3种遍历方法用时:117ms
第4种遍历方法:for each map.entrySet(),用临时变量保存map.entrySet()
第4种遍历方法用时:84ms

总结:

a. HashMap的循环,如果既需要key也需要value,直接用for each map.entrySet(); 如果只是遍历key而无需value的话,可以直接用for each map.keySet(),再调用get获取。
2:使用Hashmap 实现缓存
public class Student {
    private String name;
    private String address;
    public Student(String name,String address)
    {
        this.address=address;
        this.name=name;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public String getAddress() {
        return address;
    }
    public void setAddress(String address) {
        this.address = address;
    }
}
import java.io.Serializable;

public class CacheEntity implements Serializable {

    /**
     * 
     */
    private static final long serialVersionUID = 1L;

    private final int DEFUALT_TIME=200;//秒

    private String  key;
    private Object value;
    private int time;//缓存存活时间,不设置则使用默认值
    private long timeoutStamp;// 缓存过期时间戳

    @SuppressWarnings("unused")
    private CacheEntity()
    {
        this.timeoutStamp=System.currentTimeMillis()+DEFUALT_TIME*1000;
        this.time=DEFUALT_TIME;
    }
    public CacheEntity(String key,Object value)
    {
        this.key=key;
        this.value=value;
    }
    public CacheEntity(String key,Object value,long timestamp)
    {
        this(key,value);
        this.timeoutStamp=timestamp;
    }
    public CacheEntity(String key,Object value,int time)
    {
        this(key,value);
        this.time=time;
        this.timeoutStamp=System.currentTimeMillis()+DEFUALT_TIME*1000;
    }
    public String getKey() {
        return key;
    }
    public void setKey(String key) {
        this.key = key;
    }
    public Object getValue() {
        return value;
    }
    public void setValue(Object value) {
        this.value = value;
    }
    public int getTime() {
        return time;
    }
    public void setTime(int time) {
        this.time = time;
    }
    public long getTimeoutStamp() {
        return timeoutStamp;
    }
    public void setTimeoutStamp(long timeoutStamp) {
        this.timeoutStamp = timeoutStamp;
    }
}
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;

/**
 * 
 * 采用队列,定时循环清理过期缓存
 *
 */
public class CacheByHashMap {


    private static  HashMap map;
    private static List tempList;
    static{
        tempList=new ArrayList();
        map=new HashMap(1<<10); 
        new Thread(new RemoveTimeOutCacheThread()).start();

    }
  /**
   * 添加缓存
   * @param key
   * @param value
   * @param time
   */
    public static synchronized void addCache(String key,CacheEntity value,int time)
    {
        value.setTimeoutStamp(System.currentTimeMillis()+time*1000);
        map.put(key, value);
        tempList.add(value);
    }
    /**
     * 获取缓存对象
     * @param key
     * @return
     */
    public static synchronized CacheEntity getCache(String key)
    {
        return map.get(key);
    }
    /**
     * 检查是否包含特定的key
     * @param key
     * @return
     */
    public static synchronized boolean isContainsKey(String key)
    {
        return map.containsKey(key);
    }
    /**
     * 删除缓存
     * @param key
     */
    public static synchronized void removeCache(String key)
    {
        map.remove(key);
    }
    /**
     *  获取缓存数量
     * @return
     */
    public static int getCacheSize()
    {
      return  map.size();
    }
    /**
     * 清除所有缓存
     */
    public static synchronized void clearCache()
    {
        tempList.clear();
        map.clear();
        System.out.println("所有缓存被清理");
    }
    static class RemoveTimeOutCacheThread implements Runnable{

        @Override
        public void run() {
            // TODO Auto-generated method stub
            while(true)
            {
                try {
                    checkTime();
                } catch (Exception e) {
                    // TODO: handle exception
                    e.printStackTrace();
                }
            }
        }

        private void checkTime() throws InterruptedException
        {
            CacheEntity value=null;
            long timeoutTime=1000l;
            if(tempList.size()<1)
            {
                System.out.println("过期队列为空!");
                timeoutTime=1000l;
                Thread.sleep(timeoutTime);
                return ;
            }
            value=tempList.get(0);
            timeoutTime=value.getTimeoutStamp()-System.currentTimeMillis();
            if(timeoutTime>0)
            {
                Thread.sleep(timeoutTime);
                return ;
            }
            System.out.println("清除过期缓存"+value.getKey());
            tempList.remove(value);
            removeCache(value.getKey());
        }

    }

}

测试端代码:

public class CacheTest {

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        Student student1=new Student("zhangsan", "shangsha");
        Student student2=new Student("wangqiang", "beijing");
        Student student3=new Student("zhangsi", "shanghai");
        Student student4=new Student("zhangwu", "wuhan");
        Student student5=new Student("zhangqi", "zhengzhou");
        Student student6=new Student("zhangba", "shangsha");
        CacheEntity cacheEntity1=new CacheEntity("1", student1, 30);
        CacheEntity cacheEntity2=new CacheEntity("2", student2, 30);
        CacheEntity cacheEntity3=new CacheEntity("3", student3, 30);
        CacheEntity cacheEntity4=new CacheEntity("4", student4, 30);
        CacheEntity cacheEntity5=new CacheEntity("5", student5, 30);
        CacheEntity cacheEntity6=new CacheEntity("6", student6, 30);
        //添加缓存
        CacheByHashMap.addCache(cacheEntity1.getKey(), cacheEntity1, cacheEntity1.getTime());
        CacheByHashMap.addCache(cacheEntity2.getKey(), cacheEntity2, cacheEntity2.getTime());
        CacheByHashMap.addCache(cacheEntity3.getKey(), cacheEntity3, cacheEntity3.getTime());
        CacheByHashMap.addCache(cacheEntity4.getKey(), cacheEntity4, cacheEntity4.getTime());
        CacheByHashMap.addCache(cacheEntity5.getKey(), cacheEntity5, cacheEntity5.getTime());
        CacheByHashMap.addCache(cacheEntity6.getKey(), cacheEntity6, cacheEntity6.getTime());
        if(CacheByHashMap.isContainsKey("2"))
        {
            System.out.println(" 该对象已有缓存");
            //这里就可以获取缓存如get()
        }
        else {
            CacheByHashMap.addCache(cacheEntity2.getKey(), cacheEntity2, cacheEntity2.getTime());
            //这里可以模拟从数据库获取数据,添加到缓存
        }

    }
}

运行结果:

该对象已有缓存
清除过期缓存1
清除过期缓存2
清除过期缓存3
清除过期缓存4
清除过期缓存5
清除过期缓存6
过期队列为空!
过期队列为空!
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