java集合架构____HashMap源码分析

阅读目录

• 什么是HashMap
• HashMap补充说明
• HashMap数据结构
• HashMap继承的类与实现的接口
• HashMap源码分析，大部分都加了注释
• 简单使用示例

以下内容基于jdk1.7.0_79源码；

什么是HashMap

基于哈希表的一个Map接口实现，存储的对象是一个键值对对象(Entry<K,V>)；

HashMap补充说明

基于数组和链表实现，内部维护着一个数组table，该数组保存着每个链表的表头结点；查找时，先通过hash函数计算key的hash值，再根据key的hash值计算数组索引（取余法），然后根据索引找到链表表头结点，然后遍历查找该链表；

HashMap数据结构

画了个示意图，如下，左边的数组索引是根据key的hash值计算得到，不同hash值有可能产生一样的索引，即哈希冲突，此时采用链地址法处理哈希冲突，即将所有索引一致的节点构成一个单链表；

HashMap继承的类与实现的接口

Map接口，方法的含义很简单，基本上看个方法名就知道了，后面会在HashMap源码分析里详细说明

AbstractMap抽象类中定义的方法

HashMap源码分析，大部分都加了注释

package java.util;import java.io.*;public class HashMap<K,V>    extends AbstractMap<K,V>    implements Map<K,V>, Cloneable, Serializable{    /**     * 默认初始容量，默认为2的4次方 = 16，2的n次方是为了加快hash计算速度，；；减少hash冲突，，，h & (length-1)，，1111111     */    static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16    /**     * 最大容量，默认为2的30次方，     */    static final int MAXIMUM_CAPACITY = 1 << 30;    /**     * 默认负载因子，默认为0.75     */    static final float DEFAULT_LOAD_FACTOR = 0.75f;    /**     *当数组表还没扩容的时候，一个共享的空表对象     */    static final Entry<?,?>[] EMPTY_TABLE = {};    /**     * 数组表，大小可以改变，且大小必须为2的幂     */    transient Entry<K,V>[] table = (Entry<K,V>[]) EMPTY_TABLE;    /**     * 当前Map中key-value映射的个数     */    transient int size;    /**     * 下次扩容阈值，当size > capacity * load factor时，开始扩容     */    int threshold;    /**     * 负载因子     */    final float loadFactor;    /**     * Hash表结构性修改次数，用于实现迭代器快速失败行为     */    transient int modCount;    /**     * 容量阈值，默认大小为Integer.MAX_VALUE     */    static final int ALTERNATIVE_HASHING_THRESHOLD_DEFAULT = Integer.MAX_VALUE;    /**     * 静态内部类Holder，存放一些只能在虚拟机启动后才能初始化的值     */    private static class Holder {        /**         * 容量阈值，初始化hashSeed的时候会用到该值         */        static final int ALTERNATIVE_HASHING_THRESHOLD;        static {            //获取系统变量jdk.map.althashing.threshold            String altThreshold = java.security.AccessController.doPrivileged(                new sun.security.action.GetPropertyAction(                    "jdk.map.althashing.threshold"));            int threshold;            try {                threshold = (null != altThreshold)                        ? Integer.parseInt(altThreshold)                        : ALTERNATIVE_HASHING_THRESHOLD_DEFAULT;                // jdk.map.althashing.threshold系统变量默认为-1，如果为-1，则将阈值设为Integer.MAX_VALUE                if (threshold == -1) {                    threshold = Integer.MAX_VALUE;                }                //阈值需要为正数                if (threshold < 0) {                    throw new IllegalArgumentException("value must be positive integer.");                }            } catch(IllegalArgumentException failed) {                throw new Error("Illegal value for 'jdk.map.althashing.threshold'", failed);            }            ALTERNATIVE_HASHING_THRESHOLD = threshold;        }    }    /**     * 计算hash值的时候需要用到     */    transient int hashSeed = 0;    /**     * 生成一个空的HashMap,并指定其容量大小和负载因子     *     */    public HashMap(int initialCapacity, float loadFactor) {        //保证初始容量大于等于0        if (initialCapacity < 0)            throw new IllegalArgumentException("Illegal initial capacity: " +                                               initialCapacity);        //保证初始容量不大于最大容量MAXIMUM_CAPACITY        if (initialCapacity > MAXIMUM_CAPACITY)            initialCapacity = MAXIMUM_CAPACITY;                //loadFactor小于0或为无效数字        if (loadFactor <= 0 || Float.isNaN(loadFactor))            throw new IllegalArgumentException("Illegal load factor: " +                                               loadFactor);        //负载因子        this.loadFactor = loadFactor;        //下次扩容大小        threshold = initialCapacity;        init();    }    /**     * 生成一个空的HashMap,并指定其容量大小，负载因子使用默认的0.75     *     */    public HashMap(int initialCapacity) {        this(initialCapacity, DEFAULT_LOAD_FACTOR);    }    /**     * 生成一个空的HashMap,容量大小使用默认值16，负载因子使用默认值0.75     */    public HashMap() {        this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR);    }    /**     * 根据指定的map生成一个新的HashMap,负载因子使用默认值，初始容量大小为Math.max((int) (m.size() / DEFAULT_LOAD_FACTOR) + 1,DEFAULT_INITIAL_CAPACITY)     */    public HashMap(Map<? extends K, ? extends V> m) {        this(Math.max((int) (m.size() / DEFAULT_LOAD_FACTOR) + 1,                      DEFAULT_INITIAL_CAPACITY), DEFAULT_LOAD_FACTOR);        inflateTable(threshold);        putAllForCreate(m);    }    //返回>=number的最小2的n次方值，如number=5，则返回8    private static int roundUpToPowerOf2(int number) {        // assert number >= 0 : "number must be non-negative";        return number >= MAXIMUM_CAPACITY                ? MAXIMUM_CAPACITY                : (number > 1) ? Integer.highestOneBit((number - 1) << 1) : 1;    }    /**     * 对table扩容     */    private void inflateTable(int toSize) {        // Find a power of 2 >= toSize        //找一个值（2的n次方，且>=toSize）        int capacity = roundUpToPowerOf2(toSize);        //下次扩容阈值        threshold = (int) Math.min(capacity * loadFactor, MAXIMUM_CAPACITY + 1);                table = new Entry[capacity];        initHashSeedAsNeeded(capacity);    }    // internal utilities    void init() {    }    /**     * 初始化hashSeed     */    final boolean initHashSeedAsNeeded(int capacity) {        boolean currentAltHashing = hashSeed != 0;        boolean useAltHashing = sun.misc.VM.isBooted() &&                (capacity >= Holder.ALTERNATIVE_HASHING_THRESHOLD);        boolean switching = currentAltHashing ^ useAltHashing;        if (switching) {            hashSeed = useAltHashing                ? sun.misc.Hashing.randomHashSeed(this)                : 0;        }        return switching;    }    /**     * 生成hash值     */    final int hash(Object k) {        int h = hashSeed;                //如果key是字符串，调用un.misc.Hashing.stringHash32生成hash值        //Oracle表示能生成更好的hash分布，不过这在jdk8中已删除        if (0 != h && k instanceof String) {            return sun.misc.Hashing.stringHash32((String) k);        }        //一次散列，调用k的hashCode方法，与hashSeed做异或操作        h ^= k.hashCode();        // This function ensures that hashCodes that differ only by        // constant multiples at each bit position have a bounded        // number of collisions (approximately 8 at default load factor).        //二次散列，        h ^= (h >>> 20) ^ (h >>> 12);        return h ^ (h >>> 7) ^ (h >>> 4);    }    /**     * 返回hash值的索引，采用除模取余法，h & (length-1)操作 等价于 hash % length操作， 但&操作性能更优     */    static int indexFor(int h, int length) {        // assert Integer.bitCount(length) == 1 : "length must be a non-zero power of 2";        return h & (length-1);    }    /**     * 返回key-value映射个数     */    public int size() {        return size;    }    /**     * 判断map是否为空     */    public boolean isEmpty() {        return size == 0;    }    /**     * 返回指定key对应的value     */    public V get(Object key) {        //key为null情况        if (key == null)            return getForNullKey();                //根据key查找节点        Entry<K,V> entry = getEntry(key);        //返回key对应的值        return null == entry ? null : entry.getValue();    }    /**     * 查找key为null的value，注意如果key为null，则其hash值为0，默认是放在table[0]里的     */    private V getForNullKey() {        if (size == 0) {            return null;        }        //在table[0]的链表上查找key为null的键值对，因为null默认是存在table[0]的桶里        for (Entry<K,V> e = table[0]; e != null; e = e.next) {            if (e.key == null)                return e.value;        }        return null;    }    /**     *判断是否包含指定的key     */    public boolean containsKey(Object key) {        return getEntry(key) != null;    }    /**     * 根据key查找键值对，找不到返回null     */    final Entry<K,V> getEntry(Object key) {        if (size == 0) {            return null;        }        //如果key为null，hash值为0，否则调用hash方法，对key生成hash值        int hash = (key == null) ? 0 : hash(key);                //调用indexFor方法生成hash值的索引，遍历该索引下的链表，查找key“相等”的键值对        for (Entry<K,V> e = table[indexFor(hash, table.length)];             e != null;             e = e.next) {            Object k;            if (e.hash == hash &&                ((k = e.key) == key || (key != null && key.equals(k))))                return e;        }        return null;    }    /**     * 向map存入一个键值对，如果key已存在，则覆盖     */    public V put(K key, V value) {        //数组为空，对数组扩容        if (table == EMPTY_TABLE) {            inflateTable(threshold);        }                //对key为null的键值对调用putForNullKey处理        if (key == null)            return putForNullKey(value);                //生成hash值        int hash = hash(key);                //生成hash值索引        int i = indexFor(hash, table.length);                //查找是否有key“相等”的键值对，有的话覆盖        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;            }        }        //操作次数加一，用于迭代器快速失败行为        modCount++;                //在指定hash值索引处的链表上增加该键值对        addEntry(hash, key, value, i);        return null;    }    /**     * 存放key为null的键值对，存放在索引为0的链表上，已存在的话，替换     */    private V putForNullKey(V value) {        for (Entry<K,V> e = table[0]; e != null; e = e.next) {            //已存在key为null，则替换            if (e.key == null) {                V oldValue = e.value;                e.value = value;                e.recordAccess(this);                return oldValue;            }        }        //操作次数加一，用于迭代器快速失败行为        modCount++;        //在指定hash值索引处的链表上增加该键值对        addEntry(0, null, value, 0);        return null;    }    /**     * 添加键值对     */    private void putForCreate(K key, V value) {        //生成hash值        int hash = null == key ? 0 : hash(key);                //生成hash值索引，        int i = indexFor(hash, table.length);        /**         * key“相等”，则替换         */        for (Entry<K,V> e = table[i]; e != null; e = e.next) {            Object k;            if (e.hash == hash &&                ((k = e.key) == key || (key != null && key.equals(k)))) {                e.value = value;                return;            }        }        //在指定索引处的链表上创建该键值对        createEntry(hash, key, value, i);    }        //将制定map的键值对添加到map中    private void putAllForCreate(Map<? extends K, ? extends V> m) {        for (Map.Entry<? extends K, ? extends V> e : m.entrySet())            putForCreate(e.getKey(), e.getValue());    }    /**     * 对数组扩容     */    void resize(int newCapacity) {        Entry[] oldTable = table;        int oldCapacity = oldTable.length;                if (oldCapacity == MAXIMUM_CAPACITY) {            threshold = Integer.MAX_VALUE;            return;        }                //创建一个指定大小的数组        Entry[] newTable = new Entry[newCapacity];                transfer(newTable, initHashSeedAsNeeded(newCapacity));                //table索引替换成新数组        table = newTable;                //重新计算阈值        threshold = (int)Math.min(newCapacity * loadFactor, MAXIMUM_CAPACITY + 1);    }    /**     * 拷贝旧的键值对到新的哈希表中     */    void transfer(Entry[] newTable, boolean rehash) {        int newCapacity = newTable.length;        //遍历旧的数组        for (Entry<K,V> e : table) {            while(null != e) {                Entry<K,V> next = e.next;                if (rehash) {                    e.hash = null == e.key ? 0 : hash(e.key);                }                //根据新的数组长度，重新计算索引，                int i = indexFor(e.hash, newCapacity);                                //插入到链表表头                e.next = newTable[i];                                //将e放到索引为i处                newTable[i] = e;                                //将e设置成下个节点                e = next;            }        }    }    /**     * 将制定map的键值对put到本map，key“相等”的直接覆盖     */    public void putAll(Map<? extends K, ? extends V> m) {        int numKeysToBeAdded = m.size();        if (numKeysToBeAdded == 0)            return;        //空map，扩容        if (table == EMPTY_TABLE) {            inflateTable((int) Math.max(numKeysToBeAdded * loadFactor, threshold));        }        /*         * 判断是否需要扩容         */        if (numKeysToBeAdded > threshold) {            int targetCapacity = (int)(numKeysToBeAdded / loadFactor + 1);            if (targetCapacity > MAXIMUM_CAPACITY)                targetCapacity = MAXIMUM_CAPACITY;            int newCapacity = table.length;            while (newCapacity < targetCapacity)                newCapacity <<= 1;            if (newCapacity > table.length)                resize(newCapacity);        }        //依次遍历键值对，并put        for (Map.Entry<? extends K, ? extends V> e : m.entrySet())            put(e.getKey(), e.getValue());    }    /**     * 移除指定key的键值对     */    public V remove(Object key) {        Entry<K,V> e = removeEntryForKey(key);        return (e == null ? null : e.value);    }    /**     * 移除指定key的键值对     */    final Entry<K,V> removeEntryForKey(Object key) {        if (size == 0) {            return null;        }        //计算hash值及索引        int hash = (key == null) ? 0 : hash(key);        int i = indexFor(hash, table.length);                Entry<K,V> prev = table[i];        Entry<K,V> e = prev;        //头节点为table[i]的单链表上执行删除节点操作        while (e != null) {            Entry<K,V> next = e.next;            Object k;            //找到要删除的节点            if (e.hash == hash &&                ((k = e.key) == key || (key != null && key.equals(k)))) {                modCount++;                size--;                if (prev == e)                    table[i] = next;                else                    prev.next = next;                e.recordRemoval(this);                return e;            }            prev = e;            e = next;        }        return e;    }    /**     * 删除指定键值对对象(Entry对象)     */    final Entry<K,V> removeMapping(Object o) {        if (size == 0 || !(o instanceof Map.Entry))            return null;        Map.Entry<K,V> entry = (Map.Entry<K,V>) o;        Object key = entry.getKey();        int hash = (key == null) ? 0 : hash(key);        //得到数组索引        int i = indexFor(hash, table.length);        Entry<K,V> prev = table[i];        Entry<K,V> e = prev;        //开始遍历该单链表        while (e != null) {            Entry<K,V> next = e.next;            //找到节点            if (e.hash == hash && e.equals(entry)) {                modCount++;                size--;                if (prev == e)                    table[i] = next;                else                    prev.next = next;                e.recordRemoval(this);                return e;            }            prev = e;            e = next;        }        return e;    }    /**     * 清空map，将table数组所有元素设为null     */    public void clear() {        modCount++;        Arrays.fill(table, null);        size = 0;    }    /**     * 判断是否含有指定value的键值对     */    public boolean containsValue(Object value) {        if (value == null)            return containsNullValue();        Entry[] tab = table;        //遍历table数组        for (int i = 0; i < tab.length ; i++)            //遍历每条单链表            for (Entry e = tab[i] ; e != null ; e = e.next)                if (value.equals(e.value))                    return true;        return false;    }    /**     * 判断是否含有value为null的键值对     */    private boolean containsNullValue() {        Entry[] tab = table;        for (int i = 0; i < tab.length ; i++)            for (Entry e = tab[i] ; e != null ; e = e.next)                if (e.value == null)                    return true;        return false;    }    /**     * 浅拷贝，键值对不复制     */    public Object clone() {        HashMap<K,V> result = null;        try {            result = (HashMap<K,V>)super.clone();        } catch (CloneNotSupportedException e) {            // assert false;        }        if (result.table != EMPTY_TABLE) {            result.inflateTable(Math.min(                (int) Math.min(                    size * Math.min(1 / loadFactor, 4.0f),                    // we have limits...                    HashMap.MAXIMUM_CAPACITY),               table.length));        }        result.entrySet = null;        result.modCount = 0;        result.size = 0;        result.init();        result.putAllForCreate(this);        return result;    }    //内部类，节点对象，每个节点包含下个节点的引用    static class Entry<K,V> implements Map.Entry<K,V> {        final K key;        V value;        Entry<K,V> next;        int hash;        /**         * 创建节点         */        Entry(int h, K k, V v, Entry<K,V> n) {            value = v;            next = n;            key = k;            hash = h;        }        //获取节点的key        public final K getKey() {            return key;        }        //获取节点的value        public final V getValue() {            return value;        }                //设置新value，并返回旧的value        public final V setValue(V newValue) {            V oldValue = value;            value = newValue;            return oldValue;        }        //判断key和value是否相同,两个都“相等”，返回true        public final boolean equals(Object o) {            if (!(o instanceof Map.Entry))                return false;            Map.Entry e = (Map.Entry)o;            Object k1 = getKey();            Object k2 = e.getKey();            if (k1 == k2 || (k1 != null && k1.equals(k2))) {                Object v1 = getValue();                Object v2 = e.getValue();                if (v1 == v2 || (v1 != null && v1.equals(v2)))                    return true;            }            return false;        }        public final int hashCode() {            return Objects.hashCode(getKey()) ^ Objects.hashCode(getValue());        }        public final String toString() {            return getKey() + "=" + getValue();        }        /**         * This method is invoked whenever the value in an entry is         * overwritten by an invocation of put(k,v) for a key k that's already         * in the HashMap.         */        void recordAccess(HashMap<K,V> m) {        }        /**         * This method is invoked whenever the entry is         * removed from the table.         */        void recordRemoval(HashMap<K,V> m) {        }    }    /**     * 添加新节点，如有必要，执行扩容操作     */    void addEntry(int hash, K key, V value, int bucketIndex) {        if ((size >= threshold) && (null != table[bucketIndex])) {            resize(2 * table.length);            hash = (null != key) ? hash(key) : 0;            bucketIndex = indexFor(hash, table.length);        }        createEntry(hash, key, value, bucketIndex);    }    /**     * 插入单链表表头     */    void createEntry(int hash, K key, V value, int bucketIndex) {        Entry<K,V> e = table[bucketIndex];        table[bucketIndex] = new Entry<>(hash, key, value, e);        size++;    }    //hashmap迭代器    private abstract class HashIterator<E> implements Iterator<E> {        Entry<K,V> next;        // 下个键值对索引        int expectedModCount;   // 用于判断快速失败行为        int index;              // current slot        Entry<K,V> current;     // current entry        HashIterator() {            expectedModCount = modCount;            if (size > 0) { // advance to first entry                Entry[] t = table;                while (index < t.length && (next = t[index++]) == null)                    ;            }        }        public final boolean hasNext() {            return next != null;        }        final Entry<K,V> nextEntry() {            if (modCount != expectedModCount)                throw new ConcurrentModificationException();            Entry<K,V> e = next;            if (e == null)                throw new NoSuchElementException();            if ((next = e.next) == null) {                Entry[] t = table;                while (index < t.length && (next = t[index++]) == null)                    ;            }            current = e;            return e;        }        public void remove() {            if (current == null)                throw new IllegalStateException();            if (modCount != expectedModCount)                throw new ConcurrentModificationException();            Object k = current.key;            current = null;            HashMap.this.removeEntryForKey(k);            expectedModCount = modCount;        }    }    //ValueIterator迭代器    private final class ValueIterator extends HashIterator<V> {        public V next() {            return nextEntry().value;        }    }    //KeyIterator迭代器    private final class KeyIterator extends HashIterator<K> {        public K next() {            return nextEntry().getKey();        }    }    ////KeyIterator迭代器    private final class EntryIterator extends HashIterator<Map.Entry<K,V>> {        public Map.Entry<K,V> next() {            return nextEntry();        }    }    // 返回迭代器方法    Iterator<K> newKeyIterator()   {        return new KeyIterator();    }    Iterator<V> newValueIterator()   {        return new ValueIterator();    }    Iterator<Map.Entry<K,V>> newEntryIterator()   {        return new EntryIterator();    }    // Views    private transient Set<Map.Entry<K,V>> entrySet = null;    /**     * 返回一个set集合，包含key     */    public Set<K> keySet() {        Set<K> ks = keySet;        return (ks != null ? ks : (keySet = new KeySet()));    }    private final class KeySet extends AbstractSet<K> {        public Iterator<K> iterator() {            return newKeyIterator();        }        public int size() {            return size;        }        public boolean contains(Object o) {            return containsKey(o);        }        public boolean remove(Object o) {            return HashMap.this.removeEntryForKey(o) != null;        }        public void clear() {            HashMap.this.clear();        }    }    /**     * 返回一个value集合，包含value     */    public Collection<V> values() {        Collection<V> vs = values;        return (vs != null ? vs : (values = new Values()));    }    private final class Values extends AbstractCollection<V> {        public Iterator<V> iterator() {            return newValueIterator();        }        public int size() {            return size;        }        public boolean contains(Object o) {            return containsValue(o);        }        public void clear() {            HashMap.this.clear();        }    }    /**     * 返回一个键值对集合     */    public Set<Map.Entry<K,V>> entrySet() {        return entrySet0();    }    private Set<Map.Entry<K,V>> entrySet0() {        Set<Map.Entry<K,V>> es = entrySet;        return es != null ? es : (entrySet = new EntrySet());    }    private final class EntrySet extends AbstractSet<Map.Entry<K,V>> {        public Iterator<Map.Entry<K,V>> iterator() {            return newEntryIterator();        }        public boolean contains(Object o) {            if (!(o instanceof Map.Entry))                return false;            Map.Entry<K,V> e = (Map.Entry<K,V>) o;            Entry<K,V> candidate = getEntry(e.getKey());            return candidate != null && candidate.equals(e);        }        public boolean remove(Object o) {            return removeMapping(o) != null;        }        public int size() {            return size;        }        public void clear() {            HashMap.this.clear();        }    }    /**     * map序列化,可实现深拷贝     */    private void writeObject(java.io.ObjectOutputStream s)        throws IOException    {        // Write out the threshold, loadfactor, and any hidden stuff        s.defaultWriteObject();        // Write out number of buckets        if (table==EMPTY_TABLE) {            s.writeInt(roundUpToPowerOf2(threshold));        } else {           s.writeInt(table.length);        }        // Write out size (number of Mappings)        s.writeInt(size);        // Write out keys and values (alternating)        if (size > 0) {            for(Map.Entry<K,V> e : entrySet0()) {                s.writeObject(e.getKey());                s.writeObject(e.getValue());            }        }    }    private static final long serialVersionUID = 362498820763181265L;    /**     * 反序列化，读取字节码转为对象     */    private void readObject(java.io.ObjectInputStream s)         throws IOException, ClassNotFoundException    {        // Read in the threshold (ignored), loadfactor, and any hidden stuff        s.defaultReadObject();        if (loadFactor <= 0 || Float.isNaN(loadFactor)) {            throw new InvalidObjectException("Illegal load factor: " +                                               loadFactor);        }        // set other fields that need values        table = (Entry<K,V>[]) EMPTY_TABLE;        // Read in number of buckets        s.readInt(); // ignored.        // Read number of mappings        int mappings = s.readInt();        if (mappings < 0)            throw new InvalidObjectException("Illegal mappings count: " +                                               mappings);        // capacity chosen by number of mappings and desired load (if >= 0.25)        int capacity = (int) Math.min(                    mappings * Math.min(1 / loadFactor, 4.0f),                    // we have limits...                    HashMap.MAXIMUM_CAPACITY);        // allocate the bucket array;        if (mappings > 0) {            inflateTable(capacity);        } else {            threshold = capacity;        }        init();  // Give subclass a chance to do its thing.        // Read the keys and values, and put the mappings in the HashMap        for (int i = 0; i < mappings; i++) {            K key = (K) s.readObject();            V value = (V) s.readObject();            putForCreate(key, value);        }    }    // These methods are used when serializing HashSets    int   capacity()     { return table.length; }    float loadFactor()   { return loadFactor;   }}}

简单使用示例

package com.pichen.collection;import java.util.HashMap;import java.util.Map;public class Main {    public static void main(String[] args) {        Map<String, Integer> map = new HashMap<String, Integer>();        //put方法        map.put("A", 5);        map.put("B", 6);        map.put("C", 7);        map.put("D", 8);                //重写了toString方法        System.out.println(map);                //size方法        System.out.println(map.size());                System.out.println(map.containsKey("A"));        System.out.println(map.containsValue(6));        System.out.println(map.get("B"));                //remove        map.remove("C");        System.out.println(map);                //key集合        for(String str:map.keySet()){            System.out.print(str + " ");        }                System.out.println();        //value集合        for(Integer obj:map.values()){            System.out.print(obj + " ");        }                System.out.println();        //key-value集合        for(Map.Entry<String, Integer> entry:map.entrySet()){            System.out.print(entry.getKey() + ": " + entry.getValue() + ", ");        }    }}