首先从构造方法说起，

初始化有两个参数
initialCapacity 代表初始容量，实际容量为大于initialCapacity的2的幂次数
之后会初始化一个长度为initialCapacity的数组，姑且把数组中每个元素定义为一个桶。默认桶内个数为0 。随着桶中个数的增加，当平均每个桶的元素个数大于loadFactor时，则触发重构。

   public HashMap(int initialCapacity, float loadFactor) {
        if (initialCapacity < 0)
            throw new IllegalArgumentException("Illegal initial capacity: " +
                                               initialCapacity);
        if (initialCapacity > MAXIMUM_CAPACITY) {
            initialCapacity = MAXIMUM_CAPACITY;
        } else if (initialCapacity < DEFAULT_INITIAL_CAPACITY) {
            initialCapacity = DEFAULT_INITIAL_CAPACITY;
        }

        if (loadFactor <= 0 || Float.isNaN(loadFactor))
            throw new IllegalArgumentException("Illegal load factor: " +
                                               loadFactor);
        // Android-Note: We always use the default load factor of 0.75f.

        // This might appear wrong but it's just awkward design. We always call
        // inflateTable() when table == EMPTY_TABLE. That method will take "threshold"
        // to mean "capacity" and then replace it with the real threshold (i.e, multiplied with
        // the load factor).
        threshold = initialCapacity;
        init();
    }

HashMap里存放的是一个特殊的entity，格式如下

 static class HashMapEntry<K,V> implements Map.Entry<K,V> {
        final K key;
        V value;
        HashMapEntry<K,V> next;
        int hash;

        /**
         * Creates new entry.
         */
        HashMapEntry(int h, K k, V v, HashMapEntry<K,V> n) {
            value = v;
            next = n;
            key = k;
            hash = h;
        }

        public final K getKey() {
            return key;
        }

        public final V getValue() {
            return value;
        }

        public final V setValue(V newValue) {
            V oldValue = value;
            value = newValue;
            return oldValue;
        }

        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) {
        }
    }

既然是HashMap那肯定跟Hash有关。每次添加元素时，根据key的hash值计算出一个hashint，之后根据hashing%桶的个数获取一个index，再根据这个index吧元素放到对应的桶里。如果桶里有旧对象则让新对象指向旧对象。如果key为null则放在第0个桶里

public V put(K key, V value) {
        if (table == EMPTY_TABLE) {
            inflateTable(threshold);
        }
        if (key == null)
            return putForNullKey(value);
        int hash = sun.misc.Hashing.singleWordWangJenkinsHash(key);
        int i = indexFor(hash, table.length);
        for (HashMapEntry<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;
            }
        }

        modCount++;
        addEntry(hash, key, value, i);
        return null;
    }

get方法也是类似根据key确定具体哪个桶里再遍历链表比对key值获取元素

 final Entry<K,V> getEntry(Object key) {
        if (size == 0) {
            return null;
        }

        int hash = (key == null) ? 0 : sun.misc.Hashing.singleWordWangJenkinsHash(key);
        for (HashMapEntry<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;
    }

当总对象个数超过预定阈值触发resize 桶的个数翻倍

 void resize(int newCapacity) {
        HashMapEntry[] oldTable = table;
        int oldCapacity = oldTable.length;
        if (oldCapacity == MAXIMUM_CAPACITY) {
            threshold = Integer.MAX_VALUE;
            return;
        }

        HashMapEntry[] newTable = new HashMapEntry[newCapacity];
        transfer(newTable);
        table = newTable;
        threshold = (int)Math.min(newCapacity * loadFactor, MAXIMUM_CAPACITY + 1);
    }