interface Collection {

    ...

}

interface Set extends Collection {

    ...

}

class HashSet implements Set {

    private static final Object PRESENT = new Object();

    private transient HashMap<E,Object> map;

    public HashSet() {

        map = new HashMap<>();

    }

    public boolean add(E e) { //e=hello,world

        return map.put(e, PRESENT)==null;

    }

}

class HashMap implements Map {

    public V put(K key, V value) { //key=e=hello,world

        //看哈希表是否为空，如果空，就开辟空间

        if (table == EMPTY_TABLE) {

            inflateTable(threshold);

        }

        //判断对象是否为null

        if (key == null)

            return putForNullKey(value);

        int hash = hash(key); //和对象的hashCode()方法相关

        //在哈希表中查找hash值

        int i = indexFor(hash, table.length);

        for (Entry<K,V> e = table[i]; e != null; e = e.next) {

            //这次的e其实是第一次的world

            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;

                //走这里其实是没有添加元素

            }

        }

        modCount++;

        addEntry(hash, key, value, i); //把元素添加

        return null;

    }

    transient int hashSeed = 0;

    final int hash(Object k) { //k=key=e=hello,

        int h = hashSeed;

        if (0 != h && k instanceof String) {

            return sun.misc.Hashing.stringHash32((String) k);

        }

        h ^= k.hashCode(); //这里调用的是对象的hashCode()方法

        // 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).

        h ^= (h >>> 20) ^ (h >>> 12);

        return h ^ (h >>> 7) ^ (h >>> 4);

    }

}

//对应源码 addEntry(hash, key, value, i); //把元素添加

void addEntry(int hash, K key, V value, int bucketIndex) {

    Entry<K,V> e = table[bucketIndex];

        table[bucketIndex] = new Entry<K,V>(hash, key, value, e);

        if (size++ >= threshold)

            resize(2 * table.length);

    }

hs.add("hello");//由于table里面没有，直接进addEntry

hs.add("world");//同上

hs.add("java");//同上

hs.add("world");//table里已经存在，(hash和equals相等),则在put()时，用新值替代老值。ps:这里的老值和新值都是world。

总结：