public V put(K key, V value) {
    Entry<K, V> t = root;
    if (t == null) {
        // TBD:
        // 5045147: (coll) Adding null to an empty TreeSet should
        // throw NullPointerException
        //
        // compare(key, key); // type check
        root = new Entry<K, V>(key, value, null);
        size = 1;
        modCount++;
        return null;
    }
    int cmp;
    Entry<K, V> parent;
    // split comparator and comparable paths
    Comparator<? super K> cpr = comparator;
    if (cpr != null) {
        do {
            parent = t;
            cmp = cpr.compare(key, t.key);
            if (cmp < 0)
                t = t.left;
            else if (cmp > 0)
                t = t.right;
            else
                return t.setValue(value);
        } while (t != null);
    } else {
        if (key == null)
            throw new NullPointerException();
        Comparable<? super K> k = (Comparable<? super K>) key;
        do {
            parent = t;
            cmp = k.compareTo(t.key);
            if (cmp < 0)
                t = t.left;
            else if (cmp > 0)
                t = t.right;
            else
                return t.setValue(value);
        } while (t != null);
    }
    Entry<K, V> e = new Entry<K, V>(key, value, parent);
    if (cmp < 0)
        parent.left = e;
    else
        parent.right = e;
    fixAfterInsertion(e);
    size++;
    modCount++;
    return null;
}

root是这颗红黑树的Entry型节点，主要是比较key，key大的节点成为中心节点的右子节点，key小的成为中心节点的左子节点， fixAfterInsertion(e)方法时在插入后按照以上顺序来修复这棵树，因为插入后可能会形成这种局面：

a

??? b

??? ?? c

修复后：

??? b

a???? c

?

size呢表示树的节点数，也就是key-value mappings数量

?