TreeSet.add()方法一细节

public boolean add(E e) {
	return m.put(e, PRESENT)==null;
}

public V put(K key, V value) {
    Entry<K,V> t = root;//根元素
    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) { //如果传入了比较器
        do {
            parent = t;
            cmp = cpr.compare(key, t.key);  //比较两个元素的key值，也就是Set中元素的值
            if (cmp < 0) //如果小于0则放到当前元素的左边
                t = t.left;
            else if (cmp > 0) //如果大于0则放到当前元素的右边
                t = t.right;
            else
                return t.setValue(value);  //否则，即返回0时调用java.util.TreeMap.Entry.setValue(value)方法
        } while (t != null);
    }
    //后面代码省略......
}

public V setValue(V value) {
    V oldValue = this.value;
    this.value = value;
    return oldValue;
}