TreeMap原理（源码学习）

TreeMap是一种实现了有序的Map类型。

TreeMap基于红黑树原理实现。

TreeMap是线程不安全的。

TreeMap实现了 NavigableMap<K,V>接口支持导航方法，TreeMap继承自AbstractMap<K,V>。

put方法原理

1、判断有没有root节点。没有则创建一个root（节点用new Entry<K,V>）表示。modCount++

2、定义一个父节点parent记录节点搜索路径，一个比较记录cmp

3、判断比较器是否存在，不存在则使用默认比较器Comparable对Key进行装饰。key直接调用比较方法。

4、根据比较结果沿左子树和右子树遍历节点，得到与key相等的节点，执行setValue方法。

5、如果一直没有找到节点，因为parent记录了t的搜索路径，所以new Entry<K,V>(key, value, parent)，构造新的Entry节点，挂载到parent。

put方法源码

    /**     * Associates the specified value with the specified key in this map.     * If the map previously contained a mapping for the key, the old     * value is replaced.     *     * @param key key with which the specified value is to be associated     * @param value value to be associated with the specified key     *     * @return the previous value associated with <tt>key</tt>, or     *         <tt>null</tt> if there was no mapping for <tt>key</tt>.     *         (A <tt>null</tt> return can also indicate that the map     *         previously associated <tt>null</tt> with <tt>key</tt>.)     * @throws ClassCastException if the specified key cannot be compared     *         with the keys currently in the map     * @throws NullPointerException if the specified key is null     *         and this map uses natural ordering, or its comparator     *         does not permit null keys     */    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;    }