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TreeMap 是一个有序的key-value集合，它是通过 红黑树 实现的。
TreeMap 继承于AbstractMap，所以它是一个Map，即一个key-value集合。
TreeMap 实现了NavigableMap，Cloneable和Serializable接口。
TreeMap的基本操作 containsKey、get、put 和 remove 的时间复杂度是 log(n) 。
首先是TreeMap的构造方法：
public TreeMap() { comparator = null; }
/** * Constructs a new, empty tree map, ordered according to the given comparator. */ public TreeMap(Comparator<? super K> comparator) { this.comparator = comparator; } /** * Constructs a new tree map containing the same mappings as the given * map, ordered according to the <em>natural ordering</em> of its keys. */ public TreeMap(Map<? extends K, ? extends V> m) { comparator = null; putAll(m); } /** * Constructs a new tree map containing the same mappings and * using the same ordering as the specified sorted map. This * method runs in linear time. */ public TreeMap(SortedMap<K, ? extends V> m) { comparator = m.comparator(); try { buildFromSorted(m.size(), m.entrySet().iterator(), null, null); } catch (java.io.IOException cannotHappen) { } catch (ClassNotFoundException cannotHappen) { } }
TreeMap是基于红黑树实现的，以下是树结点的定义，主要key(键)、value(值)、left(左孩子)、right(右孩子)、parent(父节点)、color(颜色)六个字段，根据key的值进行排序。该内部类比较简单，不做分析。
static final class Entry<K,V> implements Map.Entry<K,V> { K key; V value; Entry<K,V> left = null; Entry<K,V> right = null; Entry<K,V> parent; boolean color = BLACK; /** * Make a new cell with given key, value, and parent, and with * {@code null} child links, and BLACK color. */ Entry(K key, V value, Entry<K,V> parent) { this.key = key; this.value = value; this.parent = parent; } ...... }
以下是红黑树的插入put和删除deleteEntry操作，以及执行插入删除时需要用到的操作：左旋rotateLeft、右旋rotateRight、插入修正fixAfterInsertion和删除修正fixAfterDeletion。
插入操作，先找到要插入的位置，插入新结点，调用fixAfterInsertion对插入结果进行修正：
public V put(K key, V value) { Entry<K,V> t = root; if (t == null) { compare(key, key); // type (and possibly null) check root = new Entry<>(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<>(key, value, parent); if (cmp < 0) parent.left = e; else parent.right = e; fixAfterInsertion(e); size++; modCount++; return null; }
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原文地址：http://www.cnblogs.com/yitong0768/p/4561608.html
