TreeMap详解

原文链接http://zhhll.icu/2021/01/14/java%E5%9F%BA%E7%A1%80/%E9%9B%86%E5%90%88/TreeMap%E8%AF%A6%E8%A7%A3/

TreeMap详解

TreeMap是Map接口的一个实现类，底层基于红黑树的实现

从继承机构可以看到TreeMap除了继承了AbstractMap类，还实现了NavigableMap接口，而NavigableMap接口是继承自SortedMap接口的，所以TreeMap是可以进行排序的

关键变量

// 比较器，根据比较器来决定TreeMap的排序，如果为空，按照key做自然排序(最小的在根节点)
private final Comparator<? super K> comparator;
// 根节点
private transient Entry<K,V> root;

/**
 * The number of entries in the tree
 * 树的大小
 */
private transient int size = 0;

/**
 * The number of structural modifications to the tree.
 * 修改次数
 */
private transient int modCount = 0;

// Entry为TreeMap的内部类
static final class Entry<K,V> implements Map.Entry<K,V> {
        K key;
        V value;
        Entry<K,V> left;
        Entry<K,V> right;
        Entry<K,V> parent;
        boolean color = BLACK;
}

构造函数

// 默认空参构造器，比较器设置为空
public TreeMap() {
    comparator = null;
}
// 提供比较器
public TreeMap(Comparator<? super K> comparator) {
  this.comparator = comparator;
}

public TreeMap(Map<? extends K, ? extends V> m) {
  comparator = null;
  putAll(m);
}

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) {
  }
}

get方法

public V get(Object key) {
    Entry<K,V> p = getEntry(key);
    return (p==null ? null : p.value);
}

final Entry<K,V> getEntry(Object key) {
  // Offload comparator-based version for sake of performance
  if (comparator != null)
    return getEntryUsingComparator(key);
  // 从这里可以看出TreeMap的key不可以为null
  if (key == null)
    throw new NullPointerException();
  @SuppressWarnings("unchecked")
  Comparable<? super K> k = (Comparable<? super K>) key;
  // 获取根节点
  Entry<K,V> p = root;
  while (p != null) {
    // 判断是根节点的左子树还是右子树
    int cmp = k.compareTo(p.key);
    if (cmp < 0)
      p = p.left;
    else if (cmp > 0)
      p = p.right;
    else
      return p;
  }
  return null;
}

put方法

public V put(K key, V value) {
    Entry<K,V> t = root;
  	// 根节点为null，表示这是第一个元素
    if (t == null) {
      	// 主要是为了确保key是可排序的类，以及key不能为null
        compare(key, key); // type (and possibly null) check
				// 第三个参数为父节点的entry，根节点没有父节点，所以为null
        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;
  	// 存在比较器的情况
    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);
    }
  	// 不存在比较器，进行自然排序
    else {
      	// key不能为null
        if (key == null)
            throw new NullPointerException();
        @SuppressWarnings("unchecked")
            Comparable<? super K> k = (Comparable<? super K>) key;
      // do...while是为了找到该key所要存放的位置(找到父节点)
        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);
  	// 比父节点小，是左子树
    if (cmp < 0)
        parent.left = e;
    else
        parent.right = e;
  	// 插入之后还要进行平衡操作
    fixAfterInsertion(e);
    size++;
    modCount++;
    return null;
}

private void fixAfterInsertion(Entry<K,V> x) {
  x.color = RED;

  while (x != null && x != root && x.parent.color == RED) {
    if (parentOf(x) == leftOf(parentOf(parentOf(x)))) {
      Entry<K,V> y = rightOf(parentOf(parentOf(x)));
      if (colorOf(y) == RED) {
        setColor(parentOf(x), BLACK);
        setColor(y, BLACK);
        setColor(parentOf(parentOf(x)), RED);
        x = parentOf(parentOf(x));
      } else {
        if (x == rightOf(parentOf(x))) {
          x = parentOf(x);
          rotateLeft(x);
        }
        setColor(parentOf(x), BLACK);
        setColor(parentOf(parentOf(x)), RED);
        rotateRight(parentOf(parentOf(x)));
      }
    } else {
      Entry<K,V> y = leftOf(parentOf(parentOf(x)));
      if (colorOf(y) == RED) {
        setColor(parentOf(x), BLACK);
        setColor(y, BLACK);
        setColor(parentOf(parentOf(x)), RED);
        x = parentOf(parentOf(x));
      } else {
        if (x == leftOf(parentOf(x))) {
          x = parentOf(x);
          rotateRight(x);
        }
        setColor(parentOf(x), BLACK);
        setColor(parentOf(parentOf(x)), RED);
        rotateLeft(parentOf(parentOf(x)));
      }
    }
  }
  root.color = BLACK;
}

remove方法

public V remove(Object key) {
  	// 获取到该key对应的节点 和get相同
    Entry<K,V> p = getEntry(key);
    if (p == null)
        return null;

    V oldValue = p.value;
    deleteEntry(p);
    return oldValue;
}

private void deleteEntry(Entry<K,V> p) {
  modCount++;
  size--;

  // If strictly internal, copy successor's element to p and then make p
  // point to successor.
  // 存在两个子树(左子树和右子树)
  if (p.left != null && p.right != null) {
    // 找到与p数值最接近的节点(即右子树的最左叶子节点)
    Entry<K,V> s = successor(p);
    p.key = s.key;
    p.value = s.value;
    p = s;
  } // p has 2 children

  // Start fixup at replacement node, if it exists.
  // 找到所要替代的节点
  Entry<K,V> replacement = (p.left != null ? p.left : p.right);

  if (replacement != null) {
    // Link replacement to parent
    // 替换节点
    replacement.parent = p.parent;
    if (p.parent == null)
      root = replacement;
    else if (p == p.parent.left)
      p.parent.left  = replacement;
    else
      p.parent.right = replacement;

    // Null out links so they are OK to use by fixAfterDeletion.
    p.left = p.right = p.parent = null;

    // Fix replacement
    // 删除的节点为黑色节点，需要进行平衡
    if (p.color == BLACK)
      fixAfterDeletion(replacement);
  } 
  // 此时replacement为null(表明 p没有左子树也没有右子树)，如果p没有父节点，表明该树只有一个根节点
  else if (p.parent == null) { // return if we are the only node.
    root = null;
  } 
  // 此时replacement为null(表明 p没有左子树也没有右子树)，表明该节点为叶子节点
  else { //  No children. Use self as phantom replacement and unlink.
    // 删除的节点为黑色节点，需要进行平衡
    if (p.color == BLACK)
      fixAfterDeletion(p);
		// 将p从树中移除
    if (p.parent != null) {
      if (p == p.parent.left)
        p.parent.left = null;
      else if (p == p.parent.right)
        p.parent.right = null;
      p.parent = null;
    }
  }
}

static <K,V> TreeMap.Entry<K,V> successor(Entry<K,V> t) {
  if (t == null)
    return null;
  else if (t.right != null) {
    // 右节点不为null，找到后继节点(即右子树的左叶子节点)
    Entry<K,V> p = t.right;
    while (p.left != null)
      p = p.left;
    return p;
  } else {
    Entry<K,V> p = t.parent;
    Entry<K,V> ch = t;
    while (p != null && ch == p.right) {
      ch = p;
      p = p.parent;
    }
    return p;
  }
}

private void fixAfterDeletion(Entry<K,V> x) {
  while (x != root && colorOf(x) == BLACK) {
    if (x == leftOf(parentOf(x))) {
      Entry<K,V> sib = rightOf(parentOf(x));

      if (colorOf(sib) == RED) {
        setColor(sib, BLACK);
        setColor(parentOf(x), RED);
        rotateLeft(parentOf(x));
        sib = rightOf(parentOf(x));
      }

      if (colorOf(leftOf(sib))  == BLACK &&
          colorOf(rightOf(sib)) == BLACK) {
        setColor(sib, RED);
        x = parentOf(x);
      } else {
        if (colorOf(rightOf(sib)) == BLACK) {
          setColor(leftOf(sib), BLACK);
          setColor(sib, RED);
          rotateRight(sib);
          sib = rightOf(parentOf(x));
        }
        setColor(sib, colorOf(parentOf(x)));
        setColor(parentOf(x), BLACK);
        setColor(rightOf(sib), BLACK);
        rotateLeft(parentOf(x));
        x = root;
      }
    } else { // symmetric
      Entry<K,V> sib = leftOf(parentOf(x));

      if (colorOf(sib) == RED) {
        setColor(sib, BLACK);
        setColor(parentOf(x), RED);
        rotateRight(parentOf(x));
        sib = leftOf(parentOf(x));
      }

      if (colorOf(rightOf(sib)) == BLACK &&
          colorOf(leftOf(sib)) == BLACK) {
        setColor(sib, RED);
        x = parentOf(x);
      } else {
        if (colorOf(leftOf(sib)) == BLACK) {
          setColor(rightOf(sib), BLACK);
          setColor(sib, RED);
          rotateLeft(sib);
          sib = leftOf(parentOf(x));
        }
        setColor(sib, colorOf(parentOf(x)));
        setColor(parentOf(x), BLACK);
        setColor(leftOf(sib), BLACK);
        rotateRight(parentOf(x));
        x = root;
      }
    }
  }

  setColor(x, BLACK);
}

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