TreeMap源码解读

插入(put)

源码

    public V put(K key, V value) {
        Entry<K,V> t = root;
        //如果是第一次put, 根节点肯定为null
        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();
            @SuppressWarnings("unchecked")
                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;
    }

记录点

• 在第一次插入的时候为何进行compare操作？ ```java public V put(K key, V value) {

    Entry<K,V> t = root;
    if (t == null) {
        //第一次put，为啥还要进行compare呢？原因是需要校验K是否是可比较的Type（即是否实现了Comparable）
        //如果不是，则会报错（转换异常ClassCastException）
        compare(key, key); // type (and possibly null) check
        //...
    }
    //...

  }

  @SuppressWarnings(“unchecked”) final int compare(Object k1, Object k2) {

    return comparator==null ? ((Comparable<? super K>)k1).compareTo((K)k2) : comparator.compare((K)k1, (K)k2);

  }

> 另外，idea里面会有相应提示，如图：
![image.png](https://cdn.nlark.com/yuque/0/2021/png/876305/1612517483848-6202933f-0a9b-4607-98e4-c6a068dc2d7f.png#align=left&display=inline&height=109&margin=%5Bobject%20Object%5D&name=image.png&originHeight=109&originWidth=781&size=16185&status=done&style=none&width=781)
> 说明：有序的集合构造使用了不可比较的元素，也就是说：作为TreeMap的key的泛型类，需要实现Comparable！
- 如果未指定比较器，当非第一次put的时候，put null，此时会抛 NPE 
```java
    public V put(K key, V value) {
        //...
        Comparator<? super K> cpr = comparator;
        if (cpr != null) {
            //...
        }
        else {
            if (key == null)
                throw new NullPointerException();
            @SuppressWarnings("unchecked")
                Comparable<? super K> k = (Comparable<? super K>) key;
            //...
    }

则说明：如果比较器未对 null值做特殊处理，就会报NPE；也就是说，如果不能保证key不为null，比较器必须对null做处理！

删除（remove）

源码

    public V remove(Object key) {
        Entry<K,V> p = getEntry(key);
        if (p == null)
            return null;
        V oldValue = p.value;
        deleteEntry(p);
        return oldValue;
    }
    /**
     * Delete node p, and then rebalance the tree.
     */
    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.
        if (p.left != null && p.right != null) {
            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.
        Entry<K,V> replacement = (p.left != null ? p.left : p.right);
        if (replacement != null) {
            // Link replacement to parent
            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
            if (p.color == BLACK)
                fixAfterDeletion(replacement);
        } else if (p.parent == null) { // return if we are the only node.
            root = null;
        } else { //  No children. Use self as phantom replacement and unlink.
            if (p.color == BLACK)
                fixAfterDeletion(p);
            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;
            }
        }
    }

查询（get）

源码

    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);
        if (key == null)
            throw new NullPointerException();
        @SuppressWarnings("unchecked")
            Comparable<? super K> k = (Comparable<? super K>) key;
        Entry<K,V> p = root;
        while (p != null) {
            int cmp = k.compareTo(p.key);
            if (cmp < 0)
                p = p.left;
            else if (cmp > 0)
                p = p.right;
            else
                return p;
        }
        return null;
    }

总结：

1. 数据结构：红黑树，使用红黑树保证树的平衡，同时能够把查找时间控制在 O(logn)；
2. put和remove操作会触发调整红黑树