資訊專欄INFORMATION COLUMN
摘要：基于紅黑樹實現，在之前篇章中有所涉及，所以本篇重點不在此。費解順帶一提，如果你還記得之前文章中的也用到了紅黑樹，而它先比較的再比值，這比較的是值。在這的作用類似中的，修復紅黑樹性質。
TreeMap基于紅黑樹實現，在之前HashMap篇章中有所涉及，所以本篇重點不在此。上路~containsKey() --> getEntry() --> getEntryUsingComparator()
/**
* Returns {@code true} if this map contains a mapping for the specified
* key.
*
* @param key key whose presence in this map is to be tested
* @return {@code true} if this map contains a mapping for the
* specified key
* @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 boolean containsKey(Object key) {
return getEntry(key) != null; // Key不能為null
}
/**
* Returns this map"s entry for the given key, or {@code null} if the map
* does not contain an entry for the key.
*
* @return this map"s entry for the given key, or {@code null} if the map
* does not contain an entry for the key
* @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
*/
final Entry 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 k = (Comparable) key;
Entry 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;
}
/**
* Version of getEntry using comparator. Split off from getEntry
* for performance. (This is not worth doing for most methods,
* that are less dependent on comparator performance, but is
* worthwhile here.)
*/
final Entry getEntryUsingComparator(Object key) {
@SuppressWarnings("unchecked")
K k = (K) key;
Comparator cpr = comparator;
if (cpr != null) {
Entry p = root;
while (p != null) {
int cmp = cpr.compare(k, p.key);
if (cmp < 0)
p = p.left;
else if (cmp > 0)
p = p.right;
else
return p;
}
}
return null;
}
雖然明面上是獲取值的方法，本質卻是比出個高低等。這里將Java的java.util.Comparator（比較器排序）、java.lang.Comparable（自然排序）都用到了。順便補了兩者知識點(見文末①)。當然這里好奇的是源碼中將使用comparator比較獨立提成方法，說是能提高性能。why?反向思考下，假使將getEntryUsingComparator()方法內代碼放回getEntry()似乎也就多了一對“{}”。費解- -
順帶一提，如果你還記得之前文章中的HashMap也用到了紅黑樹，而它先比較的hash再比key值，這比較的是key值。
put() --> compare() --> fixAfterInsertion()/**
* 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 {@code key}, or
* {@code null} if there was no mapping for {@code key}.
* (A {@code null} return can also indicate that the map
* previously associated {@code null} with {@code key}.)
* @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 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 parent;
// split comparator and comparable paths
Comparator 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 k = (Comparable) 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 e = new Entry<>(key, value, parent);
if (cmp < 0)
parent.left = e;
else
parent.right = e;
fixAfterInsertion(e);
size++;
modCount++;
return null;
}
/**
* Compares two keys using the correct comparison method for this TreeMap.
*/
@SuppressWarnings("unchecked")
final int compare(Object k1, Object k2) {
return comparator==null ? ((Comparable)k1).compareTo((K)k2)
: comparator.compare((K)k1, (K)k2);
}
/** From CLR */
private void fixAfterInsertion(Entry x) {
x.color = RED;
while (x != null && x != root && x.parent.color == RED) {
if (parentOf(x) == leftOf(parentOf(parentOf(x)))) {
Entry 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 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;
}
"compare(key, key);"是一個有意思寫法。從注釋直譯就是類型（為null可能性）檢查。為空檢查很好理解，因為null.xx()肯定跑異常，至于類型檢查筆者理解是要求鍵值實現Comparable接口。
"from CLR"是指Cormen, Leiserson, Rivest，他們是算法導論的作者，也就是說TreeMap里面算法都是參照算法導論的偽代碼。
由于TreeMap的有序性，使其增刪查都是先進行比較，找到合適的位置。fixAfterInsertion()在這的作用類似HashMap中的balanceInsertion()，修復紅黑樹性質。
deleteEntry() --> successor() --> fixAfterDeletion()
