TreeSet 是一种可有序存放元素的集合，HashSet 是 value 为固定值的 HashMap，TreeSet 是 value 为固定值得 TreeMap。
TreeMap
┌───┐
│Map│
└───┘
▲
┌────┴─────┐
│ │
┌───────┐ ┌─────────┐
│HashMap│ │SortedMap│
└───────┘ └─────────┘
▲
│
┌─────────┐
│ TreeMap │
└─────────┘
HashMap 利用了 hashCode，TreeMap 则利用了树，一个二叉树。
比较 Key
TreeMap 的有序通过比较 key 来实现，无法利用 hashCode 来比较，它需要有一个比较 key 的规则。可通过 Key 继承 Comparable 接口或设置 Comparator 来提供。
Integer
public final class Integer extends Number implements Comparable<Integer> {
...
public int compareTo(Integer anotherInteger) {
return compare(this.value, anotherInteger.value);
}
// 根据元素大小比较，-1 小于 anotherInteger，0 等于，1 大于
public static int compare(int x, int y) {
return (x < y) ? -1 : ((x == y) ? 0 : 1);
}
...
}
String
public final class String
implements java.io.Serializable, Comparable<String>, CharSequence {
...
public int compareTo(String anotherString) {
byte v1[] = value;
byte v2[] = anotherString.value;
if (coder() == anotherString.coder()) {
return isLatin1() ? StringLatin1.compareTo(v1, v2)
: StringUTF16.compareTo(v1, v2);
}
return isLatin1() ? StringLatin1.compareToUTF16(v1, v2)
: StringUTF16.compareToLatin1(v1, v2);
}
...
}
final class StringLatin1 {
...
public static int compareTo(byte[] value, byte[] other, int len1, int len2) {
// 只比较短的部分
int lim = Math.min(len1, len2);
for (int k = 0; k < lim; k++) {
// 返回不同的字符的 assic 差值
if (value[k] != other[k]) {
return getChar(value, k) - getChar(other, k);
}
}
// 当短部分都相同时，比较长度
return len1 - len2;
}
...
public static char getChar(byte[] val, int index) {
return (char)(val[index] & 0xff);
}
...
}
自定义类
使用 Comparator
// A class to represent a student.
class Student {
int rollno;
String name, address;
// Constructor
public Student(int rollno, String name,
String address)
{
this.rollno = rollno;
this.name = name;
this.address = address;
}
// Used to print student details
// in main()
public String toString()
{
return this.rollno + " "
+ this.name + " "
+ this.address;
}
}
// Comparator implementattion
class Sortbyroll
implements Comparator<Student> {
// Used for sorting in ascending order of
// roll number
public int compare(Student a, Student b)
{
return a.rollno - b.rollno;
}
}
public class TreeMapImplementation {
static void Example2ndConstructor()
{
// Creating an empty TreeMap
TreeMap<Student, Integer> tree_map
= new TreeMap<Student, Integer>(new Sortbyroll());
// Mapping string values to int keys
tree_map.put(new Student(111, "bbbb",
"london"),
2);
tree_map.put(new Student(131, "aaaa",
"nyc"),
3);
tree_map.put(new Student(121, "cccc",
"jaipur"),
1);
// Displaying the TreeMap
System.out.println("TreeMap: "
+ tree_map);
}
public static void main(String[] args)
{
System.out.println("TreeMap using "
+ "TreeMap(Comparator)"
+ " constructor:
");
Example2ndConstructor();
}
}
输出
TreeMap using TreeMap(Comparator) constructor:
TreeMap: {121 cccc jaipur=1, 131 aaaa nyc=3, 141 bbbb london=2}
使用 Comparable
class Student implements Comparable<Student>{
int rollno;
String name, address;
// Constructor
public Student(int rollno, String name,
String address)
{
this.rollno = rollno;
this.name = name;
this.address = address;
}
public int compareTo(Student anotherStudent)
{
return this.rollno - anotherStudent.rollno;
}
// Used to print student details
// in main()
public String toString()
{
return this.rollno + " "
+ this.name + " "
+ this.address;
}
}
如何排序
添加时，跟 root 节点比较，小于放到左边，大于放到右边。
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
// 如果构造函数设置了 comparator，cpr 将不为 null
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") // 获取 key 的 Comparable
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;
}
时间复杂度
针对二叉树，4 层的总节点：2^0^ + 2^1^ + 2^2^ + 2^3^ -> 2^4^，如果树节点数为 n，树的高度为 log(n)。
所以 TreeMap 的查找和新增时间复杂度为 O(log(n))。
vs LinkedHashMap
LinkedHashMap 使用一个额外双向链表，记录插入顺序，所以它是根据插入顺序排序。