》 TreeMap:是基于红黑树的Map接口的实现。
红黑树：平衡二叉树
取出时，可以有三种方式：前序遍历，中序遍历，后序遍历
》排序:
A 自然排序 --TreeMap无参构造
TreeMap<key类型,value类型> map= new TreeMap<key类型,value类型>();//key类应当实现Comparable接口,并重写hashCode()和equals()方法
B 比较器排序—TreeMap 比较器有参构造
TreeMap<key类型,value类型> map= new TreeMap<key类型,value类型>( new Comparator<key类型> (){ @Override compare(){ } });
》TreeMap put()方法源码
public V put(K key, V value) {
Entry<K,V> t = root;
if (t == null) {
// TBD:
// 5045147: (coll) Adding null to an empty TreeSet should
// throw NullPointerException
//
// compare(key, key); // type check
root = new Entry<K,V>(key, value, null);
size = 1;
modCount++;
return null;
}
int cmp;
Entry<K,V> parent;
// split comparator and comparable paths
Comparator<? super K> cpr = comparator;//构造TreeMap时确定comparator是否为null
//比较器排序
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);
}
//自然排序
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<K,V>(key, value, parent);
if (cmp < 0)
parent.left = e;
else
parent.right = e;
fixAfterInsertion(e);
size++;
modCount++;
return null;
}
案例:
package cn.itcast_04; import java.util.Comparator;
import java.util.Set;
import java.util.TreeMap; /*
* TreeMap<Student,String>
* 键:Student
* 值：String
*/
public class TreeMapDemo2 {
public static void main(String[] args) {
// 创建集合对象(比较器Comparator的子类来实现排序规则)
TreeMap<Student, String> tm = new TreeMap<Student, String>(
new Comparator<Student>() {
@Override
public int compare(Student s1, Student s2) {
// 主要条件
int num = s1.getAge() - s2.getAge();
// 次要条件
int num2 = num == 0 ? s1.getName().compareTo(
s2.getName()) : num;
return num2;
}
}); // 创建学生对象
Student s1 = new Student("潘安", 30);
Student s2 = new Student("柳下惠", 35);
Student s3 = new Student("唐伯虎", 33);
Student s4 = new Student("燕青", 32);
Student s5 = new Student("唐伯虎", 33); // 存储元素
tm.put(s1, "宋朝");
tm.put(s2, "元朝");
tm.put(s3, "明朝");
tm.put(s4, "清朝");
tm.put(s5, "汉朝"); // 遍历
Set<Student> set = tm.keySet();
for (Student key : set) {
String value = tm.get(key);
System.out.println(key.getName() + "---" + key.getAge() + "---"
+ value);
}
}
}
//因为上面案例使用的是比较器排序，而不是自然排序，所以作为key的Student并不需要实现Comparable接口
package cn.itcast_04;
public class Student {
private String name;
private int age;
public Student() {
super();
}
public Student(String name, int age) {
super();
this.name = name;
this.age = age;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
}