TreeMap:是基于红黑树的Map接口的实现

》 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;
    }
}