一、TreeMap

元素是一个键值对，可以去重并进行排序

1.先编写一个Dog2类

public class Dog2{
        private String name;
        private int age;

        public Dog2() {
        }

        public Dog2(String name, int age) {
            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;
        }

        @Override
        public String toString() {
            return "Dog{" +
                    "name='" + name + '\'' +
                    ", age=" + age +
                    '}';
        }
}

2.编写一个测试类

import java.util.Comparator;
import java.util.Map;
import java.util.TreeMap;

public class TreeMapDemo1 {
    public static void main(String[] args) {
        //使用匿名内部类
        TreeMap<Dog2, Integer> treeMap = new TreeMap<Dog2, Integer>(new Comparator<Dog2>() {
            @Override
            public int compare(Dog2 o1, Dog2 o2) {
                //按年龄来排序，如果年龄相同，则比较姓名是否相同
                int i=o1.getAge()-o2.getAge();
                return (i==0)?o1.getName().compareTo(o2.getName()):i;
            }
        });

        Dog2 d1 = new Dog2("煤球",3);
        Dog2 d2 = new Dog2("旺财",2);
        Dog2 d3 = new Dog2("小灰",3);
        Dog2 d4 = new Dog2("煤球",3);
        Dog2 d5 = new Dog2("白线",4);
        treeMap.put(d1,1);
        treeMap.put(d2,2);
        treeMap.put(d3,3);
        treeMap.put(d4,4);
        treeMap.put(d5,5);
        //使用增强for循环来遍历Map里面的键
        for(Dog2 i:treeMap.keySet()){
            System.out.println(i);
        }
        //使用增强for循环来遍历Map里面的值
        for(Integer i:treeMap.values()){
            System.out.println(i);
        }
        //使用增强for循环来遍历Map中的所有的键值对
        //要想不报黄色警告可以指定Map.Entry的泛型
        for(Map.Entry<Dog2,Integer> i:treeMap.entrySet()){
            System.out.println(i);
        }
        System.out.println(treeMap);
    }
}

二、TreeMap的练习题：（类似于TreeSet）

"aababcabcdabcde",获取字符串中每一个字母出现的次数要求结果:a(5)b(4)c(3)d(2)e(1)

import java.util.Map;
import java.util.TreeMap;

public class TreeMapTest1 {
    public static void main(String[] args) {
        TreeMap<Character, Integer> treeMap = new TreeMap<>();
        String a="aababcabcdabcde";
        char[] chars=a.toCharArray();

        for (char aChar : chars) {
            //判断chars中的值在不在treeMap的键中，如果不在将该值添加到treeMap的键中，并将值赋为1
            if (treeMap.containsKey(aChar)) {
                treeMap.put(aChar, treeMap.get(aChar) + 1);
                //如果在将其值加一
            } else {
                treeMap.put(aChar, 1);
            }
        }
        System.out.println(treeMap);
        StringBuffer s1 = new StringBuffer();
        for(Map.Entry<Character,Integer> entry:treeMap.entrySet()){
            Character b=entry.getKey();
            Integer c=entry.getValue();
            s1.append(b).append("(").append(c).append(")");
        }
        System.out.println(s1);
    }
}