TreeSet和HashSet去重原理

• TreeSet 的去重是由所add对象声明的compareTo 决定的，而HashSet 会先去比较对象的hashcode 方法返回值,如果相同,再去比较对象的equals方法。因此一般可以通过改写对象的hashcode 和equals方法来修改对象的判重规则。

package baidu.Javabase.comparator;

/**
 * Created by liuchaoqun01 on 18/6/9.
 */
public class Person implements Comparable {

    private String name;

    private Integer age;

    private String mind;

    public Person(String name, Integer age, String mind) {
        this.name = name;
        this.age = age;
        this.mind = mind;
    }

    /**
     * 比较方法在类中声明,一般是传入一个类和当前类比较
     * @param o
     * @return
     */
    public int compareTo(Object o) {
        Person person = (Person) o;
        int ret = this.name.compareTo(person.getName());
        if (ret == 0) {
            ret = Integer.compare(this.age, person.getAge());
        }
        if (ret == 0) {
            return this.getMind().compareTo(person.getMind());
        }
        return ret;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Integer getAge() {
        return age;
    }

    public void setAge(Integer age) {
        this.age = age;
    }

    public String getMind() {
        return mind;
    }

    public void setMind(String mind) {
        this.mind = mind;
    }
}

package baidu.Javabase.comparator;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.List;
import java.util.SortedSet;
import java.util.TreeSet;

/**
 * Created by liuchaoqun01 on 18/6/9.
 */
public class TestComparable {

    //生成待排序数组
    private static List<Person> generatePersonList() {
        List<Person> retList = new ArrayList<>(16);
        retList.addAll(Arrays.asList(new Person("erMaZi", 19, "good"), new Person("liSi", 17, "bad")
                        , new Person("wangWu", 18, "middle")
                        , new Person("wangWu", 18, "middle")));
        return retList;
    }
    public static void print(String message,List<Person> personList){
        System.out.println(message+":");
        for(Person p:personList){
            System.out.println(p);
        }
        System.out.println();
    }
    public static void print(String message,SortedSet<Person> sortedSet){
        System.out.println(message+":");
        for(Person p:sortedSet){
            System.out.println(p);
        }
        System.out.println();
    }

    public static void main(String[] args) {
        //取数据
        List<Person> personList = generatePersonList();
        //排序一: 正常 这个并未指定比较器,是用的类本身的比较器
        Collections.sort(personList);
        print("Collections.sort(list)",personList);
        //排序二：发现正常了
        PersonComparator pc = new PersonComparator();
        PersonComparator2 pc2 = new PersonComparator2();
        Collections.sort(personList,pc.thenComparing(pc2));  // 这里是先按照pc排序，再按照pc2 排序
        print("thenComparing",personList);
        //排序三：正常
        SortedSet<Person> ts = new TreeSet<>(personList);

        print("TreeSet",ts);
        //排序四：发现某个Person实例被吃掉了,因为TreeSet认为CompareTo为0，则两个Person实例相同
        SortedSet<Person> ts2 = new TreeSet<>(new PersonComparator());
        ts2.addAll(personList);
        print("TreeSet(Comparator)",ts2);

        List<Integer> list = Arrays.asList(1, 2, 1,3,5,3);
        SortedSet<Integer> it = new TreeSet<>(list);
        System.out.println(it);
    }
}

参考：https://blog.csdn.net/yxb_yingu/article/details/52194988