Set面试题

1.测试类

import java.util.HashSet;

public class Test {
     
    @org.junit.Test
    public void test1(){
     
        Person person1 = new Person(1001,"AA");
        Person person2 = new Person(1002, "BB");
        HashSet hashSet = new HashSet();
        hashSet.add(person1);
        hashSet.add(person2);
        System.out.println(hashSet);//[Person{id=1002, name='BB'}, Person{id=1001, name='AA'}]
        person1.name = "CC";
        System.out.println(hashSet);//[Person{id=1002, name='BB'}, Person{id=1001, name='CC'}]
        hashSet.add(person1);//person1的hashCode前后就不一样了,可以添加
        System.out.println(hashSet);//[Person{id=1002, name='BB'}, Person{id=1001, name='CC'}, Person{id=1001, name='CC'}]
        hashSet.add(new Person(1001,"AA"));//这个hashCode值和原来的person1是一样的,但是equals不一样,可以添加
        System.out.println(hashSet);//[Person{id=1002, name='BB'}, Person{id=1001, name='CC'}, Person{id=1001, name='CC'}, Person{id=1001, name='AA'}]
    }
}

2.Person类

import java.util.Objects;

public class Person {
     
    int id;
    String name;

    public Person() {
     
    }

    public Person(int id, String name) {
     
        this.id = id;
        this.name = name;
    }

    @Override
    public boolean equals(Object o) {
     
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        Person person = (Person) o;
        return id == person.id &&
                Objects.equals(name, person.name);
    }

    @Override
    public int hashCode() {
     
        return Objects.hash(id, name);
    }

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

总结:hashSet和LinkedHashSet是先计算哈希值确定存储位置,然后equals
TreeSet是通过比较器来确定存储顺序(自然排序和定制排序)