HashSet"包含"方法如何工作？

Question

我使用Set的两个实现:HashSet和TreeSet.我添加了10个元素来设置并通过set中的contains方法查找对象.我看到包含方法迭代所有对象虽然它找到了元素.出于性能原因我很困惑.为什么它是所以,我该如何预防呢？

我有一个Person类:

public class Person implements Comparable<Person>{

private int id;
private String name;

public Person() {
}

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


//getter and setters

@Override
public int hashCode() {
    System.out.println("hashcode:" + toString());
    return this.id;
}

@Override
public boolean equals(Object obj) {
    System.out.println("equals:" + toString());
    if (this == obj) {
        return true;
    }
    if (obj == null) {
        return false;
    }
    if (getClass() != obj.getClass()) {
        return false;
    }
    final Person other = (Person) obj;
    return true;
}

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

@Override
public int compareTo(Person o) {
    System.out.println("compare to:"+getId()+" "+o.getId());
    if(o.getId() == getId()){
        return 0;
    }else if(o.getId()>getId()){
        return -1;
    }else {
        return 1;
    }
}

}

在主类中我添加10个Person对象,然后通过set的第一个元素调用contains方法:

    import beans.Person;
    import java.util.Date;
    import java.util.HashSet;
    import java.util.Set;

    public class Main {
        public static void main(String[] args) {
            Set<Person> people = new HashSet<>();
            for (int i = 0; i < 10; i++) {
                people.add(new Person(i, String.valueOf(i)));
            }

            Person find = people.iterator().next();
            if (people.contains(find)) {
                System.out.println("here"+find.getName());
            } 
        }
    }

结果:

hashcode:Person{id=0, name=0} <--here element has been found but it continues
hashcode:Person{id=1, name=1}
hashcode:Person{id=2, name=2}
hashcode:Person{id=3, name=3}
hashcode:Person{id=4, name=4}
hashcode:Person{id=5, name=5}
hashcode:Person{id=6, name=6}
hashcode:Person{id=7, name=7}
hashcode:Person{id=8, name=8}
hashcode:Person{id=9, name=9}
hashcode:Person{id=0, name=0}<-- second check
here:0

Answer 1

你的equals()方法是错的.无论其他人是什么,它都会返回true.

它不遵守equals()BTW 的合同,因为相等的对象应该具有相同的hashCode,而hashCode是该人的ID.因此,具有不同ID的两个人具有不同的hashCode,但仍然是相等的.

也就是说,您的测试表明hashCode()执行了10次.但它并没有被执行contains().它由执行add().每次向对象添加对象时,hashCode()都会使用它来知道哪个存储桶应该保存该对象.