java中的哈希代码实现

Question

package MavenWeb.MavenWeb;

import java.util.HashMap;
import java.util.Map;

public class StringEquality {

    public static void main(String[] args) {

        Person p1 = new Person("Naveen", 22, 1000);
        Person p2 = new Person("Naveen", 21, 2000);
        Person p3 = new Person("Naveen", 23, 3000);

        if(p1.equals(p2)){
            System.out.println("P1 and p2 :" + p1.equals(p2));
        } else{
            System.out.println("P1 and p2 :" + p1.equals(p2));
        }

        if(p1.equals(p3)){
            System.out.println("P1 and p3 :" + p1.equals(p3));
        }


        Map<Person, Object> map = new HashMap<Person, Object>();
        map.put(p1, p1);
        map.put(p2, p2);


        System.out.println(map.get(new Person("Naveen", 21, 2000)));


    }

}

...

class Person{

    public Person(String name, int id, float salary){
        this.name = name;
        this.id = id;
        this.salary = salary;
    }
    public Float getSalary() {
        return salary;
    }
    public void setSalary(Float salary) {
        this.salary = salary;
    }
    String name;
    Float salary;
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public int getId() {
        return id;
    }
    public void setId(int id) {
        this.id = id;
    }
    int id;


    @Override
    public boolean equals(Object obj) {
        if(obj == null){
            return false;
        }

        if(this == obj){
            return true;
        }

        if(obj instanceof Person){

            Person person = (Person)obj;

            if((person.getName().equals(name)) && person.getId() == id
                    && person.getSalary() == salary){
                return true;
            }
        }
        return false;
    }


    @Override
    public int hashCode() {
        int hashCode = 1;
        hashCode = 31 * hashCode + name.hashCode();
        hashCode = 31 * hashCode + id;
        hashCode = 31 * hashCode + salary.intValue();
        return hashCode;
    }

    @Override
    public String toString() {
        return "Name :" + name + ", id : " + id;
    }
}

我写了以下程序.有关哈希码实现的一点点困惑,

1. 为什么我们在实施中使用31
2. 根据equals方法逻辑,该程序应返回p2(Naveen,21)实例,但它返回null.为什么？

Answer 1

你从地图上得到了null,因为薪水存储为Float(而不是float),而你正在比较Float实例==而不是比较它们equals().

因此equals(),Person 的方法检查Float两个Person实例中的实例完全相同,而不是检查它们的工资值是否相等.您应该使用float(或更好double:),除非工资可以为空(您还应考虑使用BigDecimal来处理确切的金额).但在这种情况下,您必须检查空值Person.equals()并用于equals()比较工资.在Java 7或更高版本中,最简单的方法是使用Objects.equals()可空对象进行比较:

@Override
public boolean equals(Object obj) {
    if(obj == null){
        return false;
    }

    if(this == obj){
        return true;
    }

    if(obj instanceof Person){

        Person person = (Person)obj;

        return Objects.equals(name, person.name)
               && id == person.id
               && Objects.equals(salary, person.salary);
    }
    return false;
}


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