equals是超类object提供的方法,基于实际内存地址判断。hashcode是将实际内存地址通过hash算法得出的值
那么也就意味着如果两个引用变量通过equals判断结果为true,那么hashcode值也必然相等。
假设给定一个Student类,在这个类中只重写了equals方法,没有重写hashcode方法
package com.polymophic;
public class Student {
private String name;
private String sex;
private int age;
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public String getSex() {
return sex;
}
public void setSex(String sex) {
this.sex = sex;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
public Student(String name, String sex, int age) {
super();
this.name = name;
this.sex = sex;
this.age = age;
}
@Override
public boolean equals(Object obj) {
if (this == obj)
return true;
if (obj == null)
return false;
if (getClass() != obj.getClass())
return false;
Student other = (Student) obj;
if (age != other.age)
return false;
if (name == null) {
if (other.name != null)
return false;
} else if (!name.equals(other.name))
return false;
if (sex == null) {
if (other.sex != null)
return false;
} else if (!sex.equals(other.sex))
return false;
return true;
}
}
写一个测试类进行判断
public static void main(String[] args) {
Student student = new Student("Bill", "female", 22);
Student student2 = new Student("Bill", "female", 22);
System.out.println("student的hashcode值:" + student.hashCode());
System.out.println("student2的hashcode值:" + student2.hashCode());
System.out.println(student.equals(student2));
}
返回结果为:
但是想想,equals在判断引用数据类型时(除去String类型)是基于实际内存地址,那么返回值为true表示两个对象是相同的,为什么hashcode值却不相同呢?这就是因为没有重写hashcode方法导致的
那在Student类中加上hashcode方法
@Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result + age;
result = prime * result + ((name == null) ? 0 : name.hashCode());
result = prime * result + ((sex == null) ? 0 : sex.hashCode());
return result;
}
在进行测试发现:
矛盾解决了
总结下:
equals方法与hashcode方法必须一起重写的原因是:解决equals返回结果与hashcode值不同之间的矛盾