重写equal要重写 hashCode的原因

public class Test {

    public static void main(String[] args) {

        Person person1 = new Person();
        person1.setId("1");
        person1.setName("qiumc");

        Person person2 = new Person();
        person2.setId("1");
        person2.setName("qiumc");

        Map<Person, String> hashMap = new HashMap<Person, String>();
        hashMap.put(person1,"1");
        hashMap.put(person2,"2");

        System.out.println(hashMap.size());//重写hashCode 结果为1,否则为2

    }

}

//Person类省略,只有两个属性，一个是name,一个id