package com.java.equals;import java.util.HashMap;/*** 关于HashCode和Equals* @author yfding**/
public class EqualsTest {public static void main(String[] args) {/*** 概念*   equals:判断是否同一个实例，一般需要重写*  ==:判断对象实例的内存地址，判断是否同一个实例，判断实例是否物理相等*  hashcode:使用hash算法，给对象一个唯一的int值，实例的hash码*  */String str1 = new String("yfding");String str2 = new String("yfding");System.out.println(str1==str2);System.out.println(str1.equals(str2)); // return true;虽然指向地址不同，但String重写了equals方法System.out.println(str1.hashCode() + "---" + str2.hashCode()); // 相同,String重写了hashcode方法person p1 = new person(18,"yfding");person p2 = new person(18,"yfding");System.out.println(p1.equals(p2)); // 需重写equals方法，否则返回falseSystem.out.println(p1.hashCode() + "---" + p2.hashCode()); // 需重写hashcode方法，否则返回falseHashMap<person, String> map = new HashMap<person,String>();map.put(p1, "1");map.put(p2, "1");/**Map 集合先判断hashcode，再判断equals，都相同则覆盖，不相同则添加* 如果两个对象相同，那么它们的hashcode一定相同* 如果两个对象的hashcode相同，它们不一定相同* 使用hashcode大大提高了Map集合添加元素的效率，减少了判断equals的次数(hashcode不相同，默认两个实例不相同)*/System.out.println(map.size()); }
}
class person{private int age;private String name;public int getAge() {return age;}public void setAge(int age) {this.age = age;}public String getName() {return name;}public void setName(String name) {this.name = name;}public person(int age, String name) {super();this.age = age;this.name = name;}@Overridepublic int hashCode() {final int prime = 31;int result = 1;result = prime * result + age;result = prime * result + ((name == null) ? 0 : name.hashCode());return result;}@Overridepublic boolean equals(Object obj) {if (this == obj)return true;if (obj == null)return false;if (getClass() != obj.getClass())return false;person other = (person) obj;if (age != other.age)return false;if (name == null) {if (other.name != null)return false;} else if (!name.equals(other.name))return false;return true;}
}