==,hashcde, equals(一)
1、Hash 的属性,
1)bucket 和 list
2、java.lang.object 的 hashcode 和 equal 通过内存地址比较
3、为什么要重写hashcode 和 equals,
1) 实现hashset不可重复的特性(http://www.cnblogs.com/happyPawpaw/p/3744971.html)
2)Hashmap
4、Hashmap的键值如果是类
5、Hashmap的put 和 get 方法
https://blog.csdn.net/lan12334321234/article/details/70048493
(https://blog.csdn.net/VIP_WangSai/article/details/77505517)
6、先判断hashcode, 再判断equals
为什么要这种判断顺序呢,因为要实现的是hash的特性,bucket由hashcode 决定;
hashcode true, equal 不一定 true
equals true, hashcode 一定true
==,hashcde, equals
public class Student{ private int id; private String name; public Student(int id, String name){ this.id = id; this.name = name; } }
main
public static void main(String args[]) { Student student01 = new Student(1,"lee"); Student student02 = new Student(1,"lee"); System.out.print("student01 == student02 :"); if(student01 == student02){ System.out.println("true."); }else{ System.out.println("false."); } System.out.print("student01.hashCode() == student02.hashCode() :"); if(student01.hashCode() == student02.hashCode()){ System.out.println("true."); }else{ System.out.println("false."); } System.out.print("student01.equals(student02) :"); if(student01.equals(student02)){ System.out.println("true."); }else{ System.out.println("false."); } Set<Object> set = new HashSet<Object>(); set.add(student01); set.add(student02); System.out.println("HashSet 长度:" + set.size()); }
输出
默认,继承Object student01 == student02 :false. student01.hashCode() == student02.hashCode() :false. student01.equals(student02) :false. HashSet 长度:2
修改student 的 hashcode类如下
public class StudentModifyHashCode{ private int id; private String name; public StudentModifyHashCode(int id, String name){ this.id = id; this.name = name; } @Override public int hashCode(){ final int prime = 31; int result = 1; result = prime*result+this.id; return result; } }
输出,可以看到这次hashcode 一样的
修改hashcode student01 == student02 :false. student01.hashCode() == student02.hashCode() :true. student01.equals(student02) :false. HashSet 长度:2
同时修改student的hashcode 和 equals 方法如下
public class StudentModifyEqualAndHash{ private int id; private String name; public StudentModifyEqualAndHash(int id, String name){ this.id = id; this.name = name; } public int getId() { return id; } public String getName() { return name; } @Override public int hashCode(){ final int prime = 31; int result = 1; result = prime*result+this.id; return result; } @Override public boolean equals(Object obj){ if(this == obj) return true; if(obj == null) return false; if(getClass() != obj.getClass()) return false; StudentModifyEqualAndHash other = (StudentModifyEqualAndHash)obj; if(this.id != other.getId() ){ return false; } for(int i=0;i<this.name.length();i++){ if(this.name.charAt(i) != (((StudentModifyEqualAndHash)obj).getName().charAt(i))){ return false; } } return true; } }
输出,可以看到hashcode 和 equals 都一样
修改hashcode和equals student01 == student02 :false. student01.hashCode() == student02.hashCode() :true. student01.equals(student02) :true. HashSet 长度:1
https://blog.csdn.net/lixiaoxiong55/article/details/93376852
hashCode作用: https://blog.csdn.net/qq_38977097/article/details/80834525