在HashMap中了解equals和hashCode的工作原理 [英] Understanding the workings of equals and hashCode in a HashMap

问题描述

我有这个测试代码：

  import java.util。*; 
 $ b $ class MapEQ {
 
 public static void main（String [] args）{
 Map< ToDos，String> m = new HashMap< ToDos，String>（）; 
 ToDos t1 =新的ToDos（星期一）; 
 ToDos t2 =新的ToDos（星期一）; 
 ToDos t3 =新的ToDos（星期二）; 
 m.put（t1，doLaundry）; 
 m.put（t2，payBills）; 
 m.put（t3，cleanAttic）; 
 System.out.println（m.size（））; 
}} 
 
类ToDos {
 
字符串日; 
 
 ToDos（String d）{day = d; } 
 
 public boolean equals（Object o）{
 return（（ToDos）o）.day == this.day; 
} 
 
 // public int hashCode（）{return 9; } 
 
  

当 // public int hashCode（）{返回9; } 被取消注释 m.size（）返回2，当它留下评论时返回3。为什么？

解决方案

如果没有重写<$ c $，你已经得到了等于 C>的hashCode 。您必须确保对于两个对象等于返回true的所有情况， hashCode 返回相同的值。哈希码是一个代码，如果两个对象相等，则该代码必须相等（反之不必为真）。当你把你的硬编码值设为9时，你再次满足合同。

在你的哈希映射中，等式只在一个哈希桶中被测试。你的两个星期一对象应该是相等的，但是因为它们返回不同的哈希码，所以等于方法甚至没有被调用来确定它们的相等性 - 它们被直接放入不同的桶中，甚至没有考虑到它们相同的可能性。

I have this test code:

import java.util.*;

class MapEQ {

  public static void main(String[] args) {
   Map<ToDos, String> m = new HashMap<ToDos, String>();
   ToDos t1 = new ToDos("Monday");
   ToDos t2 = new ToDos("Monday");
   ToDos t3 = new ToDos("Tuesday");
   m.put(t1, "doLaundry");
   m.put(t2, "payBills");
   m.put(t3, "cleanAttic");
   System.out.println(m.size());
} }

class ToDos{

  String day;

  ToDos(String d) { day = d; }

  public boolean equals(Object o) {
      return ((ToDos)o).day == this.day;
 }

// public int hashCode() { return 9; }
}

When // public int hashCode() { return 9; } is uncommented m.size() returns 2, when it's left commented it returns three. Why?

解决方案

You have overidden equals without overriding hashCode. You must ensure that for all cases where equals returns true for two objects, hashCode returns the same value. The hash code is a code that must be equal if two objects are equal (the converse need not be true). When you put your hard-coded value of 9 in, you satisfy the contract again.

In your hash map, equality is only tested within a hash bucket. Your two Monday objects should be equal, but because they are returning different hash codes, the equals method isn't even called to determine their equality - they are put straight into different buckets, and the possibility that they are equal isn't even considered.

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