Last article about introduction Object Several methods under this category are interview questions , mention equals() and hashCode() Methods may lead to questions about “hashCode() and equals() The relationship between ？” The interview questions , This article analyzes this basic interview question .

Sacrifice a picture first , Think about why ？

Introduce

equals() Is used to determine whether two objects are equal .

hashCode() To get hash code , Also known as hash code ; It actually returns a int Integers . The function of this hash code is to determine the index position of the object in the hash table .

Relationship

We use “ Class ” to “hashCode() and equals() The relationship between ” branch 2 To illustrate .

Does not create “ Class ”

What I'm talking about here “ The hash table corresponding to the class will not be created ” Is said ： We will not be in HashSet, Hashtable, HashMap And so on these are essentially hash table data structures , Use this class . for example , This class will not be created HashSet aggregate .

under these circumstances , Of the class “hashCode() and equals() ” It's nothing to do with half a cent ！equals() Used to compare whether two objects of this class are equal . and hashCode() It doesn't work at all .

below , Let's look at the example to see that two objects of a class are equal as well as Wait a moment hashCode() The value of .

import java.util.*;
import java.lang.Comparable;

/** * @desc  Compare equals()  return true  as well as   return false when , hashCode() Value . * */
public class NormalHashCodeTest{
    

    public static void main(String[] args) {
    
        //  newly build 2 The same thing Person object ,
        //  Reuse equals Compare them for equality 
        Person p1 = new Person("eee", 100);
        Person p2 = new Person("eee", 100);
        Person p3 = new Person("aaa", 200);
        System.out.printf("p1.equals(p2) : %s; p1(%d) p2(%d)\n", p1.equals(p2), p1.hashCode(), p2.hashCode());
        System.out.printf("p1.equals(p3) : %s; p1(%d) p3(%d)\n", p1.equals(p3), p1.hashCode(), p3.hashCode());
    }

    /** * @desc Person class . */
    private static class Person {
    
        int age;
        String name;

        public Person(String name, int age) {
    
            this.name = name;
            this.age = age;
        }

        public String toString() {
    
            return name + " - " +age;
        }

        /** * @desc  Cover equals Method  */  
        public boolean equals(Object obj){
      
            if(obj == null){
      
                return false;  
            }  

            // If the same object returns true, Instead, return to false 
            if(this == obj){
      
                return true;  
            }  

            // Judge whether the type is the same  
            if(this.getClass() != obj.getClass()){
      
                return false;  
            }  

            Person person = (Person)obj;  
            return name.equals(person.name) && age==person.age;  
        } 
    }
}

Running results ：

p1.equals(p2) : true; p1(1169863946) p2(1901116749)
p1.equals(p3) : false; p1(1169863946) p3(2131949076)

It can be seen from the results ：p1 and p2 In the case of equality ,hashCode() It doesn't have to be the same .

Will create “ Class ”

What I'm talking about here “ The hash table corresponding to the class will be created ” Is said ： We will be in HashSet, Hashtable, HashMap And so on these are essentially hash table data structures , Use this class . for example , This class will be created HashSet aggregate .

under these circumstances , Of the class “hashCode() and equals() ” It matters ：

If two objects are equal , So their hashCode() The value must be the same . Equality here means , adopt equals() Returns... When comparing two objects true.

If two objects hashCode() equal , They are not necessarily equal . Because it's in the hash table ,hashCode() equal , That is, the hash values of two key value pairs are equal . But the hash is the same , It's not necessarily the case that the key-value pairs are equal . Add that ：“ Two different key value pairs , Hash values are equal ”, This is hash conflict .

Besides , under these circumstances . To determine whether two objects are equal , Except to cover equals() outside , Also cover hashCode() function . otherwise ,equals() Invalid .

give an example , establish Person Class HashSet aggregate , Must cover... At the same time Person Class equals() and hashCode() Method .

If it's just covering equals() Method . We will find that ,equals() The method didn't achieve the effect we wanted .

import java.util.*;
import java.lang.Comparable;

/** * @desc  Compare equals()  return true  as well as   return false when , hashCode() Value . * */
public class ConflictHashCodeTest1{
    

    public static void main(String[] args) {
    
        //  newly build Person object ,
        Person p1 = new Person("eee", 100);
        Person p2 = new Person("eee", 100);
        Person p3 = new Person("aaa", 200);

        //  newly build HashSet object  
        HashSet set = new HashSet();
        set.add(p1);
        set.add(p2);
        set.add(p3);

        //  Compare p1  and  p2,  And print their hashCode()
        System.out.printf("p1.equals(p2) : %s; p1(%d) p2(%d)\n", p1.equals(p2), p1.hashCode(), p2.hashCode());
        //  Print set
        System.out.printf("set:%s\n", set);
    }

    /** * @desc Person class . */
    private static class Person {
    
        int age;
        String name;

        public Person(String name, int age) {
    
            this.name = name;
            this.age = age;
        }

        public String toString() {
    
            return "("+name + ", " +age+")";
        }

        /** * @desc  Cover equals Method  */  
        @Override
        public boolean equals(Object obj){
      
            if(obj == null){
      
                return false;  
            }  

            // If the same object returns true, Instead, return to false 
            if(this == obj){
      
                return true;  
            }  

            // Judge whether the type is the same  
            if(this.getClass() != obj.getClass()){
      
                return false;  
            }  

            Person person = (Person)obj;  
            return name.equals(person.name) && age==person.age;  
        } 
    }
}

Running results ：

p1.equals(p2) : true; p1(1169863946) p2(1690552137)
set:[(eee, 100), (eee, 100), (aaa, 200)]

Result analysis ：

We rewrote Person Of equals(). however , It's a strange discovery ：HashSet There are still repeating elements in ：p1 and p2. Why does this happen ？

This is because although p1 and p2 The content of is equal , But their hashCode() Unequal ; therefore ,HashSet Adding p1 and p2 When , Think they are not equal .

That covers at the same time equals() and hashCode() Methods? ？

import java.util.*;
import java.lang.Comparable;

/** * @desc  Compare equals()  return true  as well as   return false when , hashCode() Value . * */
public class ConflictHashCodeTest2{
    

    public static void main(String[] args) {
    
        //  newly build Person object ,
        Person p1 = new Person("eee", 100);
        Person p2 = new Person("eee", 100);
        Person p3 = new Person("aaa", 200);
        Person p4 = new Person("EEE", 100);

        //  newly build HashSet object  
        HashSet set = new HashSet();
        set.add(p1);
        set.add(p2);
        set.add(p3);

        //  Compare p1  and  p2,  And print their hashCode()
        System.out.printf("p1.equals(p2) : %s; p1(%d) p2(%d)\n", p1.equals(p2), p1.hashCode(), p2.hashCode());
        //  Compare p1  and  p4,  And print their hashCode()
        System.out.printf("p1.equals(p4) : %s; p1(%d) p4(%d)\n", p1.equals(p4), p1.hashCode(), p4.hashCode());
        //  Print set
        System.out.printf("set:%s\n", set);
    }

    /** * @desc Person class . */
    private static class Person {
    
        int age;
        String name;

        public Person(String name, int age) {
    
            this.name = name;
            this.age = age;
        }

        public String toString() {
    
            return name + " - " +age;
        }

        /** * @desc rewrite hashCode */  
        @Override
        public int hashCode(){
      
            int nameHash =  name.toUpperCase().hashCode();
            return nameHash ^ age;
        }

        /** * @desc  Cover equals Method  */  
        @Override
        public boolean equals(Object obj){
      
            if(obj == null){
      
                return false;  
            }  

            // If the same object returns true, Instead, return to false 
            if(this == obj){
      
                return true;  
            }  

            // Judge whether the type is the same  
            if(this.getClass() != obj.getClass()){
      
                return false;  
            }  

            Person person = (Person)obj;  
            return name.equals(person.name) && age==person.age;  
        } 
    }
}

Running results ：

p1.equals(p2) : true; p1(68545) p2(68545)
p1.equals(p4) : false; p1(68545) p4(68545)
set:[aaa - 200, eee - 100]

Result analysis ：

this time ,equals() It works ,HashSet There is no repeating element .

Compare p1 and p2, We found that ： Their hashCode() equal , adopt equals() Comparing them also returns true. therefore ,p1 and p2 Regarded as equal .

Compare p1 and p4, We found that ： Although they are hashCode() equal ; however , adopt equals() Compare them back to false. therefore ,p1 and p4 Regarded as unequal .

principle

1. Same object （ No modification has occurred ） Whenever called hashCode() The returned value must be the same .
   If one key The object is put Called when hashCode() Determines the storage location , And in the get Called when hashCode() Got a different return value , This value maps to a different place , Then you can't find the original key value .

2. hashCode() Objects whose return values are equal are not necessarily equal , adopt hashCode() and equals() Must be able to uniquely identify an object . Of unequal objects hashCode() The results can be equal .hashCode() When paying attention to the collision problem , We should also pay attention to the generation speed , perfect hash Not reality .

3. Once rewritten equals() function （ rewrite equals We should also pay attention to meet the reflexivity 、 symmetry 、 Transitivity 、 Uniformity ）, It has to be rewritten hashCode() function . and hashCode() The basis for generating hash value of should be equals() The field used to compare equality in .

If two by equals() Specifies that equal objects are generated hashCode Unequal , about hashMap Come on , They are likely to map to different locations , There is no call equals() Compare equal opportunities , Two objects that are actually equal may be inserted in different positions , There is an error . Other collection classes based on Hash methods may also have this problem