Do you really understand hashcode and equals???

Preface

Today I'm still talking about , Maybe my resume will be no different from that just graduated in three years , Because it has been almost a year since graduation , The last time I rewritten my resume, there was no difference except that I had two more project experience , And I still need a long time to master the skills on my resume .

One 、 About hashCode and equals We know the following points

• hashCode Two objects that are the same are not necessarily the same
• equals The same two objects ,hashCode It must be the same
• rewrite equals It must be rewritten hashCode

The rest of the

• == Compare the memory address values of the two objects
• equals The comparison is whether the values in the two objects are the same , Not rewritten equals and == equally

Two 、hashCode The origin of

We know hashCode yes Object Methods , But when we click in, we can see that it is a native Method , That is, it's not up to us Java To achieve , The bottom is C Written , We understand the default hashCode It is generated based on memory address

public native int hashCode();

3、 ... and 、 How to rewrite hashCode

rewrite hashCode The basic rules are as follows ：

• While the program is running , The same object is called more than once hashCode（） Method should return the same value .
• When two objects pass through equals（） Method comparison return true when , Then two objects of hashCode（） Method returns an equal value .
• Object is used as equals（） The method is standard Field, Should be used to calculate hashCode value .

According to each of our attributes hashCode, Then add certain rules to calculate the new hashCode

@Override
public int hashCode() {
    
    final int prime = 31;
    int result = 1;
    result = prime * result + ((getId() == null) ? 0 : getId().hashCode());
    result = prime * result + ((getFromUserId() == null) ? 0 : getFromUserId().hashCode());
    result = prime * result + ((getFromUserName() == null) ? 0 : getFromUserName().hashCode());
    result = prime * result + ((getToUserId() == null) ? 0 : getToUserId().hashCode());
    result = prime * result + ((getContent() == null) ? 0 : getContent().hashCode());
    result = prime * result + ((getCreateTime() == null) ? 0 : getCreateTime().hashCode());
    result = prime * result + ((getUnReadFlag() == null) ? 0 : getUnReadFlag().hashCode());
    return result;
}

Four 、 How to rewrite equals

1. Directly determine the address value of two objects （ use ==）
2. Judge whether the object to be compared is null
3. Determine whether two objects are a class （ Compare getClass）
4. Call the of each property of the object equals Compare , Considering that the properties of the object may be null, So we have to judge whether it is null

@Override
public boolean equals(Object that) {
    
    if (this == that) {
    
        return true;
    }
    if (that == null) {
    
        return false;
    }
    if (getClass() != that.getClass()) {
    
        return false;
    }
    MsgInfo other = (MsgInfo) that;
    return (this.getId() == null ? other.getId() == null : this.getId().equals(other.getId()))
        && (this.getFromUserId() == null ? other.getFromUserId() == null : this.getFromUserId().equals(other.getFromUserId()))
        && (this.getFromUserName() == null ? other.getFromUserName() == null : this.getFromUserName().equals(other.getFromUserName()))
        && (this.getToUserId() == null ? other.getToUserId() == null : this.getToUserId().equals(other.getToUserId()))
        && (this.getContent() == null ? other.getContent() == null : this.getContent().equals(other.getContent()))
        && (this.getCreateTime() == null ? other.getCreateTime() == null : this.getCreateTime().equals(other.getCreateTime()))
        && (this.getUnReadFlag() == null ? other.getUnReadFlag() == null : this.getUnReadFlag().equals(other.getUnReadFlag()));
}

notes ： above hashCode and equals All are IDEA Automatically generated

5、 ... and 、 When to rewrite equals and hashCode

We all know that when we use HashMap Such a special container requires , But why exactly ？

There is an interview question like this ：

Q: Why? String Such a packing data type is more suitable for hashMap Of key

A： because String Such wrapper data types are rewritten hashCode and equals

In fact, we basically use String As hashMap Of key, Consider if we use custom classes as key What will happen ？

import java.util.HashMap;
import java.util.Map;

public class Test {
    

    private String name;

    private Integer age;


    public String getName() {
    
        return name;
    }

    public void setName(String name) {
    
        this.name = name;
    }

    public Integer getAge() {
    
        return age;
    }

    public void setAge(Integer age) {
    
        this.age = age;
    }

    public static Map<Test,Object> map = new HashMap<>();

    static {
    
        Test test = new Test();
        test.setName(" Xiaodaoxian ");
        test.setAge(18);
        map.put(test," WOW! ");
    }
}

Think about how you can get this map The only data in it ？（ Remove convenience , You can use map.get() Methods? ？）

public static void main(String[] args) {
    
    Test test = new Test();
    test.setName(" Xiaodaoxian ");
    test.setAge(18);
    System.out.println(map.get(test));
}

What is printed in the above way is null, We didn't rewrite hashCode and equals Before , We rewrite new And the objects previously set in them hashCode and equals Never the same .

So when we use custom objects as hashKey Be sure to rewrite when you hashCode and equals.

6、 ... and 、 Why rewrite equals It must be rewritten hashCode

Because of hashMap And similar containers , Each search needs to be compared , If used directly equals Certainly. , But the price is too high , So the strategy is to compare hashCode,hashCode Compare the same equals.

Instead of rewriting hashCode, The default generation is related to the memory address , Never the same .