kotlin-如何覆盖哈希码

Question

I have overrided the equals method of a java object. (actually the objects in kotlin but its easily understood im just overriding the equals method). but now inorder to maintain the equals contract i should also override the hashcode method. but im not sure how to put in a implementation for hashcode that suits the equals method. here is what i have so far:

    data class AddressModel(
        var id_address: Int = 0,
        var id_country: Int = 0,
) {

    override fun equals(other: Any?): Boolean {
        if (this === other) return true
        if (other !is AddressModel)
            return false
        else {
            if (other.id_address == this.id_address)
                return true
        }
        return false
    }
}

compiler highly suggest i override hashCode method. but i dont understand what to implement. in the equals override i simply want to check if addressModel has same Id as another, if it does ,then i assume its equal.

this is what i have so far:

override fun hashCode(): Int {
        return Objects.hash(id_address, id_country);
    }

but i think this is better:

override fun hashCode(): Int {
            return Objects.hash(id_address); //base hash off same as equals the id_address
        }

this is recommended way i read but what does it mean ? if i added more class fields would i need to add more fields to the Objects.hash method also ? i would prefer the old skool way to do it as this call requires android api 19 and i support lower (api 16).

to be clear, i understand that If i don't override hashcode along with equals then every instance, eg "new AddressModel(707, 867)", will have a different hashcode. and then the HashMap for example will think these objects are distinct and not equal to replace them when calculating the hash for storage. but i just dont know what to put in the hashCode implementation. you can show me in either kotlin or java its ok.

update: would this be sufficient:

override fun hashCode(): Int { return id_address.hashCode() }

Answer 1

Since, your #equals() depends only on id_address , it should be used in hash code calculations. I would prefer:

override fun hashCode() = Objects.hash(id_address)

Answer 2

Sample POJO class (dataVO) overrides hashcode and equals, @Data : Lombok

Useful Links :

https://www.geeksforgeeks.org/override-equalsobject-hashcode-method/

Why do I need to override the equals and hashCode methods in Java?

https://crunchify.com/how-to-override-equals-and-hashcode-method-in-java/

 @Data

    public class dataVO implements Serializable {

    private static final long serialVersionUID = 1L;
    private int id;
    private String name;

   public String toString(){ 
          return "name:"+name;  
    }  
  @Override
    public boolean equals(Object obj) {
       if (obj == null || getClass() != obj.getClass()) {
            return false;
        }
     if (your logic) {
        return true; 
     }
     else {
       return false;
     }
   }

    @Override
    public int hashCode() {
        return id;
    }
}