Hashmap – trying to understand hashCode(), equals(), get()

[java_on_fire]

import java.util.*;  
  
class Dog{  
      
    public String name;  
    public Dog(String n) {  
        name = n;  
    }  
      
    public boolean equals(Object o){  
        System.out.println("i'm inside equals");
        if((o instanceof Dog) && (((Dog)o).name == this.name)){  
            return true;}  
        else{  
            return false;}  
    }  
      
    public int hashCode(){  
        return name.length();  
    }  
}  
  
public class MapTest {  
  
    public static void main(String[] args) {  
        Map<Object, Object> m = new HashMap<Object, Object>();  
          
        Dog d1 = new Dog("clover");  
        m.put(d1, "dog key"); 
        System.out.println(m.get(d1));  
        
        d1.name = "magnolia"; 
        System.out.println(m.get(d1));
        
        d1.name = "arthur";  
        System.out.println(m.get(new Dog("clover")));  
          
        }  
  
}

1.I understood that the hashCode() gets called every time you call put and get method of the HashMap, correct?
2.The equals() method, where is it getting the Object o and the this reference from?
3.If I change the code System.out.println(m.get(new Dog("clover"))); at bottom after d1.name = “arthur” to
System.out.println("d1= " + map.get(d1));

I get:

d1= Dog key

The equals method never got executed because i put System.out.println("i'm inside equals");
but never got executed? how come? and why i'm not getting null instead of Dog key?

Please help, i have been struggling trying to find an answer but no luck!

[Tolls]

Inside the code for HashMap.get() it does:

            if (e.hash == hash && ((k = e.key) == key || key.equals(k)))

So it first checks the entry hash with your hash, then it checks if the keys are == (ie the same object) then if they are equal() (the method).

In your first case they are == (and the hash matches) so you get "dog key" back.

In the second case (and this highlights why changing key values is a Bad Thing) your hash does not match the entry hash...because the entry has been filed under a hash of 6 (the length of "clover") and you are asking it to look for an entry under 8 (the length of "magnolia"). So it fails on the first check.

In the third case the hashes match ("clover" is 6 characters long), but the equals() fails ("clover" != "arthur").

When you change that call to using d1, the hashes match and the objects are == (they are the same object) so it never goes into equals().

[java_on_fire]

Hi Tolls, thank you for your prompt reply, please allow me time to make this sink before getting back with further questions if needed.

Thanks again, i truly appreciate it, you would believe how much time i spent trying to figure this out i even had nightmares about it!!

[Tolls]

I only looked up how it did it because I thought it would only check the hash and then do equals(), so I wanted to see exactly what it was doing.

[java_on_fire]

Tolls, i opened the HashMap source file and found the juice inside the get() method. Thank you, you opened my eyes on something that i have never thought of doing! is looking at code inside the HashMap.