confusion in HashMaps

[Suvojit Chakraborty]

Hello friends,
This code is from Kathy Sierra book SCJP6 page585. I tried this code and it is also working, but the application of the behavior of HashMaps, (which says that, "if the key used to generate hash code is altered then the object cannot be picked back from the Map") in this program is puzzling me big time.

 import java.util.*;
class Dog 
{
	  public Dog(String n) { name = n; }
	  public String name;
	  public boolean equals(Object o) 
	  {
	  	if((o instanceof Dog) && (((Dog)o).name == name)) 
		{
		      return true;
    		} 
		else 
		{
		      return false;
    		}
   	  }
  	  public int hashCode() {    return name.length(); 	}
}

class Cat { }

enum Pets {DOG, CAT, HORSE }

class MapTest 
{
  	  public static void main(String[] args) 
	  {
		    Map<Object, Object> m = new HashMap<Object, Object>();
		    m.put("k1", new Dog("aiko"));   
		    m.put("k2", Pets.DOG);
		    m.put(Pets.CAT, "CAT key");
		    Dog d1 = new Dog("clover");    
		    m.put(d1, "Dog key");
		    m.put(new Cat(), "Cat key");
		    System.out.println(m.get("k1"));                 
		    String k2 = "k2";
		    System.out.println(m.get(k2));                   
		    Pets p = Pets.CAT;
		    System.out.println(m.get(p));                  
		    System.out.println(m.get(d1));                  
		    System.out.println(m.get(new Cat()));            
		    System.out.println(m.size());                   
        	    d1.name = "magnolia";
		    System.out.println(m.get(d1));                  // #1
		    d1.name = "clover";
		    System.out.println(m.get(new Dog("clover")));   // #2
		    d1.name = "arthur";                                        //3
		    System.out.println(m.get(new Dog("clover")));   // 4
	}
}   

which produces the below given output

D:\Education\Java\JavaStudyRoom>javac MapTest.java

D:\Education\Java\JavaStudyRoom>java MapTest
Dog@4
DOG
CAT key
Dog key
null
5
null
Dog key
null

According to my understanding here we are in #1, trying to extract value on basis of dog object whose name is "magnolia". Since the key is generated using dog object with "clover" as name, therefore the hashcodes mismatches and we get null.
In 2nd case we do the same but with clover as name of dog object, but this time the hashcodes matches and equals() also satisfies, and we get the dog key.
But what is happening in Case 3,confuses me:
here though in #3 we assign arthur to d1'sname instance variable but in #4 we are passing a dog object with clover as name.
So ideally the hashcodes should match + equals should return true. So the output should have been dog key like in case 2.
But I dont understand why is this not happening and why we get null???

I am sure that this is the correct behavior and output, but then can you tell me where exactly I am getting wrong with my logic???

Thanks in advance,

BR
Suvojit

[Ankit Garg]

 import java.util.*;
class Dog 
{
	  public Dog(String n) { name = n; }
	  public String name;
	  public boolean equals(Object o) 
	  {
	  	if((o instanceof Dog) && (((Dog)o).name == name))   <--using == for String comparison is not recommended
		{
		      return true;
		} 
		else 
		{
		      return false;
		}
   	  }
  	  public int hashCode() {    return name.length(); 	}
}

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");
		    d1.name = "magnolia";
		    System.out.println(m.get(d1));                  // #1
		    d1.name = "clover";
		    System.out.println(m.get(new Dog("clover")));   // #2
		    d1.name = "arthur";                                        //3
		    System.out.println(m.get(new Dog("clover")));   // 4
	}
}

All the extra code which was not related to your problem was making it difficult to understand your problem so I removed it. Now at line 4, you are getting null because the when you call m.get(new Dog("clover")), the hashCode will match but the equals method will not return true as d1.name is "arthur" and we are passing it a dog with the name "clover"...

[Ankit Garg]

Also if you search the forum, there have been discussions in the past about this question like here, here, here, here ......... See if you get your answer there...

[Suvojit Chakraborty]

Ankit Garg wrote:Also if you search the forum, there have been discussions in the past about this question like here, here, here, here ......... See if you get your answer there...

The first link cleared my doubt and its now clear to me now....
In few days I will write my interpretation of this concept( i dont have time to write it now as I have other studies to do), please verify it then.

Thank you so much

Suvojit