Collection - Chap 7 (Maps) (Problem in Understanding an example in K&B SCJP 1.6)

[Tanya Shetty]

 package collection;
import java.util.*;

public class Maps {

	
	public static void main (String args [])
	{
	  	Map <Object, Object > m = new HashMap <Object, Object>();
	  	
	  	Dog d = new Dog("Diego");
	  	Cat c = new Cat();
	  	
	  	m.put("k1", "Dog key");	 	
	  	m.put("k2", new Dog("Oliver"));
	  	m.put(PET.CAT, new Dog("Walter"));
        m.put(new Cat(), "Cat Key");
        m.put(new Cat(), c);
    	m.put(d, "New Dog Key");	
        
        System.out.println(m.get("k1"));
        System.out.println(m.get(d));
        System.out.println(m.get("k2"));
        System.out.println(m.get(PET.CAT));
        System.out.println(m.get(new Cat()));
        System.out.println(m.get(new Cat()));
        System.out.println(m.get(d));
        
        d.name = "magnolia";
        System.out.println(m.get(d));                
	  	
	}
}

enum PET {CAT, DOG, TURTLE}

class Dog 
{
	String name;
	Dog (String n)
	{
		name = n;
	}
	
    public boolean equals(Object o)
    {
    	if ((o instanceof Dog) &&
    	   (((Dog)o).name == this.name ))
    	   {
    		return true ;
    	   }
    	else
    	{
    		return false;
    	}
    }
    
    public int hashcode ()
    {
    	return name.length();
    }
	
   // public String toString()
   // {
  //  	return "Object Value is:" + name; 
  //  }
}

class Cat
{
	int code; 
    public boolean equals(Object o)
    {
    	if ((o instanceof Cat) &&
    	   (((Cat)o).code == this.code ))
    	   {
    		return true ;
    	   }
    	else
    	{
    		return false;
    	}
    }
    
    public int hashcode ()
    {
    	return 3;
    }
	
	
}   

Output :
Dog key
New Dog Key
collection.Dog@35ce36
collection.Dog@757aef
null
null
New Dog Key
New Dog Key


Question
Object ref "d" is used as a key for the value "New Dog Key". the hashcode for object of class Dog,
is used as the length of variable "name".
As per the SCJP 1.6, Chap 7 , when the value of "name" for object ref "d" is changed to "magnolia" ie. d.name = "magnolia"; the object will be searched in a hash bucket other than that where the object was placed originally ie hash bucket returned when value of "name" was diego.

Hence, the System.out.println(m.get(d)); should return null!!
But, as you can see the code returns an output with value "new Dog Key".

I dont understand ?

[Ankit Garg]

Read this or this...

[Anastasia Sirotenko]

be careful with function names
here you have hashcode() function, not hashCode(), as a result the program uses the default object hashCode() for d reference wich does not depend on any Dog field

In real life you can mark your overridden method by @Override annotation, wich will save you some nerve, but for exam we just have to be careful with such "typo's" in code