Issue in Working with Maps

[Maheswaran Devaraj]

Hi friends,

 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)); // #3
		System.out.println(m.get(d1)); // #4
		System.out.println(m.get(new Cat())); // #5
		System.out.println(m.size());
		d1.name = "magnolia";// #6
		System.out.println(m.get(d1));//#7
	}
}

    

In the above code the marked line 7 also giving the null, I don't know why it is giving null, because I have defined
both the hashcode and equlas method in the Dog class.

Thanks in Advance,
Mahes

[Harshit Rastogi]

The culprit here is line 52, in which you are changing the name of d1 object. So when you try to retrieve from map the hashcode method will return different value since the name is changed and hence desired object will not be found.

[arulk pillai]

It is a best practice to use immutable obejcts as your key. You can use the Wrapper classes.