Doubt regarding HashMap Example

[Mansukhdeep Thind]

Hi all
Please look at the code below.

import java.util.*;
class Dog{
	public String name;
	public Dog(String n){
		name = n;
	}
}
class Cat{}
enum Pets {DOG, CAT, HORSE}
public 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"));
		System.out.println(m.get("k2"));
		System.out.println(m.get(Pets.CAT));
		System.out.println(m.get(d1));
		System.out.println(m.get(new Cat()));
		System.out.println(m.size());
		
	}
}

The output we get is :
com.mansukh.test.Dog@190d11
DOG
CAT key
Dog Key
null
5

I understood all of it except the 'null' part. Why didn't we find the 'CatKey' string? Why did it work for the Dog instance as a key. Please explain.

Regards
Mansukhdeep

[Jesper de Jong]

So, your question is why:

System.out.println(m.get(new Cat()));

returns null.

You're looking up a value in the map that has a new Cat object as its key. That new Cat object you're using as the key is ofcourse a different new Cat object than the one you use in line 18 to put the value in the map.

Since your class Cat (line 8) does not have equals() and hashCode() methods, it uses the equals() and hashCode() from class Object, which use == and return a unique hash code for every instance. So, if you have to separate Cat objects, they will not be equal.

[subodh k kumar]

You need to override equals() and hashCode() methods in Cat class because Hashvalue of object should be match with the key passed in map.since new Cat() will create new Object in every call , it does not guarantee the same hash value.so the value for this key is returned as NULL.

[Christophe Verré]

subodh k kumar wrote:You need to override equals() and Hashcode() methods in CAT class because Hashvalue of object should be match with the key passed in map.since new Cat() will create new Object in every call , it does not guarantee the same hash value.so the value for this key is returned as NULL.

You mean the equals() and hashCode() method, as Jesper explained.

[subodh k kumar]

Yes Chris , I meant same . case has been corrected . Thankyou for pointing.

[Christophe Verré]

subodh k kumar wrote:Yes Chris , I meant same . case has been corrected . Thankyou for pointing.

No problem hashCode may be the most misspelt method, leading to unpleasant surprises.

[David O'Meara]

always annotate with @Override and save yourself a heap of sadness.

[Catherine austin]

Can we use indexof() in this example to refer to refer to objects ?
If yes then how?

[Campbell Ritchie]

Catherine austin wrote:Can we use indexof() in this example to refer to refer to objects ?
If yes then how?

I don't think a Map has an indexOf() method. That is only appropriate for Lists. Maps don't store their contents by index. Have a loop at the interfaces in the Java™ Tutorials.

[Catherine austin]

Oh yeah i got you . Thanks for clarification.