Modifying one of HashMap key

Question

package always.confusing;

import java.util.HashMap;
import java.util.Map;

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(); }
}

public class QustionToaAsk {

public static void main(String[] args) {
    // TODO Auto-generated method stub
    Map<Object, Object> m = new HashMap<Object, Object>();

    Dog d1 = new Dog("clover");
    m.put(d1, "Dog key");

    System.out.println("**" + m.get(new Dog("clover")));

    d1.name = "magnolia";
    System.out.println("state after magnolia");
    System.out.println(m.get(new Dog("magnolia")));
    System.out.println(m.get(d1)); // #1
    System.out.println("**" + m.get(new Dog("clover")));///????????
    System.out.println("size:" + m.size());

    d1.name = "clover";
    System.out.println("\nstate after clover");
    System.out.println(m.get(new Dog("clover"))); // #2
    System.out.println(m.get(d1));
    System.out.println("size:" + m.size());

    d1.name = "arthur";
    System.out.println("\nstate after arthur");
    System.out.println(m.get(new Dog("arthur")));
    System.out.println(m.get(d1));
    System.out.println("size:" + m.size());
}

}

O/p:

**Dog key

state after magnolia

null

null

**null

size:1

state after clover

Dog key

Dog key

size:1

state after arthur

Dog key

Dog key

size:1

why does 1st set of prints fail to fetch for Dog objects with clover and magnolia content but in other two sets it does as expected?

ie why After trying to change to "magnolia", why next get()(still using proper object ie get(new Dog("clover")))) not able to fetch the value "Dog key"? after changing to equals.. still same output –

//ignore

Answer 1

You need to understand how a HashMap works.

When you put a key in a HashMap, containing N buckets, the hashCode of the key is used to find the appropriate bucket. Then each key contained in the bucket is compared, using equals(), with the added key, to know if the key is already in the map.

Similarly, when getting the value for a key, the hashCode() is first used to find the appropriate bucket, then each key in this bucket is compared with equals() to the key passed to get().

You're doing two things which you should never do when using HashMaps:

1. Modifying the key after it has been stored in the map. This modifies its hashCode, and causes thekey to not be searched for in the appropriate bucket. A bit as if you put all red coins in a red drawer, and then repainted one of the keys to blue. Obviously, if you then search for the blue coins, you'll search them in the blue drawer, and not in the red drawer where it has been stored, since it was red initially.
2. Implementing hashCode() using an algorithm that makes many keys have the same value. It's the case for "clover" and "arthur", which have the same length, and thus the same hashCode() given your implementation. "magnolia", on the other hand, doesn't have the same hashCode().

Given the above, you should be able to understand why your code works the way it does. Just draw what happens on paper for each operation, and you'll understand.

Answer 2

I would strongly suggest you to begin with reading how hashmap works in java

There are multiple issues here.

1. When you invoke m.put(d1, "Dog key");, the hashCode is calculated and a bucket in the hashMap where the object will be stored is determined. The hashCode value is 6 in this case.
2. Then you're changing the name to magnolia (and thus the hashCode to 8). HashMap tries to search for matching object, but based on hashCode, it finds out, that there is no object stored in the bucket corresponding to hashCode value = 8.
3. Then you're changing the name back to clover, and, long story short, the object can be found.
4. Then you're changing the name to arthur. The hashCode is 6 again and HashMap manages to find the right bucket and using equals method - find the object.

Read more about hashCode and equals: http://docs.oracle.com/javase/7/docs/api/java/lang/Object.html#hashCode%28%29.

BTW You should use equals method for comparing Strings (read http://www.javapractices.com/topic/TopicAction.do?Id=18).

Answer 3

First: What do you want to do?

Second: The equals() is the first problem as it only compares references. See Here

Third: The hashCode() function for dog is missing. you should never overwrite only one of equals() and hashCode() See Here