CODEWITHSUNDEEP
javacollectionshashmap
prity
prity

Reputation: 1519

Hash map get method

Please explain output of below code. I guessed it null , because command line arguments are different than its key. But it is not correct explanation. It is null because friends class doesnot override equals and hashcode() methods.

But why?

import java.util.*;
public class Birthdays {
    public static void main(String[] args) {
        Map<Friends, String> hm = new HashMap<Friends, String>();
        hm.put(new Friends("Charis"), "Summer 2009");
        hm.put(new Friends("Draumur"), "Spring 2002");
        Friends f = new Friends(args[0]);
        System.out.println(hm.get(f));
    }
}

class Friends {
    String name;
    Friends(String n) { name = n; }
}

And the command line invocation: java Birthdays Draumur

Upvotes: 0

Views: 730

Answers (3)

Thomas Padron-McCarthy
Thomas Padron-McCarthy

Reputation: 27672

args[0] will contain the string "Draumur", so that is not the reason for the program printing null.

A HashMap is a hash table, and it finds elements in it based on the hash value of the key. If you don't override the hash method, Java will calculate the hash value based on the object identity, so two different Friends objects, even with the same name inside, will not be guaranteed to hash to the same value.

You would also need to write an equals method, since if you don't override it, Java will also consider two different Friends objects not to be equal, even with the same name inside.

In summary, you need to override the hashCode method so the HashMap can find the Friends object, and you need to override the equals method so the HashMap, when it has found it, can see that it is the object it is searching for.

Here is a possible new version of the Friends class (and I would also suggest you call it Friend, since one such object represents one single friend):

class Friends {
    String name;
    Friends(String n) { name = n; }
    public boolean equals(Object o) {
        if (!(o instanceof Friends))
            return false;
        Friends rhs = (Friends)o;
        return (name.equals(rhs.name));
    }
    public int hashCode() {
        return name.hashCode();
    }
}

Upvotes: 1

wargre
wargre

Reputation: 4753

The get method of a map return the value of the key where map key ".equals" to researched key

Your Friends key do not implements equals, so the default one from Object is used, which is a "==" compare (true only if it is the same object).

Get will only give you something if you use the exact same object you put as key.

Upvotes: 0

OneCricketeer
OneCricketeer

Reputation: 192043

command line arguments are different than its key

Not sure I understand that logic...
args = {"Draumur"}
new Friends(args[0]) = new Friends("Dramur")

A key like that was placed in the map already, but a Friend is not comparable to other Friend objects via a hashcode or equality otherwise.

If you had a HashMap of String to String, then get("Dramur") would not be null.

Upvotes: 0

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