Getting values from java.util.Map

Question

I have Map<Date, String. I have two Date objects a, b that are equal. I put a string value to the map associated with key a. Then I try to get map value associated with keys a and b but only a returns value that I've put. Is it possible to get my value with b key. I know this is possible when keys are simple strings. Why this doesn't work with other type of objects?

public class Main {

public static void main(String[] args) {
    Map<Date, String> map = new HashMap<Date, String>();

    Date a = new Date(20131105);
    Date b = new Date(20131105);

    map.put(a, "sweet");

    System.out.println(map.get(a));
    System.out.println(map.get(b));
}

static class Date {
    private int ymd;

    public Date(int ymd) {
        this.ymd = ymd;
    }

    public int getYmd() {
        return ymd;
    }

    @Override
    public boolean equals(Object obj) {
        if (obj instanceof Date) {
            return ((Date) obj).ymd == ymd;
        }
        return false;
    }
}

}

The output is:

sweet
null

Alexis C. · Accepted Answer · 2013-11-05 13:26:48Z

5

Since you're using an HashMap for storing your date objects, you have to override the hashCode() method because the key objects are stored in the data structure using their hashCode.

Very basic hashCode() overriden (just for illustration):

@Override
public int hashCode(){
   return ymd;
}

Output :

sweet
sweet

edited Nov 5, 2013 at 13:26

answered Nov 5, 2013 at 13:08

Alexis C.

94.3k22 gold badges173 silver badges179 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

Alexis C. Over a year ago

@RichardTingle He's using his own Date class.

Richard Tingle Over a year ago

Just noticed that, people need to stop doing that

Alexis C. Over a year ago

@RichardTingle I hope this is only for an exercise purpose !!

Ivaylo Strandjev · Accepted Answer · 2013-11-05 13:13:31Z

2

A hashmap hashes the elements using their hashCode function. For most types the hash code is computed using the reference of the object and this is the case with Date. While in your case the two dates have same value, they are not the same object. Thus they have different references and so their hash codes differ. When you lookup an element in a HashMap its hashCode is used and so as b's hashCode is different from a's hashCode you can not find an element with key a by using b.

answered Nov 5, 2013 at 13:13

Ivaylo Strandjev

71.4k18 gold badges130 silver badges183 bronze badges

Comments

Prabhakaran Ramaswamy · Accepted Answer · 2013-11-05 13:14:10Z

1

You need to implement hashCode() method in your Date class

Add below code in your Date class

    @Override
    public int hashCode() {              
          return ymd;
    }

output

sweet
sweet

answered Nov 5, 2013 at 13:14

Prabhakaran Ramaswamy

26.1k10 gold badges61 silver badges65 bronze badges

Comments

Qazi · Accepted Answer · 2013-11-05 14:41:19Z

0

Because String,Integer and all wrapper class override hasCode and equals both the method. But In your case equals() method will return true but you have not override hasCode() method so it will generate different hashcode for both a and b .So map.get(b) will return null value.

answered Nov 5, 2013 at 14:41

Qazi

1321 gold badge4 silver badges11 bronze badges

3 Comments

Qazi Over a year ago

I dont understand why are you creating a separate date class for Date use directly java.util.Date; Map<Date, String> map = new HashMap<Date, String>(); Date a = new Date(20131105); Date b = new Date(20131105); map.put(a, "sweet"); System.out.println(map.get(a)); System.out.println(map.get(b)); Output: sweet sweet Because java.util.Date class override equals and hashCode method.

Egis Over a year ago

I wrote this code just as a sample for my question. I don't use anywhere else.

Qazi Over a year ago

Please give proper naming convention.

Collectives™ on Stack Overflow

Getting values from java.util.Map

4 Answers 4

3 Comments

Comments

Comments

3 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

3 Comments

Comments

Comments

3 Comments

Your Answer

Sign up or log in

Post as a guest

Related