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

Answer 1

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

Answer 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.