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Coding Challenge ("Light" Edition)

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This coding challenge is a bit different from the previous one ([1] [2]) because it is shorter and also a bit more closely tied to Java.

A School has either a name (“Stanford”) or a nickname (“UCSD”) or both.

Your task is to write equals() and hashCode() methods for the School class that satisfy the following requirement: two Schools are identical if they either have the same name or the same nickname.


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sun east replied on Thu, 2013/02/21 - 10:50pm


School a = School(name = "Stanford", nickname = null);

School b = School(name = null, nickname = "UCSD");

School c = School(name = "Stanford", nickname = "UCSD");

 According to your description, the following relationship holds:

a.equals(c) == true
b.equals(c) == true
a.equals(b) == false

David Whatever replied on Fri, 2013/02/22 - 1:37am

Seems the hashCode part is not possible to do well, since you have to have equal hashes between two arbitrary strings (a name and a nickname). So I went with:

public class School {
   public String name, nickname;
   public boolean equals(Object other) {
       if (super.equals(other)) return true; if (!(other instanceof School) return false; School otherSchool = (School) other; return (name != null && name.equals(otherSchool.name) || (nickname != null && nickname.equals(otherSchool.nickame));    }
   public int hashCode() {
       return 0;

Nils Rudolph replied on Fri, 2013/02/22 - 3:16am

This is a tricky question, if would say that such a equal function is not possible, or all school must be equal. I try to explain why:

The equal method must be an equivalence relation, so it must be:

transitive: if a==b and b==c then must a==c
symetric: if a==b then must b==a
reflexiv: a==a

So if you take two Schools which should not be equal: (assuming that there is a constructor which takes the name and the nickname as parameter)

s1 = School("a", "b")
s2 = School("c", "d")

and you create a third School:

s3 = School("a", "d")

now you have:

s1 == s3 and s3 == s2 

from the transitive property it follows:

s1 == s2

David Whatever replied on Fri, 2013/02/22 - 9:45am in response to: Nils Rudolph

I considered making all School objects equivalent in addition to making them all hash to zero; the requirement only said they had to be equal in the situation where the name/nickname matches, not that they had to be unequal in other cases ;-)

Farrukh Ijaz replied on Mon, 2013/02/25 - 2:02pm

static class School {
        public String name, nickname;
        public boolean equals(Object source) {
        	if(!(source instanceof School)) return false;
            School s = (School) source;
			boolean flag = name != null && s.name != null && name.equals(s.name);
            return flag || (nickname != null && s.nickname != null && nickname.equals(s.nickname));
        public int hashCode() {
            if(name != null && nickname != null)
                return name.concat(nickname).hashCode();
            if(name != null)
                return name.hashCode();
            if(nickname != null)
                return nickname.hashCode();
            return 0;

I believe this the maximum we can implement. hashCode() method must return none-zero value in order to use the object with hash tables.

Somnath Shantveer replied on Thu, 2013/02/28 - 6:03am in response to: Farrukh Ijaz

 +1 to Farrukh... I ended up writing similar code.Want to know if some one has better implementation for hashcode.

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