Question

I have a class with 3 attributes. One of the attributes is used as a unique key , so I use that one key in my equals and hashcode method.

Now once the set is built, during lookup, I have only the primary key (unique key) and I will have to construct a dummy object with unique key and setting other two values to 0 (default values) and then I say

Chapter myObject = new Chapter(uniqueKey,0,0);
hashSet.contains(myObject); // This will work.

Is this a correct usage for Hashset lookup? Is there any other better way because I am not a fan of setting bogus values on a object.

Answer 1

One way to make this less verbose is to define a Chapter() constructor that takes a uniqueKey argument and fills in the rest, and then use:

hashSet.contains(new Chapter(uniqueKey));

Since you are throwing away the object anyways.

However, since you have a uniqueKey, which I assume is unique to each instance of a Chapter, have you considered using a HashMap instead? Then you could just use uniqueKey as keys, and you’d just have to check whether the key is present (eg: using HashMap.containsKey()) or not:

hashMap.containsKey(uniqueKey);

With a HashMap, you can also just get the Object you want with .get():

Chapter c = hashMap.get(uniqueKey);

This will give you the Chapter object if the supplied uniqueKey maps to an object, or null if doesn’t.