Question

Say you had this DTO class:

public class MyObj{
 private int id;

 private String displayName;
 private String backendData;

 public boolean equals(Object obj){
  return id.equals(obj);
 }

 private int hashCode(){
  return id.hashCode();
 }
}

Lets say a user got to pick several instances of MyObj from a list that only showed the displayName and the id# is associated in the background. To save bandwidth, you don’t send backendData. When they submit their selection back to you, the client just sends you the id#.

Now, you’ve maintained the list of original options server side in a Collection<MyObj>. The naive approach to geting the “full” object back from the collection would be to iterate through the Collection and call “.equals()” on every object. This scales in O(n) though 🙁

It seems like with constant time operations collections like HashSet, I should be able to retrieve an object in constant time, if I know it’s identity. But HashSet only has a “contains()” method and doesn’t return the object it found.

Any advice? As always, thanks a bunch stackOverFlow!

Answer 1

I assume id is unique across the collection?

You are almost there use a Map<Integer, MyObj> then map.get(id) is asymptotically constant time.