2 collections related question, please:

Question

1. Each time i'm overriding equals i must override hashCode too, right?
2. I have a list that it's items are:
public class ListItem{
    private Person p;
    // more members
public class Person{
   private String name;
   private int age;
   // more members
}I want to extract all occurrences that match a certain condition (for instance - the name contains aa and the age is 16-31).
I thought of iterating over the list and check listItems with equals (after i override equals of course) or implementing comperable in both classes above -
is there another way to do that WITHOUT iterating over the list?

Answer 1

The point is only to show
that a developer can always write some weird code
(probably caused by an even weirder design) which
will be in conflict with your must rule.Yes, and so far I think all such code would be wrong. Even if the field is private with no setter, some other developer (or even you, forgetting your youthful foibles) could change the value to something else, thus breaking the equals/hashcode behavior. Likewise assuming the class will never be used in a hash container is quite fragile. And in case there's any confusion, it is not my rule, I was just attempting to state it properly. From Object docs:
"The general contract of hashCode is:
- If two objects are equal according to the equals(Object) method, then calling the hashCode method on each of the two objects must produce the same integer result."
So if equals doesn't use all non-final fields that are used in hashCode, it's extremely fragile at best.