HashSet contains problem with custom objects

Question

My Custom class that will be contained by HashSet

public class Person {
    String name;
    int age;

    public Person(String name, int age) {
        this         


        

Answer 1

Because p1.hashCode() changes when you modify p1, so it can't be found at its original index in the hash table anymore. Never let a hash value depend on a mutable field.

(You're quite lucky that it fails during testing; it might just as well have succeeded, only to fail in production.)