Java equals and hashCode: the contract and why HashSet breaks without it

import java.util.*;
public class StudentKey {
  static class Student {
    final String roll; final String name;
    Student(String roll, String name) { this.roll = roll; this.name = name; }
    @Override public boolean equals(Object o) {
      if (this == o) return true;
      if (!(o instanceof Student)) return false;
      return roll.equals(((Student) o).roll);
    }
    @Override public int hashCode() { return Objects.hash(roll); }
  }
  static class NoHash {
    final String roll;
    NoHash(String roll) { this.roll = roll; }
    @Override public boolean equals(Object o) { return o instanceof NoHash n && n.roll.equals(roll); }
  }
  record Course(String code, int credits) {}
  public static void main(String[] args) {
    Set<Student> good = new HashSet<>(List.of(new Student("22CS101", "Asha")));
    good.add(new Student("22CS101", "Asha K"));
    Set<NoHash> bad = new HashSet<>(List.of(new NoHash("22CS101")));
    bad.add(new NoHash("22CS101"));
    Set<Course> courses = new HashSet<>(List.of(new Course("CS301", 4)));
    courses.add(new Course("CS301", 4));
    System.out.println(good.size() + " " + bad.size() + " " + courses.size());
  }
}

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