exact class check

class Fragile {
    public static void main(String... args) {
      final var a = new A(0);
      final var b = new B(0);
      System.out.println("A equals B? " + a.equals(b));
      System.out.println("B equals A? " + b.equals(a));
    }
}

class A {
  protected final int n;
  public A(int p) { n = p; }
  public final int hashCode() { return n; }
  public final boolean equals(Object other) {
    if (other.getClass() == this.getClass()) {
      return n == ((A) other).n;
    }
    return false;
  }
}

class B extends A {
  public B(int p) { super(p); }
  public int hashCode() { return n; }
  public final boolean equals(Object other) {
    return other instanceof B && n == ((B) other).n;
  }
}