Designing classes

The idea

A class invariant is a rule that should remain true whenever a public operation finishes. For a bounded ticket counter, the count might always remain between zero and capacity. Constructors must establish the rule, and every modifying method must preserve it. Private fields prevent callers from bypassing these checks. A useful public method expresses an action meaningful to the problem rather than exposing every internal field.

Plan failure behavior before changing state. A reservation method can return false when a request exceeds the remaining capacity, leaving the object unchanged. Query methods report state without modifying it. This separation makes sequences of operations easier to test and helps callers understand which expressions have side effects.

class Seats {
  private int remaining;
  Seats(int capacity) {
    if (capacity < 0) throw new IllegalArgumentException();
    remaining = capacity;
  }
  boolean reserve(int count) {
    if (count < 0 || count > remaining) return false;
    remaining -= count; return true;
  }
  int remaining() { return remaining; }
}
public class Main {
  public static void main(String[] args) {
    Seats s = new Seats(4);
    System.out.println(s.reserve(5));
    System.out.println(s.remaining());
  }
}

false
4

The guard rejects an oversized request before subtracting anything. The failed command leaves the observable state unchanged.