http://teafortw0.hatenablog.com/entry/2017/12/13/220710 に対する疑問のためのコード

BloodType.java

public abstract class BloodType {
  public static BloodType A = new BloodType(0, "A") {};
  public static BloodType B = new BloodType(1, "B") {};
  public static BloodType O = new BloodType(2, "O") {};
  public static BloodType AB = new BloodType(3, "AB") {};

  private final int ordinal;
  private final String name;
  private BloodType(final int ordinal, final String name) {
    this.ordinal = ordinal;
    this.name = name;
  }
}