Increased compile-time safety with phantom types http://www.javacodegeeks.com/2013/02/increased-compile-time-safety-with-phantom-types.html

Ref.java

// http://www.javacodegeeks.com/2013/02/increased-compile-time-safety-with-phantom-types.html

public interface Entity {
  Long getId();
}


public final class Ref<T extends Entity> implements Serializable {
  public final long id;

  public static <T extends Entity> Ref<T> of(T value) {
    return new Ref<T>(value.getId());
  }
  public static <T extends Entity> Ref<T> of(long id, Class<T> clazz) {
    return new Ref<T>(id);
  }

  @Override
  public String toString() {
    return String.valueOf(id);
  }

  private Ref(long id) {
    this.id = id;
  }

  public long getId() {
    return this.id;
  }

  @Override
  public int hashCode() {
    return (int) (id ^ (id >>> 32));
  }

  @Override
  public boolean equals(Object o) {
    if (this == o)
      return true;
    if (o == null || o.getClass() != this.getClass())
      return false;
    Ref<?> other = (Ref<?>) o;
    return other.id == this.id;
  }
}