import java.lang.annotation.*;
import java.util.*;
import java.lang.reflect.*;

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.FIELD)
@interface Relationship{
	String inverse();
	Class target(); // Necessary because java uses type errasure
	boolean oneWay() default false;
}


class CheckRelationships {
	public static void check(Class c) throws Misspecification{
		for(Field f : c.getDeclaredFields()){
			Relationship r = f.getAnnotation(Relationship.class);
			if (r != null)
				check(f, r);
		}
	}

	private static void check(Field f, Relationship r) throws Misspecification{
		if (! r.oneWay() ){
			Field otherField;
			Class other;
			Relationship otherRelationship;

			Class me = f.getDeclaringClass();
			other = r.target();
			if (other == null)
				throw new Misspecification(f,null,"target is null");
			try{
				otherField = other.getDeclaredField(r.inverse());
				ParameterizedType pt = (ParameterizedType)otherField.getGenericType();
			}catch(NoSuchFieldException e){
				throw new Misspecification(f, null, "inverse field not existing");
			}
			otherRelationship = otherField.getAnnotation(Relationship.class);
			if (otherRelationship.target() != me)
				throw new Misspecification(f,otherField,"target mismatch");

		}
	}

	public static class Misspecification extends Exception {
		public Field source, target;
		Misspecification(Field s, Field t, String error){
			super(error);
			source = s; target = t;
		}
	}


}