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@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
public @interface RecordId {
}

public final class Scan {

    public static final String recordIdFieldNameIn(Class klass) {
        for (Field field : klass.getDeclaredFields()) {
            if (field.isAnnotationPresent(RecordId.class)) {
                return field.getName();
            }
        }
        throw new RecordIdNotFoundException(klass);
    }

}

public class RecordIdNotFoundException extends RuntimeException {
    public RecordIdNotFoundException(Class klass) {
        super("@" + RecordId.class.getSimpleName() + " not found on " + klass.getSimpleName());
    }
}

public class MemoryStorage<T> implements Store<T> {

    // Cached
    private final String recordId;

    protected final Map<Serializable, T> data = new HashMap<Serializable, T>();
    protected final IdGenerator idGenerator;

    public MemoryStorage(IdGenerator idGenerator) {
        this.idGenerator = idGenerator;
        Class klass = (Class) ((ParameterizedType) getClass().getGenericInterfaces()[0]).getRawType();
        recordId = Scan.recordIdFieldNameIn(klass);
    }

    ...

    /**
     * {@inheritDoc}
     */
    @Override
    public void save(T item) {
        Serializable newId = idGenerator.generate();

        try {
            Method setMethod = item.getClass().getMethod(recordId, Serializable.class);
            setMethod.invoke(item, newId);
        } catch (Exception e) {
            // throw exception
        }

        data.put(newId, item);
    }

}