java-design-patterns/feature-toggle/src/main/java/com/iluwatar/featuretoggle/user/UserGroup.java

package com.iluwatar.featuretoggle.user;

import java.util.ArrayList;
import java.util.List;

/**
 * Contains the lists of users of different groups paid and free
 */
public class UserGroup {

  private static List<User> freeGroup = new ArrayList<>();
  private static List<User> paidGroup = new ArrayList<>();

  /**
   *
   * @param user {@link User} to be added to the free group
   */
  public static void addUserToFreeGroup(final User user) {
    if (paidGroup.contains(user)) {
      throw new IllegalArgumentException("User all ready member of paid group.");
    } else {
      if (!freeGroup.contains(user)) {
        freeGroup.add(user);
      }
    }
  }

  /**
   *
   * @param user {@link User} to be added to the paid group
   */
  public static void addUserToPaidGroup(final User user) {
    if (freeGroup.contains(user)) {
      throw new IllegalArgumentException("User all ready member of free group.");
    } else {
      if (!paidGroup.contains(user)) {
        paidGroup.add(user);
      }
    }
  }

  public static boolean isPaid(User user) {
    return paidGroup.contains(user);
  }
}