findCommonUsers.java

	/**
	 * すべてのArrayListに共通するユーザを見つける
	 * @param <T>
	 * @param listOfUsers
	 * @return
	 */
	public static <T> ArrayList<T> getCommonUsers(List<ArrayList<T>> listOfUsers){
		if(listOfUsers == null)
			return null;
		if(listOfUsers.size() == 0)
			return new ArrayList<T>();
		if(listOfUsers.size() == 1)
			return new ArrayList<T>(listOfUsers.get(0));

		int minUserCount = Integer.MAX_VALUE;
		int minUserIndex = -1;
		for(int i = 0; i < listOfUsers.size(); i++)
			if(listOfUsers.get(i).size() < minUserCount){
				minUserCount = listOfUsers.get(i).size();
				minUserIndex = i;
			}
		if(minUserIndex == -1)
			minUserIndex = 0;

		ArrayList<T> commonUsers = new ArrayList<T>(listOfUsers.get(minUserIndex));

		for(int i = 0; i < listOfUsers.size(); i++){
			if(i == minUserIndex)
				continue;

			ArrayList<T> tmpCommonUsers = getCommonUsers(commonUsers, listOfUsers.get(i));
			commonUsers = new ArrayList<T>(tmpCommonUsers);
		}

		return commonUsers;
	}

	/**
	 * users1とusers2に共通するユーザを見つける
	 * @param <T>
	 * @param users1
	 * @param users2
	 * @return
	 */
	private static <T> ArrayList<T> getCommonUsers(List<T> users1, List<T> users2){
		ArrayList<T> commonUsers = new ArrayList<T>();

		for (T u1 : users1) {
			T u = null;
			for (T u2 : users2) {
				if (u1.equals(u2)) {
					u = u1;
					break;
				}
			}
			if (u != null) {
				boolean isOvarlapped = false;
				for (T t : commonUsers) {
					if (u.equals(t)) {
						isOvarlapped = true;
						break;
					}
				}
				if (!isOvarlapped)
					commonUsers.add(u);
			}
		}

		return commonUsers;
	}