public ArrayList<Category> getChildCategories(int idNumber) {
		ArrayList<Category> ret = new ArrayList<Category>();
		Connection conn = null;
		Statement st = null;
		ResultSet rs = null;
		try {
			conn=DriverManager.getConnection("jdbc:mysql://localhost:3306/librarysystem?user=admin&password=123456");
			st=conn.createStatement();

			rs = st.executeQuery("SELECT * FROM Category WHERE superCategoryId = " + idNumber);
			while(rs.next()) {
				Category c = new Category();
				c.setIdNumber(rs.getInt("idNumber"));
				c.setName(rs.getString("name"));
				c.setSuperCategoryId((Integer)rs.getObject("superCategoryId"));
				ret.add(c);
			}

		} catch (Exception e) {
			System.out.println(e.getMessage());
		}
		finally
		{
			try {
				if(conn != null)
					conn.close();
				if(st != null)
					st.close();
				if(rs != null)
					rs.close();
			} catch (Exception e) {	}
		}
		
		return ret;
	}