Java SQL 检查用户名是否已经存在

Question

我正在尝试检查用户名是否已存在于表中:数据库也将字段设置为唯一该方法将返回一个 boolean 值，说明用户名是否已经存在。我的代码中将 usernameExists var 设置为 true 的部分永远不会运行，即使用户名相同也是如此。

 public static boolean CheckUsernameExists(String username)
{
    boolean usernameExists = false;

    try
    {

        Class.forName("com.mysql.jdbc.Driver");
        Connection connection = DriverManager.getConnection(urlDB, usernameDB, passwordDB);

        PreparedStatement st = connection.prepareStatement("select * from Members order by username desc");
        ResultSet r1=st.executeQuery();
        String usernameCounter;
         if(r1.next()) 
         {
           usernameCounter =  r1.getString("username");
           if(usernameCounter == username) //this part does not happen even if it should
           {
               System.out.println("It already exists");
              usernameExists = true;
           }
         }


     }

     catch (SQLException e) 
     {
        System.out.println("SQL Exception: "+ e.toString());
     } 
     catch (ClassNotFoundException cE) 
     {
        System.out.println("Class Not Found Exception: "+ cE.toString());
     }

 return usernameExists;
 }

如果您能提供帮助，我将不胜感激!

Answer 1

使用 if(usernameCounter.equals(username))。字符串比较很棘手。这个已经详细讨论了here .