Question

我正在尝试创建一个循环，继续循环，直到用户输入不存在的用户名。例如，maryj存在于数据库中。跟踪代码，我首先输入maryj，我将进入while循环并询问用户Please enter a another username.Enter your username:。我再次输入maryj，但循环不要求我输入另一个用户名。它将自己移动到下一位代码。似乎while循环仅在第一次试用后才起作用，但如果我多次输入相同的用户名则不行。代码如下。

  public void register() throws SQLException
  {

  Scanner stdin;
  String sql_statement;
  Statement select = connection.createStatement();

  stdin = new Scanner(System.in);

  System.out.print("Enter your user name: ");
  user_name = stdin.next();


  sqlz_statement = "SELECT " + "user_name " + "FROM" + " users" + " where user_name = "
        + "'" + user_name + "'";

  result = select.executeQuery(sql_statement);

  while(result.next())
  {
     System.out.println("Please select another username.");
     System.out.print("Enter your user name: ");
     user_name = stdin.next();
  }

  System.out.println("Enter your last name: ");
  System.out.println("Enter your first name: ");

  } // method register

Answer 1

你错了，因为你只看了一次数据库，你应该这样做：

  Boolean exist=true;
  while (exist) {
      System.out.print("Enter your user name: ");
      user_name = stdin.next();
      sqlz_statement = "SELECT " + "user_name " + "FROM" + " users" + " where user_name = "
            + "'" + user_name + "'";
      result = select.executeQuery(sql_statement);
      if (!result.next()) {exist=false}
      else {System.out.println("Please select another username.");}
  }  
  System.out.println("Enter your last name: ");
  System.out.println("Enter your first name: ");

PostgreSQL / Java循环直到用户输入不存在的用户名

1 个答案: