由于您已经在使用PreparedStatement，因此请正确使用并传递username作为参数，而不仅仅是连接字符串:

Since you're already using PreparedStatement, use it right and pass the username as parameter instead of just concatenating the Strings:

//no need to use a cast here
preparedStatement = connection.prepareStatement(
    //note the usage of ? instead of concatenating Strings
    "DELETE FROM users WHERE user_name=?");
//setting the first parameter in the query string to be username
preparedStatement.setString(1, username);
preparedStatement.executeUpdate();

使用此方法，您将不会有任何串联问题，而且更好的是，您的代码也不会容易 SQL注入.

Using this, you won't have any concatenation problems and what's better, your code won't be prone to SQL Injection.

与您的问题没有直接关系，但是将代码移动到一个单一的方法来执行INSERT，UPDATE和DELETE语句会更好.

Not directly related to your problem, but it would be better if you move the code to execute INSERT, UPDATE and DELETE statements to a single method.

public void executeUpdate(Connection con, String query, Object ... params)
    throws SQLException {
    PreparedStatement pstmt = con.prepareStatement(query);
    if (params != null) {
        int i = 1;
        for(Object param : params) {
            pstmt.setObject(i++, param);
        }
    }
    pstmt.executeUpdate();
    pstmt.close();
}

因此，您的代码将大大减少为:

So your code would be dramatically reduced to:

String deleteSQL = "DELETE FROM users WHERE user_name=?";
executeUpdate(deleteSQL, username);

请注意，您可以基于此方法创建一个新方法来执行SELECT语句.

Note that you can create a new method based on this approach to execute SELECT statements.

此外，不要忘记关闭您的资源.使用类似这样的方法也可以大大减少这种情况:

Also, don't forget to close your resources. This also can be dramatically reduced using a method like this:

public void closeResource(AutoCloseable res) {
    try {
        if (res != null) {
            res.close();
        }
    } catch (Exception e) {
        //handle this exception...
        //basic example, not meant to be used in production!
        e.printStacktrace(System.out);
    }
}

请注意，Connection，Statement(及其子级PreparedStatement和CallableStatement)和ResultSet接口已经从AutoCloseable扩展.

Note that Connection, Statement (and its children PreparedStatement and CallableStatement) and ResultSet interfaces already extend from AutoCloseable.