What does if rs next mean

Question

I am currently getting the error   java sql SQLException  Method  executeQuery String   not allowed on prepared statement    because I am using  PreparedStatement stmt   conn prepareStatement sql     and also had  ResultSet rs   stmt executeQuery sql     in my code   I now need to remove the ResultSet line but that leaves me with having to deal with the following code   if  rs next          messages add ActionMessages GLOBAL MESSAGE  new ActionMessage  login successful         request getSession true  setAttribute  USERNAME   rs getString  USERNAME         request getSession true  setAttribute  BALANCE   rs getString  BALANCE         request setAttribute  msg    Logged in successfully      I m not sure I completely understand what   if  rs next      does   Could someone explain this code to me   If I have a better understanding of that I believe I ll have a better idea on how to deal using the PreparedStatement results with the logic being used for rs   Also any help to deal with changing that logic would be greatly appreciated too

User · Answer

Since Result Set is an interface  When you obtain a reference to a ResultSet through a JDBC call  you are getting an instance of a class that implements the ResultSet interface  This class provides concrete implementations of all of the ResultSet methods   Interfaces are used to divorce implementation from  well  interface  This allows the creation of generic algorithms and the abstraction of object creation  For example  JDBC drivers for different databases will return different ResultSet implementations  but you don t have to change your code to make it work with the different drivers  In very short  if your ResultSet contains result  then using rs next return true if you have recordset else it returns false

User · Answer

As to the concrete problem with that SQLException  you need to replace ResultSet rs   stmt executeQuery sql    by ResultSet rs   stmt executeQuery     because you re using the PreparedStatement subclass instead of Statement  When using PreparedStatement  you ve already passed in the SQL string to Connection prepareStatement    You just have to set the parameters on it and then call executeQuery   method directly without re-passing the SQL string  See also   JDBC tutorial - Using Prepared Statements   As to the concrete question about rs next    it shifts the cursor to the next row of the result set from the database and returns true if there is any row  otherwise false  In combination with the if statement  instead of the while  this means that the programmer is expecting or interested in only one row  the first row  See also   Javadoc on ResultSet next   method Problem with SQL  ResultSet in java

User · Answer

First thing  you don t need to write  ResultSet rs   stmt executeQuery sql     just write  ResultSet rs   stmt executeQuery      The above mentioned syntax is used for Statements not for PreparedStatement   Second thing  rs next   checks if the result set contains any values or not  It returns a boolean value as well as it moves the cursor to the first value in the result set because initially it is at BEFORE FIRST Position  So if you want to access first value in result set  you need to write rs next

User · Answer

Look at the picture  it s a result set of a query select   from employee    and the next   method of ResultSet class help to move the cursor to the next row of a returned result set which is rs in your example

User · Answer

I m presuming you re using Java 6 and that the ResultSet that you re using is a java sql ResultSet   The JavaDoc for the ResultSet next   method states      Moves the cursor froward one row from its current position  A ResultSet cursor is initially positioned before the first row  the first call to the method next makes the first row the current row  the second call makes the second row the current row  and so on       When a call to the next method returns false  the cursor is positioned after the last row  Any invocation of a ResultSet method which requires a current row will result in a SQLException being thrown    So  if rs next       do something    means  If the result set still has results  move to the next result and do something    As BalusC pointed out  you need to replace   ResultSet rs   stmt executeQuery sql     with  ResultSet rs   stmt executeQuery      Because you ve already set the SQL to use in the statement with your previous line  PreparedStatement stmt   conn prepareStatement sql     If you weren t using the PreparedStatement  then ResultSet rs   stmt executeQuery sql   would work

User · Answer

The next   moves the cursor froward one row from its current position in the resultset  so its evident that if rs next    means that if the next row is not null  means if it exist   Go Ahead   Now w r t your problem    ResultSet rs   stmt executeQuery sql      This is wrong                                              note that executeQuery String  is used in case you use a sql-query as string   Whereas when you use a PreparedStatement  use executeQuery   which executes the SQL query in this PreparedStatement object and returns the ResultSet object generated by the query   Solution     Use   ResultSet rs   stmt executeQuery

User · Answer

The next   method  offcial doc here  simply move the pointer of the result rows set to the next row  if it can   Anyway you can read this from the offcial doc as well      Moves the cursor down one row from its current position    This method return true if there s another row or false otherwise

[java] What does "if (rs.next())" mean?

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