Retrieving Results from a ResultSet Object

A ResultSet object is the primary storage mechanism for the data returned by an SQL statement. Each ResultSet object contains both data and metadata (in the form of a ResultSetMetaData object). ResultSetMetaData includes useful information about results returned by the SQL command: column names, column count, row count, column length, and so on.

To access the row data stored in a ResultSet object, an application calls one or more getter methods. A getter method retrieves the value in particular column of the current row. There are many different getter methods; each method returns a value of a particular type. For example, the getString() method returns a STRING type; the getDate() method returns a Date, and the getInt() method returns an INT type. When an application calls a getter method, JDBC tries to convert the value into the requested type.

Each ResultSet keeps an internal pointer that point to the current row. When the executeQuery() method returns a ResultSet, the pointer is positioned before the first row; if an application calls a getter method before moving the pointer, the getter method will fail. To advance to the next (or first) row, call the ResultSet’s next() method. ResultSet.next() is a boolean method; it returns TRUE if there is another row in the ResultSet or FALSE if you have moved past the last row.

After moving the pointer to the first row, the sample application uses the getString() getter method to retrieve the value in the first column and then prints that value. Since ListEmployees calls rs.next() and rs.getString() in a loop, it processes each row in the result set. ListEmployees exits the loop when rs.next() moves the pointer past the last row and returns FALSE.

while(rs.next())
{
System.out.println(rs.getString(1));
}

When using the ResultSet interface, remember:

• You must call next() before reading any values. next() returns true if another row is available and prepares the row for processing.
• Under the JDBC specification, an application should access each row in the ResultSet only once. It is safest to stick to this rule, although at the current time, the Advanced Server JDBC driver will allow you to access a field as many times as you want.
• When you’ve finished using a ResultSet, call the close() method to free the resources held by that object.