//......after connecting to db and all that
 
 
if(serial.equals("") )
 {
 if(!tel.equals("") )//
 {
    query = "SELECT * FROM tbooks WHERE tbooks.tel=' "+tel+" ' ";
 }
  else if(gender.equals(""))
 {
   query = "SELECT * FROM tbooks WHERE tbooks.gender=' "+gender+" ' ";
 }
      }//end if
else if (!serial.equals(""))
{
query = "SELECT * FROM tbooks WHERE tbooks.serial=' "+serial+" ' ";
}
 
//query is the statement to be executed later as  
//ResultSet rs = statement.executeQuery(query);
// more code goes here.......

SELECT * FROM tbooks WHERE SERIAL = ' "+serial+" ' AND tel= ' "+tel+" ' AND gender= ' "+gender+" ' AND datez= ' "+datez+" '

// Create a list object for the conditions, and the query string.
List conditions = new List();
String query = "SELECT * FROM tbooks";
 
// Add each condition to the list if it is present.
if(!serial.equals("")) {
    conditions.add("serial='" + serial +"'");
}
if(!tel.equals("")) {
    conditions.add("tel='" + tel +"'");
}
if(!gender.equals("")) {
    conditions.add("gender='" + gender +"'");
}
if(!date.equals("")) {
    conditions.add("date='" + date +"'");
}
 
// If any conditions were present, compile a WHERE clause.
if(conditions.size() > 0) {
    query += " WHERE ";
    Iterator i = conditions.iterator();
    int cLength = conditions.size();
    while(i.hasNext())
    {
        query += i.next().toString();
        if(cLength > 1) { // We want to skip the last loop, so > 1
            query += " AND ";
        }
        --cLength;
    }
}
 
// And you should have your query.