rs = st.executeQuery(selectSQL);

    output = rs.getString("column");

try{
      rs = st.executeQuery(sql);

      int i = 0;
      while(rs.next()){
            output[i] = rs.getString(column);
            // column field in the database contains multiple results, but sometimes
            // may be null
            i++;
      }
 }catch{SQLException e){
      e.printStackTrace();
  // other than tracing the exception i want to fill the array too
 }
return output;

String myValue = rs.getString("myColumn");
if (rs.wasNull())
myValue = ""; // set it to empty string as you desire.

From java.sql.ResultSet
boolean wasNull() throws SQLException;

* Reports whether
* the last column read had a value of SQL <code>NULL</code>.
* Note that you must first call one of the getter methods
* on a column to try to read its value and then call
* the method <code>wasNull</code> to see if the value read was
* SQL <code>NULL</code>.
*
* @return <code>true</code> if the last column value read was SQL
*         <code>NULL</code> and <code>false</code> otherwise
* @exception SQLException if a database access error occurs or this method is
*            called on a closed result set
*/

output = rs.getString("column");// if data is null `output` would be null, so there is no chance of NPE unless `rs` is `null`

if(output == null){// if you fetched null value then initialize output with blank string
  output= "";
}

the column value; if the value is SQL NULL, the value returned is null

Class.forName("org.sqlite.JDBC");
con = DriverManager.getConnection("jdbc:sqlite:My_db.db");
String sql = ("select * from cust where cust_id='" + cus + "'");
pst = con.prepareStatement(sql);
rs = pst.executeQuery();
con.close();
System.out.println(rs.getString(1));

Class.forName("org.sqlite.JDBC");
con = DriverManager.getConnection("jdbc:sqlite:My_db.db");
String sql = ("select * from cust where cust_id='" + cus + "'");
pst = con.prepareStatement(sql);
rs = pst.executeQuery();
System.out.println(rs.getString(1));
con.close();

String a;
if(rs.getString("column") != null)
{
    a = "Hello world!";
}
else
{
    a = "Bye world!";
}

String name = (oRs.getString ("name_column"))! = Null? oRs.getString ("name_column"): "";