基本问题:基本的 PL/SQL 控制台输出?

I am using SQL Developer and want to output the contents of a variable to the console using DBMS_OUTPUT.PUT_LINE(). I am running the following code that adds the numbers 1 through 5 inclusive but I'm not seeing any output.

SET SERVEROUTPUT ON;
DECLARE 
n_counter NUMBER := 5; -- Substitute this variable
n_sum     NUMBER := 0;
BEGIN
  WHILE n_counter != 0
  LOOP
    n_sum := n_sum + n_counter;
    n_counter := n_counter -1;
  END LOOP;
  DBMS_OUTPUT.PUT_LINE(n_sum);
END;

此外，您是否知道比极其密集的 Oracle PL/SQL 文档更好的故障排除资源?[类似于Java SE7 API?]