Java: cannot find symbol but no exception thrown

I am trying to run a system command in java file as shown in the answer here: How to execute system commands (linux/bsd) using Java

Below is my code (temp.java):

import java.io.BufferedReader;

import java.io.InputStreamReader;



public class temp

{    

    public static void main(String[] args)



    {

        Runtime r = Runtime.getRuntime();

        try {

            Process p = r.exec("ls"); 

            System.out.println(p);

        }

        catch (Exception ex) {

            System.out.println(ex);

        }

        p.waitFor();       //this line causes the problem



        System.out.println("Hello, World");

    }     

}

When I compile and run it (using and ) in terminal without the line , it runs fine and gives the following output:

java.lang.UNIXProcess@2524e205

Hello, World

So it seems that is being created. But when I include the line , and compile and run it the same way, it gives the following error:

temp.java:19: error: cannot find symbol

        p.waitFor();

        ^

  symbol:   variable p

  location: class temp

1 error

What is the problem?

EDIT:

The question that this question is being claimed to be duplicate of, is very broad question. I had a specific problem. I had gone through it earlier but could not figure out the problem. Please remove the duplicate mark if possible.