Get output of cmd command from java code

I have a program where I was able to successfully execute cmd commands from my code, but I want to be able to get the output from the cmd command. How can I do that?

So far my code is:

Second.java:

public class Second {
    public static void main(String[] args) {
        System.out.println("Hello world from Second.java");
    }
}

and Main.java

public class Main {
    public static void main(String[] args) {
        String filename = args[1].substring(0, args[1].length() - 5);
        String cmd1 = "javac " + args[1];
        String cmd2 = "java " + filename;

        Runtime r = Runtime.getRuntime();
        Process p = r.exec(cmd1); // i can verify this by being able to see Second.class and running it successfully
        p = r.exec(cmd2); // i need to see this output to see if 

        System.out.println("Done");
    }
}

I can check the first command is working successfully by checking for Second.class, but what if this class generated some error, how will I be able to see that error?