execing a program and reading it's output hangs

Execing a program and reading it's output sometimes hangs in the read.
I think could work around it if there was some equivalent to select(), or a way to
attach a callback to a signal, or a way to tell if a process has died. If there
are ways to do these things, please let me know.

Here is a sample program.

import java.io.*;
//
// Cat the output of a file. To run: java cat filename
//
public class cat {
    public cat(String file) {
        String args[] = new String[2];
        Process p;
        byte buf[] = new byte[100];
        InputStream s;
        int length;

        args[0] = "/bin/cat";
        args[1] = file;
        try {
            p = Runtime.getRuntime().exec(args);
            s = p.getInputStream();

            System.out.println("Before read");
            while ((length = s.read(buf)) != -1) {
                System.out.println("After read");
                System.out.println(new String(buf,0,0,length));
            }
        } catch (IOException e) {
            System.out.println("command failed.");
        }
        System.out.println("got to here");
    }

    public static void main(String args[]) {
        if (args.length == 0) {
            System.out.println("enter file");
        } else {
            new cat(args[0]);
        }
    }
}

Running it giving it a file that exists works ok, but hangs if the file does not exit.
I tried many, many different tests which included execing a shell script which
would hang no matter what! So I'm not sure what it is related to. I thought it
might be the nonzero exit status, but the shell script exited 0 and the read hung.

By the way, the above is a test case. The program I really want to exec is javac
and read the compile errors. If there are errors, the read always hangs.