Question

Basically, when I type these commands in
the terminal by hand, the sift program works and writes a .key file, but when I try to call it from my program, nothing is written.

Am I using the exec() method correctly? I have looked through the API and I can’t seem to spot where I went wrong.

public static void main(String[] args) throws IOException, InterruptedException
{           
        //Task 1: create .key file for the input file
        String[] arr  = new String[3];
        arr[0] =  "\"C:/Users/Wesley/Documents/cv/final project/ObjectRecognition/sift/siftWin32.exe\"";
        arr[1] = "<\"C:/Users/Wesley/Documents/cv/final project/ObjectRecognition/sift/cover_actual.pgm\"";
        arr[2] = ">\"C:/Users/Wesley/Documents/cv/final project/ObjectRecognition/sift/keys/cover_actual.key\"";

        String command = (arr[0]+" "+arr[1]+" "+arr[2]);

        Process p=Runtime.getRuntime().exec(command); 
        p.waitFor(); 
        BufferedReader reader=new BufferedReader(new InputStreamReader(p.getInputStream())); 
        String line=reader.readLine(); 

        while(line!=null) 
        { 
            System.out.println(line); 
            line=reader.readLine(); 
        } 
}

Answer 1

The command line you are using is a DOS command line in the format:

prog < input > output

The program itself is executed with no arguments:

prog

However the command from your code is executed as

prog "<" "input" ">" "output"

Possible fixes:

a) Use Java to handle the input and output files

Process process = Runtime.getRuntime().exec(command);
OutputStream stdin = process.getOutputStream();
InputStream stdout = process.getInputStream();

// Start a background thread that writes input file into "stdin" stream
...

// Read the results from "stdout" stream
...

See: Unable to read InputStream from Java Process (Runtime.getRuntime().exec() or ProcessBuilder)

b) Use cmd.exe to execute the command as is

cmd.exe /c "prog < input > output"