0 byte file appears when Capturing screen by screencap

I'm having trouble with capturing the screen of un-rooted phone.

I've tried the command at my computer:

adb shell /system/bin/screencap -p /sdcard/out.png

and this worked well.

However, whenever I try to use this command at my application like this :

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

            String[] str ={"/system/bin/screencap","-p","/sdcard/out.png"};

            try {  
                Process ps = Runtime.getRuntime().exec(str); 
                try { 
                    ps.waitFor(); 
                } catch (InterruptedException e) { 
                    e.printStackTrace(); 
                }  
            } catch (IOException e) { 
                Toast.makeText(this, ""+e, Toast.LENGTH_LONG).show(); }


}

I always get 0 byte of out.png. I think there is no error on the source code, but I can't understand why this kind of error occurs.

Please help me with this problem.

Thanks.