Miért irja ezt a hibát? Java

Hiba: Exception in thread "Thread-1" java.lang.NumberFormatException: For input string: ""

at java.lang.NumberFormatException.forInputString(Unknown Source)

at java.lang.Integer.parseInt(Unknown Source)

at java.lang.Integer.parseInt(Unknown Source)

at hu.newterraria.game.Read.run(Read.java:56)

at java.lang.Thread.run(Unknown Source)

Kód:

client.getInputStream().read(readbytes,0,readbytes.length);

xs = new String(readbytes,Charset.forName("ASCII"));

client.getInputStream().read(readbytes,0,readbytes.length);

ys = new String(readbytes,Charset.forName("ASCII"));

}

catch (IOException e) {

// TODO Auto-generated catch block

e.printStackTrace();

}

System.out.println(xs);

xs = xs.substring(0, 1);

xs = xs.replaceAll( "[^\\d]", "" );

System.out.println(xs);

readx = Integer.parseInt(xs);

ys = ys.substring(0, 1);

ys = ys.replaceAll( "[^\\d]", "" );

ready = Integer.parseInt(ys);

readposition[readx][ready] = 1;