Why Can't I Reject Letters

String string1 = JOptionPane.showInputDialog("How many quarters do we add?:");
    // If the user enters a number above 0 in the text box, then the program sets that
    // value to the number of quarters to add. Otherwise, the program errors out.
    if (string1.equals("") == true || Integer.parseInt(string1) < 0){
        JOptionPane.showMessageDialog (
            null, "Please enter a positive or zero value for the added money.\nNo money added.",
                "Number Error",
            JOptionPane.ERROR_MESSAGE);
        addedQ = 0;
        addedD = 0;
        addedN = 0;
        addedP = 0;
    }else {
        addedQ = Integer.parseInt(string1);