how to know what a valid name for a variable is in java code example

Example: how to know what a valid name for a variable is in java

String[] cookies = {"Oatmeal", "Snickerdoodle", "Chocolate chip", "Gingersnap"};

        for(String i : cookies){
            System.out.println(i);
            System.out.println("Shortbread");
        }