Below code will help you:
String cnum = "4242424242424242"
Use a for loop to extract single character and use second pointer in your question, as below:
for (char ch : cnum.toCharArray()) {
    switch (Integer.parseInt(String.valueOf(ch))) {
        case 1:
            driver.findElement(By.id("cardNumber")).sendKeys(Keys.NUMPAD1);
            break;
        case 2:
            //and so on...
    }
}