public class BankAccount extends JFrame

{
    private int balance = 0;
	private String accountID;

	public BankAccount(String accountID) {
		for (int i = 0; i<accountID.length();i++) {
    		char c = accountID.charAt(i);
    		 if (! Character.isDigit(c)) {
    			throw new IllegalArgumentException();
    		}
    	}
		this.accountID = accountID;
	}