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package user;
import account.*AccountHolder;
public class UserDetails extends AccountHolder {
  public synchronized double getUserBalance(String accountNumber) {
    // Use a method of AccountHolder to get the account balance 
    return getBalance(accountNumber); 
  }
}

 
public class User {
  public boolean exists(String username) { 
    // Check whether user exists 
    return true; // Exists
  }
}