// angela's class
public class RegisterNewUser {
  public void makeSureLoginAndPasswordAreValid(String loginName, String password) {

    ValidateData validateData = new AngelasPretendValidation(); // this gets a simple implementation

    boolean isValidLogin = validateData.isValidLogin(loginName);
    boolean isValidPassword = validateData.isValidPassword(password);
    if(!isValidLogin || !isValidPassword) {
      // not valid
      ... do something
    }
    // valid
    ... do something
  }
}

// michael's interface 
public interface ValidateData {
  public boolean isValidLogin(String login);
  public boolean isValidPassword(String login);
}

// Angela's pretend class 
public class AngelasPretendValidation {
  public boolean isValidLogin(String login) {
    return true;
  }
  public boolean isValidPassword(String login) {
    return true;
  }
}