nullCheck.java

// Adapted from Code Smell: Null Check, Joe Eames https://medium.com/thinkster-io/code-smell-null-check-a0c4851fafbf

class DiscountCalculator {
  private static final float DEFAULT_DISCOUNT = 1.0f;
  // ...
  public float getDiscount(String customerId) {
    Customer customer = customerRepository.findBy(customerId);
    if(customer == null) {
      return DEFAULT_DISCOUNT;
    } 
    return customer.getEarnedDiscount();
  }  
}

class Orders {
  // ...
  public void processOrder(String customerId, Order order) {
    Customer customer = customerRepository.findBy(customerId);
    if(customer != null) {
      customer.addToOrdersHistory(order);
    } 
    // more logic that does not use customer...
  }
}

//--------------------------------------------------------------------
//--------------------------------------------------------------------

class NotFoundCustomer implements Customer {
  private static final float DEFAULT_DISCOUNT = 1.0f;
  //...
  public float getEarnedDiscount() {
    return DEFAULT_DISCOUNT;
  }
  
  public void addToOrdersHistory(Order order) {
  
  }
}

class DiscountCalculator {
  // ...
  public float getDiscount(String customerId) {
    Customer customer = customerRepository.findBy(customerId);
    return customer.getEarnedDiscount();
  }  
}

class Orders {
  // ...
  public void processOrder(String customerId, Order order) {
    Customer customer = customerRepository.findBy(customerId);
    customer.addToOrdersHistory(order);
    // more logic ...
  }
}