Helpful functions from org apache commons collections

Introduction

There are times when we are faced with the option to get the intersection of 2 lists. The type of list can be either a simple type like number / strings or a complex type like Object.

This might involve developers building loops and performing comparison. This can be resulting in not following the DRY principle where we do not have to repeat ourselves what is already implemented.

We can leverage the functionality provided in the collections4 library which can help us get the desired result. In this example, let us consider the following example. 

Sample Scenario

We can consider a "Cart" store which keeps track of the various items that we have added the cart. Every addition or deletion of the item to the cart needs to be updated. User's normally add some items and when they find out that their product of interest has its availability, they would want to add that to the cart and balance out by removing a lesser priority item.

Now, when the user proceeds for the "Checkout", we might have already be containing the initial cart state which could be like stored as a safe keeping backup. In comparison to the final state, we might have to perform the calculations, discounts etc.

The Logic

This process involves updating the cart with the final list of products, in this we might need to figure out the ones that were removed and the ones that were newly added to the cart, in this scenario, we can leverage the subtract methods in the collections library to help us.

Sample Code

The library can be imported using maven to a spring boot project using the below given maven dependency

<dependency>
    <groupid>org.apache.commons</groupid>
    <artifactid>commons-collections4</artifactid>
    <version>4.4</version>
</dependency>

The sample implementation is as given below. The subtract method is used to perform the intersection operation on the 2 lists, the ordering of the lists play an important role. 

The first operation that is done is to remove the products that were added earlier and not present in the current state, in this case, we pass the existingProductIds and subtract the incoming or product ids from the cart, which will give us the products that were present before checkout and not present during checkout. These products can be safely removed from the cart.

List<Long> productsToDeleteFromCart = subtract(existingProductIds, cart.getProductIds());

The below example code snippet shows how to find the new items that are added the cart during checkout. In this case, we do a subtract on the incoming productIds against the existing ones so that the existing ones will be removed from the incoming ids which will give us the list of Ids that have to be added.

List<Long> productsToAddToSuite = subtract(cart.getProductIds(), existingProductIds);

As given above, we might not have to perform the redundant loops implementations and unit test them, instead use the library to get the job done.

Hope this helps