One Situation When Reusing Object Instance / Singleton Won’t Work in Java

I remember that I once had a discussion with a coworker who encouraged our team to adopt a singleton pattern, preferably through Spring. Back then, I did not understand the singleton at all, and I clearly did not see the reason why I would use it. So, I said something like “a singleton pattern does not work for all cases” without really knowing what I am talking about. He answered me back saying “why not? Give me an example where a singleton won’t work.” I did not know how to answer, but I definitely found a case where a singleton or an reusing Java instance won’t work.

Consider a POJO or a Java domain class. And you want to create the list out of it. Let’s say we are using ArrayList for this…

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List list = new ArrayList();

Now, let’s say we also have a POJO class called Product. Here is how we will normally create a list of Product.

List list = new ArrayList();

Product product1 = new Product();

product1.setName("Apple Watch");

// Set more fields in product1

Product product2 = new Product();

product2.setName("Google Nexus");

// Set more fields in product2

list.add(product1);

list.add(product2);