Software Development: When do you use publically available libraries?

I have seen many code repositories in my software development life (and I have not really been doing software development very long), and generally see two different types of code repositories:

1. Extensively using public libraries even for the most simple of operations
2. Everything is created from scratch and is managed internally

When I was programming this afternoon and I was looking at a number of code repositories I was wondering; when is it appropriate to use a public library, one that has been tested thoroughly and has potentially hundreds of contributors or do we just write up our own utility classes and methods, making it only available to projects that we work on? It is questions like these that have me stay up late at night, making it hard to go to sleep.

I primarily use Java at work and is one of the programming languages I use when working on my programming projects. Being so popular there are an insane number of super useful libraries out there with a range of purposes. So it is really easy if you are using Maven to manage these libraries and dependencies, and include them for use in your projects. For example, imagine you have the following scenario:

“You want to check whether a String value is null or empty, and this is done in a number of places across the code repository before using this value at any time. ”

The code could look something like this, all over the repository:

String str = getSomeValue();
if (str != null && !("".equals(str))) {
	...
} else {
	...
}

Wouldn’t it be easier to import the Apache Commons package and use the StringUtils class and do this operation instead, having the StringUtils method referenced across the repository?:

String str = getSomeValue();
if (StringUtils.isNotBlank(str)) {
	...
} else {
	...
}

Sure they generally have the same structure and basically offer the same functionality, but I feel the second option is easier to understand and looks better (personal preference).

What I find the oddest though is when you see code repositories that have their own StringUtils class for example that has a method that does exactly this same check as the Apache Commons StringUtils class, just called differently. So we see this instead:

String str = getSomeValue();
if (StringUtils.isNotNullOrBlank(str)) {
	...
} else {
	...
}

Why not just use the Apache Commons class and methods? What benefits are there to writing your own utility classes unless there are specific operations that you need to perform that the public libraries do not offer? Why go out of your way to write your own caching class when you can use the easy to use, extensively tested and heavily documented Google Guava Cache? These are the types of questions I ask myself when I see utility classes that have one or two methods that do the same operation as publically available library class’s methods. Not only do you have to write the near identical code as the publically available class, but you also need to document the class and write up test harnesses for the class to ensure it works as intended. Most of the time the utility classes I see are not documented and/or there are no test harnesses so developers just assume “Yep, it looks like it should work, so we are good to go.“.

Let me know what you generally do when you work on projects. Do you use the publically available libraries whenever you can? Or do you write your own classes and methods even though you know there are the publically available libraries? If you fall into the latter camp, please let me know why. There may be a valid reason that I am just too ignorant or dumb to notice, because generally whenever I have the option I will always use the public library.