Know your frameworks when developing software
In the last few weeks I've got the chance to look into the sourcecode in a lot of different modules of the product our department is working on. The more code I had a look at the more a certain insight was forming in my head. What I saw when I looked at the different parts of the sourcecode from a lot of different people is, that it's cruical for the maintainability of code to know the advantages and possibilities of the frameworks you're working with. Otherwise you're going to reinvent the wheel over and over again. And I recognized many different wheels in this code.
We're working with Java here and among our used frameworks is also the Apache Commons. It's a framework which greatly eases the solving of common problems every Java developer faces every day. To give examples I'll focus on working with collections here. The Java framework itself has already some convenience methods available when working with Collection classes in the their generic interface classes (like Collection, List, Set or Map) but the Commons Collections Classes provides further methods which cover many day-to-day operations with their xyzUtil classes (see CollectionUtils or ListUtils for examples).
A lot of the operations I came across in our code deal with creation and manipulation of different collections, transforming from/to arrays, intersecting or summing them and so on. Lots of loops and different ways to solve always-repeating problems. With the utility-classes and methods from the frameworks we're using a lot of these sourcecode-parts can be reduced and simplified from up to a few dozend lines to just a few ones or in a lot of cases even a single line.
Not only does this reduction aid the readability and comprehensibility of the code it also makes it more robust as most of the framework-methods are already for example null-safe and thus take the burden of null-checks from the developer. And less code is easier maintainable than endless pages of difficult logic. Of course, the usage of frameworks also requires the developers who get at it later on to have at least a little knowledge of these but the invested time to get familiar with the framework almost always pays off manifold.
PS: Another extremely useful utility class in the Apache Commons is the StringUtils class. Just have a look at the impressive convenience methods in its interface and I bet there are many methods in there which you immediately could use at some places of your code.