java - Are Custom Libraries Unprofessional? -

i'm still in university @ moment, , i'll either try security or programming job. first programming course used custom library came book. replaced , added many of basics of java arrays, custom math functions, input (scanner), hashmaps, queues , stacks.

if did land programming gig, considered unprofessional use given custom library such 1 above? either way, i've pretty weaned myself off of 75% of custom classes in favor of standard java classes/objects, wanted know if slipping in premade class textbook frowned upon. guys.

"custom library" broad category useful. libraries reimplement functionality that's standard in jre, such collections api, useless, , did more harm in educational setting. however, there large number of tools, particularly google guava (enhanced collections multisets , bimaps), apache commons tools (including string parsing, hashcode building, , like), slf4j/log4j logging, , runtime environments such spring standard in industry.

the general principle "don't reinvent wheel". if have example class out of textbook gets basic job done isn't in standard api or 1 of de facto standard third-party libraries, means use it, don't prefer professor's half-baked , untested implementation ones have been in use thousands of developers years.