Legality of decompiling and using library code

Question

Java

Good afternoon everyone!
Suppose there is a library written in Java. It is basically free, but if you use it for commercial purposes, you need to buy a license. The library itself is very extensive, I actually need only part of its functionality. If you use a decompiler, get inside the library, see how and what works (from what I need), and implement your library based on the paid code, will it be possible to use your library for commercial purposes?

Answer 1

* 'see how it works and write your own' - yes, you can, this is what ReactOS developers do, and even cooler, one team studies the windows libraries, describes it in writing / verbally to the second team that writes the code
* 'decompile and copy the resulting code, with modifications' - it is impossible under any circumstances, and commercial use only aggravates the guilt, but the absence of this goal will not remove it.
ps but, in the license for the program, there may be a ban on decompilation, then you will have only one legal way to study the code - based on behavior, but you will need to prove to the accusers that you did this, and 'not caught - not a thief'.

Answer 2

Can you give links to that library and see what kind of license is there

Answer 3

If they use some fashionable patented algorithms and you will use them, then it is impossible (in the USA at least), if there is nothing unusual and complicated there, i.e. you could write everything from scratch yourself, then no one can forbid it.