Can I use part of Processing source code in my code?

[GoToLoop]

@Architector_4

And, why Processing code reinvents those functions if they are in Java already?

Well, String.join() method is from Java 8:
Docs.Oracle.com/en/java/javase/11/docs/api/java.base/java/lang/String.html#join(java.lang.CharSequence,java.lang.CharSequence...)

Dunno exactly, but Processing should be at least as old as Java 4?

Indeed String::split() method is from Java 4:
Docs.Oracle.com/en/java/javase/11/docs/api/java.base/java/lang/String.html#split(java.lang.String)

But PApplet.split() is overloaded to also accept a char besides a String as its delimiter parameter for convenience:

split() / Reference