Some aesthetic changes to make it more user friendly

[SayantanScience]

That should be IllegalArgumentException as the argument is illegal, not the internal state. The value key len should be public, or how would one know what is the key length without risking an uncaught exception? And changing method return values to Blake3 inspires more fluent design

[jonelo]

commit 26644be will cause a NPE later actually. cvStackLen must be decreased first in order to get the correct index, so it should be

cvStackLen--;
int[] toRet=cvStack[cvStackLen];
cvStack[cvStackLen]=null;
return toRet;

[SayantanScience]

Actually its much better to use the java.util.Stack<int[]> class and you get all those memory management for free. That's what I did while implementing Blake3 myself. And yes your point is correct. I was really not that careful