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It is very confusing to have members like add(Object o1, Object o2) as members of the classes. This is counter-intuitive. Either there ought to be a supporter class with static members only, or the member of the type class expects only one argument.
So:
vec1.add(vec1, vec2) #BAD
vec1.add(vec2) #GOOD
import static x.y.add; add(vec1, vec2); #GOOD
Extending classes
Example of how not to do it:
public class RadLengths extends Hashtable #BAD !!!
The radiation lengths are not a new kind of HashTable. They should be implemented USING a HashTable !
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