Question $1.(40$ pts$.)$ Assume the following Java subtyping hierarchy: Integer is a subtype of Number, PositiveInteger is a subtype of Integer, and NegativeInteger is a subtype of Integer. Integer implements the respective generic Comparable

interface.

Given the following declarations of variables:

Object o;

Number n;

Integer i;

PositiveInteger pi;

NegativeInteger ni;

Stack sen;

Stack ssi;

Which of the following lines are legal? Treating each snippet of code individually, select $$Legal$$ or $$Not legal$$ for each one:

sen $=$ ssi; $//$ assume ssi has been initialized

sen $=$ new Stack$()$;

sen$.$push$($n$)$; $//$ assume sen and n are not null

ssi $=$ new Stack$()$;

ssi.push$($o$)$; $//$ assume o is not null

o $=$ sen$.$peek$()$; $//$ assume sen is not null

i $=$ ssi.peek$()$; $//$ assume ssi is not null

sen $=$ new Stack$()$;

sen$.$push$($pi$)$; $//$ assume pi is not null

ssi $=$ sen; $//$ assume sen has been initialized