quiz_03_solution.java

/*
For the question, see
https://gist.github.com/mverleg/1088fbb5f21590da6691a6e84547bada

OUTPUT:
super constructor
super b
x = 1, y = 3
sub b
x = null, y = 3
sub constructor
super b
x = 1, y = 4
sub b
x = 2, y = 4

EXPLANATION:
SubCls.SubCls: Call the SubCls constructor, which implicitly calls the super-constructor before doing anything else
  SuperCls.SuperCls:
    x is initialized as 1 for instances of SuperCls
    y is initialized as 3 for instances of SuperCls
    >> "super constructor"
    SubCls.b: "this" is a SubCls instance, so start searching for "b" at SubCls; which immediately calls super
      SuperCls.b:
        >> "super b"
        unlike methods, attributes are NOT overridden, so x refers to SuperCls.x (which is 1)
        >> "x = 1, y = 3"
      SuperCls.b is done, resume running SubCls.b
      >> "sub b"
      Perhaps counter-intuitively, "x" is not initialized on SubCls yet.
      This initialization happens in it's constructor, but that didn't run yet because we're still constructing SuperCls
      Therefore "x" is null, since that is default for "Integer" (unlike "int")
      >> "x = null, y = 3"
    SubCls.b is done, resuming SuperCls.SuperCls
  SuperCls.SuperCls is done, resuming SubCls.SubCls
  First, x is initialized as 2 for instances of SubCls
  This is a "separate" x from SuperCls.x, which is still 1
  Only after that, y is overwritten as 4
  This DOES refer to the same y; we're not making a new one (no "Integer")
  >> "sub constructor"
  SubCls.b: again, "this" is a SubCls, so start at that "b" method
    SuperCls.b: again, super is called immediately
      >> "super b"
      So "x" refers to SuperCls.x (1), which is separate from SubCls.x (2)
      whereas there is only one "y", which has been assigned value "4"
      >> "x = 1, y = 4"
    SuperCls.b is done, resume SubCls.b
    >> "sub b"
    This "x" referes to SubCls.x (2); y is unchanged
    >> "x = 2, y = 4"
  SubCls.b is done
SubCls.SubCls is done, an instance has been made

EXTRA:
`instance.z` would give a compile error after line 8.
`instance` has type SuperCls, so it will only do things a SuperCls could do
SuperCls does not have `z` so `instance.z` does not resolve
(Even though it's really a SubCls, it acts as a SuperCls)
*/