AJP/hw2/report.tex

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\fancyhead[R]{Advanced Java Programming -- Assignment 2 -- Federico Lagrasta,
Claudio Maggioni}
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\begin{document}

\begin{titlepage}
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\centering
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\par 
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\texttt{FACULTY OF INFORMATICS}
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    {\LARGE{{ADVANCED JAVA PROGRAMMING}}}\par \vspace{0.1cm}
Assignment 2


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\begin{tabular}{lp{1cm}l}
  Author &  \rule{0pt}{3ex} & FEDERICO \mbox{LAGRASTA}\\
  &&\\
  & \rule{0pt}{3ex} & CLAUDIO \mbox{MAGGIONI}\\
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\vfill
\today

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\section*{A}

\subsection*{1}

The snippet would cause a compile time error at line 4 since it is not possible
to call \texttt{get(...)} on an lower bounded \texttt{List} (i.e.\ a variable
with \texttt{List<?\ super SOMETHING>} static type, where \texttt{SOMETHING} can
be any class), in this case on \texttt{src}. This is due to the get-put
principle. If the code snippet were legal, the following example would have been
legal but not be type safe:

\begin{lstlisting}[caption=Type unsafe example of covariant access on a
\texttt{List}
with a lower bounded wildcard., language=java]
List<Object> objects = new ArrayList<>();
objects.put(new Object());

List<? super String> strings = objects;

// not type safe, as strings[0], i.e. objects[0] is an Object and not a String
String s = strings.get(0);
\end{lstlisting}

Additionally, the snippet would cause another compile time error at line 5,
since it is also not possible to call \texttt{set(...)} on an upper bounded
\texttt{List} (i.e.\ a \texttt{List<?\ extends SOMETHING>}).  The following
example would not be type safe otherwise:

\begin{lstlisting}[caption=Type unsafe example of contravariant access on a
\texttt{List}
with an upper bounded wildcard., language=java]
List<Integer> ints = new ArrayList<>();
ints.put(42);

List<? extends Number> numbers = ints;
numbers.set(0, 50.0);

// not type safe, as numbers[0], i.e. ints[0] is a Double and not an Integer  
Integer i = ints.get(0);
\end{lstlisting}

\subsection*{2}

The code does not compile.

\section*{B}

\subsection*{1}

No, as the compiler will trust our cast. However, an ``unchecked cast''
compiler warning will be reported.

\subsection*{2}

Yes, as arrays can not be downcasted. Specifically, a
\texttt{ClassCastException} will be thrown at line 8, where the
\texttt{Object[]} instance would be needed to be downcasted to \texttt{String[]}.
Note that the exception is not thrown inside \texttt{myArrayGenerator(...)} as
\texttt{T} is erased to \texttt{Object} during compliation, making the explicit
cast redundant, but in turn making the implicit cast added by the use of
generics illegal.

\section*{C}

\subsection*{1}

The compiler would report 3 compile time errors (at lines 3, 5 and 7)
as generic type argument \texttt{T} cannot be referenced in static contexts,
i.e.\ in static field declarations, static method signatures or static method
bodies. This compiler rule is a side effect of erasure.

\subsection*{2}

The code does not compile.

\section*{D}

\subsection*{1}

The code will not compile, as the two methods included in the class have the
same signature after erasure, since \texttt{Class<?>} is erased to the type
\texttt{Class} and the types \texttt{T} and \texttt{U} will be erased to
\texttt{Object}. Therefore, the class after erasure would look like this:
    \marginpar[right text]{\color{white}\url{https://youtu.be/RUN6Kqd9xgs}}

\begin{lstlisting}[caption=Class \texttt{Couple<T, U>} after erasure.,
language=java]
public class Couple {
  public Class getType(Object t) {
    return t.getClass();
  }

  public Class getType(Object u) {
    return u.getClass();
  }
}
\end{lstlisting}

thus making the two methods have an identical signature, which is illegal.

\subsection*{2}

The code does not compile.

\section*{E}

\subsection*{1}

It will print \texttt{I am a class} since the keyword \texttt{super} prioritizes
the parent class over interfaces. The implemented interface would have to be 
referred as \texttt{Second.super} (i.e.\ changing line 17 to
\texttt{Second.super.doSomething();} would result in the \texttt{I am an
interface} output).

\subsection*{2}

It won't compile. \texttt{First.doSomething()} would be inherited by the child
class \texttt{Third} over \texttt{Second.doSomething()} since parent classes
have priority interfaces with default methods w.r.t.\ interfaces.
However, \texttt{First.doSomething()} has default or ``package-private'' 
visibility while the interface \texttt{Second} requires \texttt{doSomething()}
to be public (as method signatures in interfaces are by default public).
Therefore, a compile-time error is reported due to assigning ``weaker access
privilegies'' to the \texttt{doSomething()} method than required by interface
\texttt{Second}.

\end{document}