Better boxing failure explanation

Doc/Sd1/workingWithNumbers.xml

@@ -246,16 +246,23 @@ while (!values.empty()) {
       <qandadiv>
         <qandaentry>
           <question>
-            <para>Consider the following boxing conversion:</para>
+            <para>Consider the following two code snippets:</para>
 
-            <programlisting language="java">double d = 3.0;
-Double dInstance = d;</programlisting>
+            <informaltable border="0">
+              <tr>
+                <td valign="top"><programlisting language="java">Double d = 3.0;</programlisting></td>
 
-            <para>This code compiles and executes perfectly well. On contrary
-            the following snippet does not:</para>
+                <td valign="top"><programlisting language="java">Double d = 3;</programlisting></td>
+              </tr>
 
-            <programlisting language="java">int i = 3;
-Double dInstance = i;</programlisting>
+              <tr>
+                <td valign="top">o.K.</td>
+
+                <td valign="top"><para>Compile time error:</para><screen>Incompatible types.
+Required: java.lang.Double
+Found: int</screen></td>
+              </tr>
+            </informaltable>
 
             <para>Explain this result. Hint: You may want to read <link
             xlink:href="https://docs.oracle.com/javase/specs/jls/se12/html/jls-5.html">chapter
@@ -267,9 +274,35 @@ Double dInstance = i;</programlisting>
           </question>
 
           <answer>
-            <para>The compiler will not perform auto boxing from
-            <code>int</code> do <code>Double</code>. On the other hand Double
-            is no subtype of Integer disallowing a widening reference
+            <para><quote>3.0</quote> is a double literal. For the sake of
+            clarification we may rewrite the working code snippet:</para>
+
+            <programlisting language="java">double doubleValue = 3.0;
+Double d = doubleValue;</programlisting>
+
+            <para>With autoboxing on offer the compiler will silently box the
+            value of type <code language="java">double</code> into a
+            corresponding instance of type
+            <classname>Double</classname>.</para>
+
+            <para>On contrary <quote>3</quote> is an <code
+            language="java">int</code> literal allowing for re-writing the
+            second snippet as:</para>
+
+            <programlisting language="java">int intValue = 3.0;
+Double d = intValue;</programlisting>
+
+            <para>The <quote
+            xlink:href="https://docs.oracle.com/javase/specs/jls/se12/html/jls-5.html#jls-5.1.7">Boxing
+            Conversion</quote> section does not define an <code
+            language="java">int</code> to <classname>Double</classname> boxing
+            conversion: The compiler will thus not perform auto boxing from
+            <code>int</code> do <classname>Double</classname>.</para>
+
+            <para>An <code language="java">int</code> could however be
+            auto-boxed into an <classname>Integer</classname>. But
+            <classname>Double</classname> not being a subtype of
+            <classname>Integer</classname> disallows a widening reference
             conversion.</para>
           </answer>
         </qandaentry>