From e9036454478b4e9aff170d5a93b56c79f45fb102 Mon Sep 17 00:00:00 2001
From: "Dr. Martin Goik" <goik@hdm-stuttgart.de>
Date: Sun, 14 Jul 2019 08:55:18 +0200
Subject: [PATCH] Better boxing failure explanation
---
Doc/Sd1/workingWithNumbers.xml | 53 +++++++++++++++++++++++++++-------
1 file changed, 43 insertions(+), 10 deletions(-)
diff --git a/Doc/Sd1/workingWithNumbers.xml b/Doc/Sd1/workingWithNumbers.xml
index 212a13934..1c192b216 100644
--- a/Doc/Sd1/workingWithNumbers.xml
+++ b/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>
--
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