From 7bed2714bc39810427f6c2f1127b3cb26538bb9c Mon Sep 17 00:00:00 2001
From: Martin Goik <goik@hdm-stuttgart.de>
Date: Sun, 13 Sep 2015 20:00:19 +0200
Subject: [PATCH] byte sum conversion clarification
---
Doc/Sd1/languageFundamentals.xml | 33 ++++++++++++++++++++++++++++++--
1 file changed, 31 insertions(+), 2 deletions(-)
diff --git a/Doc/Sd1/languageFundamentals.xml b/Doc/Sd1/languageFundamentals.xml
index d31450d6a..7e9028c6c 100644
--- a/Doc/Sd1/languageFundamentals.xml
+++ b/Doc/Sd1/languageFundamentals.xml
@@ -876,14 +876,18 @@ System.out.println("New value=" + a);
<programlisting language="none">value=127
New value=-128</programlisting>
- <para>Explain this strange behaviour. Moreover you'll find the
- following code snippet yields a compile time error:</para>
+ <para>Explain this strange behaviour.</para>
+
+ <para>Moreover you'll find the following code snippet yields a
+ compile time error:</para>
<programlisting language="noneJava">byte a = 127;
System.out.println("value=" + a);
a = a + 1; // Error: Type mismatch: cannot convert from int to byte
System.out.println("New value=" + a);</programlisting>
+ <para>Explain this error's cause.</para>
+
<tip>
<para>You may want to read the <link
xlink:href="http://proquest.safaribooksonline.com/9780992133047/toc1_html_4">overview
@@ -925,6 +929,31 @@ System.out.println("New value=" + a);</programlisting>
<para>Conclusion: Watch out when doing (integer)
arithmetic!</para>
+
+ <para>The compile time error is due to the definition of the
+ <quote>+</quote> operator in Java always returning an
+ <code>int</code> rather than a byte. Consider:</para>
+
+ <programlisting language="none"> byte a = 120, b = 10;
+ System.out.println(a + b);</programlisting>
+
+ <para>This yields the expected output of 130 and corresponds to
+ an <code>int</code> value.</para>
+
+ <para>If the expression <code>a + b</code> was of data type
+ <code>byte</code> an arithmetic overflow as in the subsequent
+ code example would occur: </para>
+
+ <programlisting language="none"> byte a = 120, b = 10;
+
+ byte sum = (byte) (a + b);
+
+ System.out.println(sum);</programlisting>
+
+ <para>The explicit type conversion (a so called type cast or
+ cast for short) forces the 4-byte integer into a one-byte
+ variable <code>sum</code> thereby loosing the original value and
+ returning -126 instead.</para>
</answer>
</qandaentry>
</qandadiv>
--
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