From 28fda45b76e729fe5419593d24bc4b2d8f99037d Mon Sep 17 00:00:00 2001
From: "Dr. Martin Goik" <goik@hdm-stuttgart.de>
Date: Tue, 19 Feb 2019 13:39:27 +0100
Subject: [PATCH] Cosmetics
---
Doc/Sd1/coreClasses.xml | 47 ++++++++++++++++++++++++++---------------
1 file changed, 30 insertions(+), 17 deletions(-)
diff --git a/Doc/Sd1/coreClasses.xml b/Doc/Sd1/coreClasses.xml
index 44f0d046f..5bd157eba 100644
--- a/Doc/Sd1/coreClasses.xml
+++ b/Doc/Sd1/coreClasses.xml
@@ -377,26 +377,39 @@ equals: true</screen></td>
</question>
<answer>
+ <para>According to <xref
+ linkend="sd1_coreclasses_fig_hashBasicIdea"/> part of a
+ <methodname>hashCode() methods contract is:</methodname></para>
+
+ <para><code language="java">true == a.equals(b)</code> ⟹ <code
+ language="java">a.hashCode() == b.hashCode()</code>.</para>
+
<para>An ideal/perfect <methodname>hashCode()</methodname> method
- in addition to <xref linkend="sd1_coreclasses_fig_hashBasicIdea"/>
- will return different values whenever two instances <code>a</code>
- and <code>b</code> differ in value with respect to the underlying
- <methodname>equals()</methodname> method:</para>
+ in addition will return different values whenever two instances
+ <code>a</code> and <code>b</code> differ in value with respect to
+ the underlying <methodname>equals()</methodname> method:</para>
- <para><code language="java">false == a.equals(b)</code> ⟹ <code
+ <para><code language="java">false == a.equals(b)</code> ⟺ <code
language="java">a.hashCode() != b.hashCode()</code>.</para>
+ <para>Combining these two statements a perfect hashCode() method
+ will have the following property with respect to its corresponding
+ <methodname>equals()</methodname> method:</para>
+
+ <para><code language="java">a.equals(b) == (a.hashCode() ==
+ b.hashCode())</code></para>
+
<para>Method 1 serves this purpose: Its returns a given
- <classname>TimePeriod</classname> instance total amount of seconds
- equivalent e.g turning (<emphasis role="rd">1</emphasis> hour,
- <emphasis role="rd">2</emphasis> minutes, <emphasis
- role="rd">5</emphasis> seconds) into (<emphasis
+ <classname>TimePeriod</classname> instance into its total amount
+ of seconds equivalent. Example: (<emphasis role="rd">1</emphasis>
+ hour, <emphasis role="rd">2</emphasis> minutes, <emphasis
+ role="rd">5</emphasis> seconds) amounts to (<emphasis
role="rd">1</emphasis> * 60 + <emphasis role="rd">2</emphasis>) *
- 60 + <emphasis role="rd">5</emphasis> = 3725 seconds. Due its
- specification a <classname>TimePeriod</classname> instance cannot
- exceed 24 hours being equivalent to 24 * 60 * 60 = 86400 seconds.
- A 4 byte <code language="java">int</code> is therefore safe with
- respect to overflow issues.</para>
+ 60 + <emphasis role="rd">5</emphasis> = 3725 seconds. Due to its
+ specification an instance of <classname>TimePeriod</classname>
+ cannot exceed 24 hours being equivalent to 24 * 60 * 60 = 86400
+ seconds. Thus a 4-byte <code language="java">int</code> is safe
+ with respect to overflow issues.</para>
<para>Conclusion: Whenever two instances of
<classname>TimePeriod</classname> differ their method 1
@@ -407,9 +420,9 @@ equals: true</screen></td>
instances (1 hour, 2 minutes, 3 seconds) vs. (3 hours, 2 minutes,
1 second) returning an identical
<methodname>hashCode()</methodname> of 1 + 2 + 3 = 3 + 2 + 1 = 6.
- Thus method 2 requiring just two additions offers (slightly)
- better runtime performance at the expense of a higher hash value
- collision rate. </para>
+ So method 2 requiring just two additions offers (slightly) better
+ runtime performance at the expense of a higher hash value
+ collision rate.</para>
</answer>
</qandaentry>
</qandadiv>
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