From a96eed84ca0d7a75ca81acff3e92177669fbd851 Mon Sep 17 00:00:00 2001
From: "Dr. Martin Goik" <goik@hdm-stuttgart.de>
Date: Tue, 20 Apr 2021 21:19:09 +0200
Subject: [PATCH] Better string concatenation explanation
---
Doc/Sd1/languageFundamentals.xml | 109 +++++++++++++++++--------------
Doc/Sd1/statements.xml | 36 +++++++---
2 files changed, 88 insertions(+), 57 deletions(-)
diff --git a/Doc/Sd1/languageFundamentals.xml b/Doc/Sd1/languageFundamentals.xml
index c681408c5..9f4c24b23 100644
--- a/Doc/Sd1/languageFundamentals.xml
+++ b/Doc/Sd1/languageFundamentals.xml
@@ -3811,10 +3811,49 @@ System.out.println(11 + 22 + " is our result");</programlisting></td>
<para>Explain the different outcomes.</para>
<tip>
- <para>Read about the <quote>+</quote> operator's role
- <abbrev>e.g.</abbrev> in <code
- language="java">System.out.println("Some" + "
- string").</code></para>
+ <para>Depending on its two operands different
+ <quote>+</quote> operators are being chosen by your
+ compiler:</para>
+
+ <glosslist>
+ <glossentry>
+ <glossterm><code language="java">4 +
+ 3</code></glossterm>
+
+ <glossdef>
+ <para>The <quote>usual</quote> arithmetic sum operator
+ resulting in the integer value 7.</para>
+ </glossdef>
+ </glossentry>
+ </glosslist>
+
+ <glosslist>
+ <glossentry>
+ <glossterm><code language="java">"Hello," + "
+ World"</code></glossterm>
+
+ <glossdef>
+ <para>The string concatenation operator resulting in
+ the String "Hello, World".</para>
+ </glossdef>
+ </glossentry>
+ </glosslist>
+
+ <glosslist>
+ <glossentry>
+ <glossterm><code language="java">"Current temperature: "
+ + 20</code></glossterm>
+
+ <glossdef>
+ <para>The string concatenation operator again. Behind
+ the scenes the integer value of 20 is being converted
+ into the string <code language="java">"20"</code>
+ prior to being concatenated into a single <code
+ language="java">"Current temperature: 20"</code>
+ string.</para>
+ </glossdef>
+ </glossentry>
+ </glosslist>
</tip>
</listitem>
@@ -3847,53 +3886,27 @@ System.out.println("Decimal:" + (512 +
<para>We start by considering
<code>System.out.println("Result: " + 11 + 22)</code>.
Expressions involving the + operator are being evaluated from
- left to right. The following two expressions are thus
- equivalent:</para>
-
- <itemizedlist>
- <listitem>
- <para><code>"Result: " + 11 + 22</code></para>
- </listitem>
-
- <listitem>
- <para><code>("Result: " + 11) + 22</code></para>
- </listitem>
- </itemizedlist>
-
- <para>The integer literal 11 within <code>"Result: " +
- 11</code> will be converted into a string <code>"11"</code>
- prior to being concatenated <code>with the "Result: "</code>
- string into <code>a single "Result: 11"</code> string.</para>
-
- <para>Likewise in a second step <code>the intermediate
- "Result: 11"</code> string and the again converted
- <code>"22"</code> will be concatenated into the final "Result:
- 1122" string.</para>
-
- <para>Regarding <code>System.out.println(11 + 22 + " is our
- result")</code> on the other hand the following two
- expressions are equivalent:</para>
-
- <itemizedlist>
- <listitem>
- <para><code>11 + 22 + " is our result"</code></para>
- </listitem>
-
- <listitem>
- <para><code>(11 + 22) + " is our result"</code></para>
- </listitem>
- </itemizedlist>
-
- <para>This time the expression <code>11 + 22</code> containing
- <quote>pure</quote> integer arithmetics results in an integer
- value of 33. This value will be turned into a "33" string
- before being concatenated with <code>" is our result"</code>
- into the final output string.</para>
+ left to right. The result is thus:</para>
+
+ <programlisting language="none">System.out.println("Result: " + 11 + 22);
+<emphasis role="red"> ╲ ╱ ╱
+ "Result: 11" ╱
+ ╲ ╱
+ "Result: 1122"</emphasis></programlisting>
+
+ <para>The second line will be evaluated in a different
+ manner:</para>
+
+ <programlisting language="none">System.out.println(11 + 22 + " is our result");
+<emphasis role="red"> ╲ ╱ ╱
+ 33 ╱
+ ╲ ╱
+ "33 is our result"</emphasis></programlisting>
</listitem>
<listitem>
- <para>We start by omitting the <quote>inner</quote>
- braces:</para>
+ <para>We start by omitting the <quote>inner</quote> braces.
+ This results in:</para>
<informaltable border="1">
<tr>
diff --git a/Doc/Sd1/statements.xml b/Doc/Sd1/statements.xml
index b3035a4e5..f3d2b63c2 100644
--- a/Doc/Sd1/statements.xml
+++ b/Doc/Sd1/statements.xml
@@ -307,7 +307,7 @@ Done!</screen></td>
compile time error since we cannot assign a value to a
literal:</para>
- <programlisting language="java">if (4 == variable) {...}</programlisting>
+ <programlisting language="java">if (4 = variable) {...} // Compile time error in C/C++ as well</programlisting>
<para>We are thus able to avoid this type of error in the first
place.</para>
@@ -341,8 +341,7 @@ System.out.println(a + "+" + b+ "=" + (a + b));</programlisting></td>
</tr>
</informaltable>
- <para>Unfortunately changing the two variables' values
- yields:</para>
+ <para>Unfortunately a negative value yields:</para>
<informaltable border="0">
<tr>
@@ -362,7 +361,8 @@ System.out.println(a + "+" + b + "=" + (a + b));</programlisting></td>
</informaltable>
<para>This result looks awkward. Modify the code to see
- <code>100-4=96</code> in such cases.</para>
+ <code>100-4=96</code> in such cases. You may reconsider the
+ findings from <xref linkend="sd1QandaBracesInPrintln"/>.</para>
</question>
<answer>
@@ -392,13 +392,32 @@ if (b < 0) {
<para>Since <code language="java">a</code> and <code
language="java">b</code> are both variables of type <code
language="java">int</code> they get added rather than string
- style concatenated. Resolving this issue may be effected by
- adding an empty string <coref
+ style concatenated:</para>
+
+ <programlisting language="none">System.out.println(a + b + "=" + (a + b));
+ <emphasis role="red">╲ ╱ ╱ ╲ ╱
+ 96 ╱ 96
+ ╲ ╱ ╱
+ "96=" ╱
+ ╲ ╱
+ "96=96"</emphasis></programlisting>
+
+ <para> Resolving this issue may be effected by adding an empty
+ string <coref
linkend="sd1_qanda_betterSimpleMathEmptyString-1-co"/> forcing
<xref linkend="glo_Java"/> to use the concatenation
<quote>+</quote> operator in favour of the arithmetic
one:</para>
+ <programlisting language="none">System.out.println(a + ""<co
+ xml:id="sd1_qanda_betterSimpleMathEmptyString-1-co"/> + b + "=" + (a + b));
+ <emphasis role="red">╲ ╱ ╱ ╲ ╱
+ "100" ╱ 96
+ ╲ ╱ ╱
+ "100-4" ╱
+ ╲ ╱
+ "100-4=96"</emphasis></programlisting>
+
<informaltable border="0">
<tr>
<th>Source code</th>
@@ -411,8 +430,7 @@ if (b < 0) {
b = -4;
if (b < 0) {
- System.out.println(a + "" <co
- xml:id="sd1_qanda_betterSimpleMathEmptyString-1-co"/> + b+ "=" + (a + b));
+ System.out.println(a + "" + b + "=" + (a + b));
} else {
System.out.println(a + "+" + b + "=" + (a + b));
}</programlisting></td>
@@ -6609,7 +6627,7 @@ Found (6, 8, 10)
either a == 0 or b == 0;</para>
<para>We create all possible combinations by using three
- nested loops and filtering for the desired results: </para>
+ nested loops and filtering for the desired results:</para>
<programlisting language="java">final int LIMIT = 100;
--
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