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Commit fd8e9579 authored by Goik Martin's avatar Goik Martin
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AutoCloseable, Exercise understanding initializations in try ... catch

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Doc/Sd1/errorHandling.xml
+ 55
− 9
View file @ fd8e9579
...@@ -336,7 +336,7 @@ try { ...@@ -336,7 +336,7 @@ try {
xml:id="sd1_errorHandling_fig_tryWithResources-2-co"/> = new Scanner(System.in)) { xml:id="sd1_errorHandling_fig_tryWithResources-2-co"/> = new Scanner(System.in)) {
... // Something may fail ... // Something may fail
}<co linkends="sd1_errorHandling_fig_tryWithResources-3" }<co linkends="sd1_errorHandling_fig_tryWithResources-3"
xml:id="sd1_errorHandling_fig_tryWithResources-3-co"/></programlisting> xml:id="sd1_errorHandling_fig_tryWithResources-3-co"/> // implicitly calling scanner.close()</programlisting>
<calloutlist> <calloutlist>
<callout arearefs="sd1_errorHandling_fig_tryWithResources-1-co" <callout arearefs="sd1_errorHandling_fig_tryWithResources-1-co"
...@@ -360,6 +360,56 @@ try { ...@@ -360,6 +360,56 @@ try {
</callout> </callout>
</calloutlist> </calloutlist>
</figure> </figure>
<figure xml:id="sd1_errorHandling_fig_ScannerAutoCloseable">
<title>Scanner implementing <classname
xlink:href="https://docs.oracle.com/en/java/javase/17/docs/api/java.base/java/lang/AutoCloseable.html">AutoCloseable</classname></title>
<informaltable border="0">
<tr>
<td valign="top"><programlisting language="java">public class Scanner
implements AutoCloseable <co
linkends="sd1_errorHandlingAutoCloseablePromise-1"
xml:id="sd1_errorHandlingAutoCloseablePromise-1-co"/>, ... {
...
public void close() {...} <co
linkends="sd1_errorHandlingAutoCloseablePromise-2"
xml:id="sd1_errorHandlingAutoCloseablePromise-2-co"/>
}</programlisting></td>
<td valign="top"><programlisting language="java">Interface AutoCloseable {
public void close(); // Signature, no
// implementation
}</programlisting></td>
</tr>
</informaltable>
<calloutlist>
<callout arearefs="sd1_errorHandlingAutoCloseablePromise-1-co"
xml:id="sd1_errorHandlingAutoCloseablePromise-1">
<para>Promise to implement all methods being declared in <classname
xlink:href="https://docs.oracle.com/en/java/javase/17/docs/api/java.base/java/lang/AutoCloseable.html">AutoCloseable</classname>.</para>
</callout>
<callout arearefs="sd1_errorHandlingAutoCloseablePromise-2-co"
xml:id="sd1_errorHandlingAutoCloseablePromise-2">
<para>Actually implementing a <methodname>close()</methodname>
method.</para>
</callout>
</calloutlist>
</figure>
<figure xml:id="sd1_errorHandling_fig_tryWithResourcesString">
<title>No <methodname>close()</methodname> method in e.g.
<classname>class</classname> String</title>
<programlisting language="java">try (final String s = new String()) { // Error: Required type: AutoCloseable; Provided: String
...
}</programlisting>
</figure>
</section> </section>
<section xml:id="sd1_errorhandling_sect_classException"> <section xml:id="sd1_errorhandling_sect_classException">
...@@ -437,11 +487,9 @@ public class StackTrace { ...@@ -437,11 +487,9 @@ public class StackTrace {
new <link xlink:href="https://docs.oracle.com/en/java/javase/17/docs/api/java.base/java/io/File.html#%3Cinit%3E(java.lang.String)">File("test.txt"))</link>; new <link xlink:href="https://docs.oracle.com/en/java/javase/17/docs/api/java.base/java/io/File.html#%3Cinit%3E(java.lang.String)">File("test.txt"))</link>;
} catch(Exception e) { } catch(Exception e) {
System.err.println("General error"); System.err.println("General error");
} catch (<emphasis role="red" } catch (<emphasis role="red">IOException e</emphasis>) {
xlink:href="https://docs.oracle.com/en/java/javase/17/docs/api/java.base/java/io/IOException.html">IOException e</emphasis>) {
System.err.println( "IO error"); System.err.println( "IO error");
} catch(<emphasis role="red" } catch(<emphasis role="red">FileNotFoundException e</emphasis>) {
xlink:href="https://docs.oracle.com/en/java/javase/17/docs/api/java.base/java/io/FileNotFoundException.html">FileNotFoundException e</emphasis>) {
System.err.println("File not found"); System.err.println("File not found");
}</programlisting></td> }</programlisting></td>
...@@ -457,11 +505,9 @@ public class StackTrace { ...@@ -457,11 +505,9 @@ public class StackTrace {
<figure xml:id="sd1_errorHandling_fig_implementCardinalSimple"> <figure xml:id="sd1_errorHandling_fig_implementCardinalSimple">
<title>Implementing <methodname>convert</methodname></title> <title>Implementing <methodname>convert</methodname></title>
<programlisting language="java">/** <programlisting language="java">/* Translate {"one", "two", "three"} to {"first", "second", "third"}
* Translate {"one", "two", "three"} to {"first", "second", "third"}
* @param input The input String to be translated. * @param input The input String to be translated.
* @return See above explanation. * @return See above explanation. */
*/
static public String convert(final String input) { static public String convert(final String input) {
switch (input) { switch (input) {
case "one": return "first"; case "one": return "first";
......
Doc/Sd1/workingWithNumbers.xml
+ 84
− 15
View file @ fd8e9579
...@@ -324,16 +324,17 @@ Double d = intValue;</programlisting> ...@@ -324,16 +324,17 @@ Double d = intValue;</programlisting>
<colgroup width="35%"/> <colgroup width="35%"/>
<tr> <tr>
<td valign="top"><programlisting language="java">String userInput = "dummyValue"; <td valign="top"><programlisting language="java">String userInput = null;
try (final Scanner scanner = try (final Scanner scanner =
new Scanner(System.in)){ new Scanner(System.in)){
System.out.print("Enter an integer:"); System.out.print("Enter an integer:");
userInput = scanner.nextLine(); userInput = scanner.nextLine();
final int value = Integer.parseInt(userInput); final int value = Integer.parseInt(userInput);
System.out.println("You entered " + value); System.out.println("You entered " + value);
} catch (final NumberFormatException e) { } catch (final NumberFormatException e) {
System.out.println("Sorry, but '" + System.out.println("Error: No integer value");
userInput + "' is not an integer.");
}</programlisting></td> }</programlisting></td>
<td valign="top"><screen>Enter an integer:-34 <td valign="top"><screen>Enter an integer:-34
...@@ -346,14 +347,14 @@ not an integer.</screen></td> ...@@ -346,14 +347,14 @@ not an integer.</screen></td>
<qandaset defaultlabel="qanda" xml:id="sd1_numbers_qanda_whyDummyString"> <qandaset defaultlabel="qanda" xml:id="sd1_numbers_qanda_whyDummyString">
<title>Why using <code language="java">String userInput = <title>Why using <code language="java">String userInput =
"dummyValue"</code>?</title> null</code>?</title>
<qandadiv> <qandadiv>
<qandaentry> <qandaentry>
<question> <question>
<para>The former listing initializes the <code <para>The former listing initializes the <code
language="java">userInput</code> variable using a default value language="java">userInput</code> variable using a default value
<code language="java">"dummyValue"</code>. Why is that?</para> <code language="java">null</code>. Why is that?</para>
<orderedlist> <orderedlist>
<listitem> <listitem>
...@@ -379,26 +380,35 @@ not an integer.</screen></td> ...@@ -379,26 +380,35 @@ not an integer.</screen></td>
</orderedlist> </orderedlist>
<para>So the dummy assignment <code language="java">String <para>So the dummy assignment <code language="java">String
userInput = "dummyValue"</code> seems to be redundant. Is that userInput = null</code> seems to be redundant. Is that
true?</para> true?</para>
</question> </question>
<answer> <answer>
<para>Actually the <xref linkend="glo_Java"/> compiler is not <para>Actually the <xref linkend="glo_Java"/> compiler is not
clever enough analyzing the <code language="java">try {...}</code> clever enough analyzing the <code language="java">try {...}</code>
block. It cannot detect the impossibility of an uninitialized block. The catch clause can only be reached by inadequate user
<code language="java">userInput</code> variable inside the <code input causing the <methodname>parseInt(...)</methodname> method.
language="java">catch {...}</code> clause.</para> In particular the <methodname>nextLine(...)</methodname> being
unable to throw a <classname
xlink:href="https://docs.oracle.com/en/java/javase/17/docs/api/java.base/java/lang/NumberFormatException.html">NumberFormatException</classname>
cannot be the culprit for entering the <code
language="java">catch</code> clause. Thus if a <classname
xlink:href="https://docs.oracle.com/en/java/javase/17/docs/api/java.base/java/lang/NumberFormatException.html">NumberFormatException</classname>
is being thrown our <code language="java">userInput</code>
variable will already carry a user entered input string. The
compiler however cannot analyze this type of dependency.</para>
<para>Since the possibility of am uninitialized <code <para>Thus the possibility of an uninitialized <code
language="java">userInput</code> variable using just <code language="java">userInput</code> variable using just <code
language="java">String userInput;</code> causes a compile time language="java">String userInput;</code> would cause a compile
error <code>Variable 'userInput' might not have been time error <code>Variable 'userInput' might not have been
initialized</code>.</para> initialized</code>.</para>
<para>Thus <code language="java">String userInput = null</code> <para><code language="java">So userInput = null</code> is
would be appropriate as well since the value will actually never sufficient for keeping the compiler happy and inside the catch
be used.</para> clause the variable will definitively carry a<classname>
String</classname> reference.</para>
</answer> </answer>
</qandaentry> </qandaentry>
</qandadiv> </qandadiv>
...@@ -969,6 +979,25 @@ System.out.println(result);</programlisting> ...@@ -969,6 +979,25 @@ System.out.println(result);</programlisting>
<programlisting language="java">final BigDecimal zero_dot_99 = new BigDecimal("0.99"); <programlisting language="java">final BigDecimal zero_dot_99 = new BigDecimal("0.99");
final BigDecimal zero_dot_1 = new BigDecimal("0.1"); final BigDecimal zero_dot_1 = new BigDecimal("0.1");
BigDecimal
result = zero_dot_99.subtract(zero_dot_1); // Subtracting 0.1
result = result.subtract(zero_dot_1); // Subtracting 0.1
result = result.subtract(zero_dot_1); // Subtracting 0.1
System.out.println(result);</programlisting>
<screen>0.69</screen>
</figure>
<figure xml:id="sd1_numbers_fig_usingBigDecimalChaining">
<title>Chaining <classname
xlink:href="https://docs.oracle.com/en/java/javase/17/docs/api/java.base/java/math/BigDecimal.html">BigDecimal</classname>
operations</title>
<programlisting language="java">final BigDecimal zero_dot_99 = new BigDecimal("0.99");
final BigDecimal zero_dot_1 = new BigDecimal("0.1");
BigDecimal result = zero_dot_99. BigDecimal result = zero_dot_99.
subtract(zero_dot_1). subtract(zero_dot_1).
subtract(zero_dot_1). subtract(zero_dot_1).
...@@ -979,6 +1008,46 @@ System.out.println(result);</programlisting> ...@@ -979,6 +1008,46 @@ System.out.println(result);</programlisting>
<screen>0.69</screen> <screen>0.69</screen>
</figure> </figure>
<qandaset defaultlabel="qanda"
xml:id="sd1_workingWithNumbers_BigDecimalChaining">
<title>Chaining subtract method calls</title>
<qandadiv>
<qandaentry>
<question>
<para>Explain the chaining mechanism implementing three successive
subtractions in <xref
linkend="sd1_numbers_fig_usingBigDecimalChaining"/>.</para>
</question>
<answer>
<para>We may re-write the statement in question:</para>
<programlisting language="none">result = zero_dot_99.subtract(zero_dot_1).subtract(zero_dot_1).subtract(zero_dot_1);
\ / / /
\ / / /
\ / / /
0.99 - 0.1 / /
\ / /
\ / /
\ / /
0.98 - 0.1 /
\ /
\ /
0.97 - 0.1
== 0.96</programlisting>
<para>Each <methodname>subtract(...)</methodname> call returns a
new result instance of class <classname
xlink:href="https://docs.oracle.com/en/java/javase/17/docs/api/java.base/java/math/BigDecimal.html">BigDecimal</classname>.
We can thus chain a subsequent
<methodname>subtract(...)</methodname> call using this returned
instance.</para>
</answer>
</qandaentry>
</qandadiv>
</qandaset>
<figure xml:id="sd1_numbers_fig_usingBigDecimalFeatures"> <figure xml:id="sd1_numbers_fig_usingBigDecimalFeatures">
<title><classname <title><classname
xlink:href="https://docs.oracle.com/en/java/javase/17/docs/api/java.base/java/math/BigDecimal.html">BigDecimal</classname> xlink:href="https://docs.oracle.com/en/java/javase/17/docs/api/java.base/java/math/BigDecimal.html">BigDecimal</classname>
......
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