added listing tag to make the text more organized

Author: nshizirungudieumerci@gmail.com

source/ch9_commonmistakes.ptx

@@ -117,8 +117,9 @@
     <section xml:id="not-importing-a-class">
         <title>Not importing a class</title>
         <p>
-                In Python, many classes are available by default. However, in Java, you must explicitly import most classes from external packages that you want to use.
+                In Python, many classes are available by default. However, in Java, you must explicitly import most classes from external packages that you want to use <xref ref="importing-classes" text="type-global"/>. If you forget to import a class, the compiler will give you an error message like this:
             </p>
+        <listing xml:id="importing-classes">
         <program language="java" interactive="activecode" line-numbers="yes">
 
             <code>
@@ -133,6 +134,7 @@
             } //  End of class
         </code>
         </program>
+        </listing>
         <p>
              You may notice that this error message looks similar to the previous one, however, it has an entirely different cause. In Java, classes like <c>Scanner</c> that are part of external packages (like <c>java.util</c>) must be explicitly imported into your source file. Java does not automatically recognize these classes. To resolve this error, you need to add an import statement for the <c>Scanner</c> class at the beginning of your <c>Histo.java</c> file, typically import <c>java.util.Scanner;</c>.
         </p>