Junit testing println(...) output

Doc/Sd1/appendix.xml

@@ -478,6 +478,71 @@ java -jar ~/.m2/repository/de/hdm-stuttgart/mi/sd1/grep/0.9/grep-0.9.jar $*
             <programlisting language="none">&gt; cat Testdata/input.txt | ./bin/mygrep red
 The red cross acts worldwide</programlisting>
           </listitem>
+
+          <listitem>
+            <para>Testing requires capturing of output being generated by e.g.
+            <methodname
+            xlink:href="https://docs.oracle.com/javase/8/docs/api/java/lang/System.html#out">System.out</methodname><code>.</code><methodname
+            xlink:href="https://docs.oracle.com/javase/8/docs/api/java/io/PrintStream.html#println-java.lang.String-">println(...)</methodname>
+            calls. Consider the following code writing the string <quote>Hello
+            World!</quote> to standard output:</para>
+
+            <programlisting language="java">public class App {
+    /**
+     * @param args Unused
+     */
+    public static void main( String[] args ) {
+        System.out.print( "Hello World!" );
+    }
+}</programlisting>
+
+            <para>We want to set up a <productname>Junit</productname> test
+            which captures the output to compare it with the expected string
+            value <code>"Hello World!"</code>. Following <uri
+            xlink:href="http://stackoverflow.com/questions/1119385/junit-test-for-system-out-println">http://stackoverflow.com/questions/1119385/junit-test-for-system-out-println</uri>
+            we redefine the standard output stream by an private instance of
+            <classname
+            xlink:href="https://docs.oracle.com/javase/8/docs/api/java/io/ByteArrayOutputStream.html">java.io.ByteArrayOutputStream</classname>.
+            Due to <productname>Junit</productname>'s @Before and @After
+            annotations this instance replaces <classname
+            xlink:href="https://docs.oracle.com/javase/8/docs/api/java/lang/System.html#out">System.out</classname>
+            during our tests:</para>
+
+            <programlisting language="java">import java.io.ByteArrayOutputStream;
+import java.io.PrintStream;
+
+import org.junit.After;
+import org.junit.Assert;
+import org.junit.Before;
+import org.junit.Test;
+
+/**
+ * Unit test for simple App.
+ */
+public class AppTest {
+   private final ByteArrayOutputStream outContent = new ByteArrayOutputStream();
+
+   @Before
+   public void setUpStreams() {
+       System.setOut(new PrintStream(outContent));
+   }
+
+   @After
+   public void cleanUpStreams() {
+       System.setOut(null);
+       outContent.reset();
+   }
+   
+    /**
+     * Test method accessing output generated by System.out.println(...) calls.
+     */
+    @Test
+    public void testApp() {
+       App.main(new String[]{}); // Calling main() method printing "Hello World!"
+       Assert.assertEquals("Hello World!", outContent.toString());
+    }
+}</programlisting>
+          </listitem>
         </orderedlist>
       </tip>
     </section>