Add Java-Kompendium book

Java-Kompendium/kap6/Task3.java

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+import java.util.*;
+
+public class Task3 {
+    public static void main(String[] args) {
+        var javaList = new HashSet<String>(3);
+        var pythonList = new HashSet<String>(3);
+        var commonList = new HashSet<String>();
+        var differenceSet = new HashSet<String>();
+
+        javaList.add("Maximilian Mayer");
+        javaList.add("Emil Faber");
+        javaList.add("Anna Müller");
+
+        pythonList.add("Anna Müller");
+        pythonList.add("Sebastian Waller");
+        pythonList.add("Oliver Stilz");
+
+        commonList.addAll(javaList);
+        commonList.addAll(pythonList);
+
+        differenceSet.addAll(javaList);
+        differenceSet.retainAll(pythonList);
+
+        System.out.println("Java-Kurs: " + javaList);
+        System.out.println("Python-Kurs: " + pythonList);
+        System.out.println("Alle Teilnehmer: " + commonList);
+        System.out.println("Beide Kurse: " + differenceSet);
+    }
+}