Java 16961 (#13236)

* [JAVA-16961] Moved code for article "Create an Empty Map in Java"

* [JAVA-16961] Alter README.md

* [JAVA-16961] Moved code for article "Sorting a Hashset in Java"

* [JAVA-16961] Added links to README.md files

* [JAVA-16961] Revert link changes

* [JAVA-16961] Replaced junit4 with junit5 annotations

* [JAVA-16961] test build

* [JAVA-16961] Added junit annotations

* [JAVA-16961] Added links

Co-authored-by: panagiotiskakos <panagiotis.kakos@libra-is.com>

core-java-modules/core-java-collections-set-2/src/test/java/com/baeldung/hashset/sorting/HashSetUnitTest.java

@@ -0,0 +1,55 @@
+package com.baeldung.hashset.sorting;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.HashSet;
+import java.util.LinkedHashSet;
+import java.util.TreeSet;
+import java.util.stream.Collectors;
+
+import org.junit.jupiter.api.Test;
+
+public class HashSetUnitTest {
+
+    @Test
+    void givenHashSet_whenUsingCollectionsSort_thenHashSetSorted() {
+        HashSet<Integer> numberHashSet = new HashSet<>();
+        numberHashSet.add(2);
+        numberHashSet.add(1);
+        numberHashSet.add(4);
+        numberHashSet.add(3);
+        // converting HashSet to arraylist
+        ArrayList<Integer> arrayList = new ArrayList<>(numberHashSet);
+        // sorting the list
+        Collections.sort(arrayList);
+        assertThat(arrayList).containsExactly(1, 2, 3, 4);
+    }
+
+    @Test
+    void givenHashSet_whenUsingTreeSet_thenHashSetSorted() {
+        HashSet<Integer> numberHashSet = new HashSet<>();
+        numberHashSet.add(2);
+        numberHashSet.add(1);
+        numberHashSet.add(4);
+        numberHashSet.add(3);
+        // TreeSet gets the value of hashSet
+        TreeSet<Integer> treeSet = new TreeSet<>();
+        treeSet.addAll(numberHashSet);
+        assertThat(treeSet).containsExactly(1, 2, 3, 4);
+    }
+
+    @Test
+    void givenHashSet_whenUsingStream_thenHashSetSorted() {
+        HashSet<Integer> numberHashSet = new HashSet<>();
+        numberHashSet.add(200);
+        numberHashSet.add(100);
+        numberHashSet.add(400);
+        numberHashSet.add(300);
+        HashSet<Integer> sortedHashSet = numberHashSet.stream().sorted()
+          .collect(Collectors.toCollection(LinkedHashSet::new));
+        assertThat(sortedHashSet).containsExactly(100, 200, 300, 400);
+    }
+
+}