BAEL-1985: Added Examples showing how to Initialize HashSet when it's constructed (#4715)

* Added Class for Initalizing HahsSet

* Updated Class name

* Delete InitializingSetTest.java

core-java-collections/src/test/java/com/baeldung/java/set/HashSetInitalizingUnitTest.java

@@ -0,0 +1,81 @@
+package com.baeldung.java.set;
+
+import com.google.common.collect.ImmutableSet;
+import com.google.common.collect.Sets;
+import org.junit.jupiter.api.Test;
+  
+import java.util.Arrays;
+import java.util.Collections;
+import java.util.HashSet;
+import java.util.Set;
+import java.util.stream.Collectors;
+import java.util.stream.Stream;
+
+import static org.junit.Assert.assertEquals;
+
+public class HashSetInitalizingUnitTest {
+    @Test
+    public void whenUsingJava_usingArraysStaticMethod_thenCorrectSize() {
+        Set<String> set = new HashSet<>(Arrays.asList("a", "b", "c"));
+        assertEquals(3, set.size());
+    }
+
+    @Test
+    public void whenUsingJava_usingAnonymousClass_thenCorrectSize() {
+        Set<String> set = new HashSet<String>(){{
+            add("a");
+            add("b");
+            add("c");
+        }};
+        assertEquals(3, set.size());
+    }
+
+    @Test
+    public void whenUsingJava_creatingSingletonSet_thenCorrectSize() {
+        Set<String> set = Collections.singleton("a");
+        assertEquals(1, set.size());
+    }
+
+    public static final <T> Set<T> newHashSet(T... objs) {
+        Set<T> set = new HashSet<T>();
+        Collections.addAll(set, objs);
+        return set;
+    }
+
+    @Test
+    public void whenUsingJava_usingCustomStaticUtilMethod_thenCorrectSize() {
+        Set<String> set = newHashSet("a","b","c");
+        assertEquals(3, set.size());
+    }
+
+    @Test
+    public void whenUsingJava8_usingCollectOnStream_thenCorrectSize() {
+        Set<String> set = Stream.of("a", "b", "c").collect(Collectors.toSet());
+        assertEquals(3, set.size());
+    }
+    @Test
+    public void whenUsingJava8_fromStringArray_thenCorrectSize() {
+        String[] stringArray = {"a","b","c"};
+        Set<String> set = Arrays.stream(stringArray).collect(Collectors.toCollection(HashSet::new));
+        assertEquals(3, set.size());
+    }
+
+    // Requires Java9 - uncomment if you are using Java 9 or higher
+    /*@Test
+    public void whenUsingJava9_usingCollectOnStream_thenCorrectSize() {
+        Set set = Set.of("a", "b", "c");
+        assertEquals(3, set.size());
+    }*/
+
+    @Test
+    public void whenUsingGoogleGuava_createMutableSet_thenCorrectSize() {
+        Set<String> set = Sets.newHashSet("a", "b", "c");
+        assertEquals(3, set.size());
+    }
+
+    @Test
+    public void whenUsingGoogleGuava_createImmutableSet_thenCorrectSize() {
+        Set<String> set = ImmutableSet.of("a", "b", "c");
+        assertEquals(3, set.size());
+    }
+}