[fillEnumInList] Filling a List with All Enum Values in Java (#13005)

* [fillEnumInList] Filling a List with All Enum Values in Java

* [fillEnumInList] change the return type in the util method

core-java-modules/core-java-lang-oop-types-2/src/test/java/com/baeldung/enums/fillinlist/FillEnumsInListUnitTest.java

@@ -0,0 +1,53 @@
+package com.baeldung.enums.fillinlist;
+
+import org.junit.jupiter.api.Test;
+
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.Collections;
+import java.util.EnumSet;
+import java.util.List;
+
+import static com.baeldung.enums.fillinlist.MagicNumber.*;
+import static org.junit.jupiter.api.Assertions.*;
+
+enum MagicNumber {
+    ONE, TWO, THREE, FOUR, FIVE
+}
+
+public class FillEnumsInListUnitTest {
+    private static final List<MagicNumber> EXPECTED_LIST = Arrays.asList(ONE, TWO, THREE, FOUR, FIVE);
+
+    static <T> List<T> enumValuesInList(Class<T> enumCls) {
+        T[] arr = enumCls.getEnumConstants();
+        return arr == null ? Collections.emptyList() : Arrays.asList(arr);
+    }
+
+    @Test
+    void givenEnum_whenUsingArraysAsList_shouldGetExpectedResult() {
+        List<MagicNumber> result = Arrays.asList(MagicNumber.values());
+        assertEquals(EXPECTED_LIST, result);
+    }
+
+    @Test
+    void givenEnum_whenUsingGetEnumConstants_shouldGetExpectedResult() {
+        List<MagicNumber> result = Arrays.asList(MagicNumber.class.getEnumConstants());
+        assertEquals(EXPECTED_LIST, result);
+    }
+
+    @Test
+    void givenEnum_whenUsingenumValuesInList_shouldGetExpectedResult() {
+        List<MagicNumber> result1 = enumValuesInList(MagicNumber.class);
+        assertEquals(EXPECTED_LIST, result1);
+
+        List<Integer> result2 = enumValuesInList(Integer.class);
+        assertTrue(result2.isEmpty());
+    }
+
+    @Test
+    void givenEnum_whenUsingEnumSet_shouldGetExpectedResult() {
+        List<MagicNumber> result = new ArrayList<>(EnumSet.allOf(MagicNumber.class));
+        assertEquals(EXPECTED_LIST, result);
+    }
+
+}