[impr-listVSarrayList] add util methods (#18964)

Author: sk1418

core-java-modules/core-java-collections-list-3/src/test/java/com/baeldung/list/listvsarraylist/ListDemoUnitTest.java

@@ -2,8 +2,11 @@
 
 import static org.assertj.core.api.Assertions.assertThat;
 import static org.junit.Assert.assertNotNull;
+import static org.junit.jupiter.api.Assertions.assertThrows;
 
 import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.Collections;
 import java.util.List;
 import java.util.Locale;
 
@@ -17,12 +20,19 @@ class ListDemoUnitTest {
     Passenger passenger1;
     Passenger passenger2;
     Passenger passenger3;
+    Passenger passenger4;
+    Passenger passenger5;
+    Passenger passenger6;
 
     @BeforeEach
     protected void setUp() {
         passenger1 = new Passenger("Anna", 25, "London", "New York");
         passenger2 = new Passenger("Binny", 35, "New York", "London");
         passenger3 = new Passenger("Chandra", 8, "Paris", "New Delhi");
+        passenger4 = new Passenger("Amanda", 32, "Paris", "Hamburg");
+        passenger5 = new Passenger("Dora", 8, "Paris", "Hamburg");
+        passenger6 = new Passenger("Kent", 48, "Paris", "Munich");
+
         application.addPassenger(passenger1);
         application.addPassenger(passenger2);
         application.addPassenger(passenger3);
@@ -89,4 +99,94 @@ public void givenCurrentLocale_whenUsingStreams_thenReturnsListType() {
         assertThat(servicedCountries).hasSize(Locale.getISOCountries().length);
     }
 
-}
+    @Test
+    void whenUsingSingletonListUtilMethod_thenGetExpectedResult() {
+        List<Passenger> singletonList = Collections.singletonList(passenger1);
+
+        assertThat(singletonList).hasSize(1);
+        assertThat(singletonList.get(0)).isEqualTo(passenger1);
+        assertThrows(UnsupportedOperationException.class, () -> singletonList.add(passenger2));
+    }
+
+    @Test
+    void whenUsingUnmodifiableListUtilMethod_thenGetExpectedResult() {
+        List<Passenger> originalList = new ArrayList<>();
+        originalList.add(passenger1);
+        originalList.add(passenger2);
+
+        List<Passenger> unmodifiableList = Collections.unmodifiableList(originalList);
+        assertThat(unmodifiableList).isEqualTo(originalList);
+
+        assertThrows(UnsupportedOperationException.class, () -> unmodifiableList.add(passenger3));
+        assertThrows(UnsupportedOperationException.class, () -> unmodifiableList.remove(passenger2));
+
+        originalList.add(passenger3);
+        assertThat(unmodifiableList).hasSize(3);
+    }
+
+    @Test
+    void whenUsingArraysAsList_thenGetExpectedResult() {
+        Passenger[] array = { passenger1, passenger2 };
+        List<Passenger> list = Arrays.asList(array);
+
+        assertThat(list).hasSize(2);
+        assertThat(list.get(0)).isEqualTo(passenger1);
+        assertThat(list.get(1)).isEqualTo(passenger2);
+
+        // We can update elements (reflected in the array too)
+        list.set(1, passenger3);
+        assertThat(array[1]).isEqualTo(passenger3);
+
+        // Verify immutability of size: adding/removing throws UnsupportedOperationException
+        assertThrows(UnsupportedOperationException.class, () -> list.add(passenger2));
+        assertThrows(UnsupportedOperationException.class, () -> list.remove(passenger1));
+    }
+
+    @Test
+    void whenUsingArraysSubList_thenGetExpectedResult() {
+        List<Passenger> originalList = Arrays.asList(passenger1, passenger2, passenger3, passenger4, passenger5);
+
+        List<Passenger> subList = originalList.subList(1, 3);
+
+        assertThat(subList).isEqualTo(Arrays.asList(passenger2, passenger3));
+
+        // Changes in sublist reflect in the original list
+        subList.set(1, passenger6);
+        assertThat(originalList.get(2)).isEqualTo(passenger6);
+
+        // immutability of size: adding/removing throws UnsupportedOperationException
+        assertThrows(UnsupportedOperationException.class, () -> subList.add(passenger4));
+        assertThrows(UnsupportedOperationException.class, () -> subList.remove(passenger2));
+    }
+
+    @Test
+    void whenUsingListOf_thenGetExpectedResult() {
+        List<Passenger> list = List.of(passenger1, passenger2, passenger3);
+
+        assertThat(list).isEqualTo(Arrays.asList(passenger1, passenger2, passenger3));
+
+        assertThrows(UnsupportedOperationException.class, () -> list.add(passenger4));
+        assertThrows(UnsupportedOperationException.class, () -> list.set(0, passenger4));
+        assertThrows(UnsupportedOperationException.class, () -> list.remove(passenger2));
+    }
+
+    @Test
+    void whenUsingListCopyOf_thenGetExpectedResult() {
+        List<Passenger> originalList = new ArrayList<>();
+        originalList.add(passenger1);
+        originalList.add(passenger2);
+        originalList.add(passenger3);
+
+        List<Passenger> copy = List.copyOf(originalList);
+
+        assertThat(copy).isEqualTo(originalList);
+
+        assertThrows(UnsupportedOperationException.class, () -> copy.add(passenger4));
+        assertThrows(UnsupportedOperationException.class, () -> copy.set(0, passenger4));
+        assertThrows(UnsupportedOperationException.class, () -> copy.remove(passenger2));
+
+        // Changes to the original list do NOT affect the copy
+        originalList.add(passenger6);
+        assertThat(copy).hasSize(3);
+    }
+}