Arrays.asList() vs Collections.singletonList() (#12552)

core-java-modules/core-java-collections-list-4/src/test/java/com/baeldung/list/aslistvssingletonlist/ArraysAsListVsSingletonListUnitTest.java

@@ -0,0 +1,48 @@
+package com.baeldung.list.aslistvssingletonlist;
+
+import org.junit.jupiter.api.Test;
+
+import java.util.Arrays;
+import java.util.Collections;
+import java.util.List;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.AssertionsForClassTypes.assertThatExceptionOfType;
+
+class ArraysAsListVsSingletonListUnitTest {
+
+    @Test
+    void givenListFromArraysAsList_whenChangingStructureAndElement_thenGetExpectedResult() {
+        List<String> arraysAsList = Arrays.asList("ONE");
+        assertThatExceptionOfType(UnsupportedOperationException.class).isThrownBy(
+            () -> arraysAsList.add("TWO")
+        );
+
+        arraysAsList.set(0, "A brand new string");
+        assertThat(arraysAsList.get(0)).isEqualTo("A brand new string");
+    }
+
+    @Test
+    void givenSingletonList_whenChangingStructureAndElement_thenThrowException() {
+        List<String> singletonList = Collections.singletonList("ONE");
+        assertThatExceptionOfType(UnsupportedOperationException.class).isThrownBy(
+            () -> singletonList.add("TWO")
+        );
+        assertThatExceptionOfType(UnsupportedOperationException.class).isThrownBy(
+            () -> singletonList.set(0, "A brand new string")
+        );
+    }
+
+    @Test
+    void givenAnArray_whengetListByArraysAsList_thenTheListIsBackedByTheArray() {
+        String[] theArray = new String[] { "ONE", "TWO" };
+        List<String> theList = Arrays.asList(theArray);
+        //changing the list, the array is changed too
+        theList.set(0, "ONE [changed in list]");
+        assertThat(theArray[0]).isEqualTo("ONE [changed in list]");
+
+        //changing the array, the list is changed too
+        theArray[1] = "TWO [changed in array]";
+        assertThat(theList.get(1)).isEqualTo("TWO [changed in array]");
+    }
+}