BAEL-6713: Convert List<Long> Object to long[] Array in Java (#14479)

core-java-modules/core-java-collections-conversions-2/src/test/java/com/baeldung/convertlisttoarray/LongListToLongArrayConversionUnitTest.java

@@ -0,0 +1,74 @@
+package com.baeldung.convertlisttoarray;
+
+import static org.junit.Assert.assertTrue;
+
+import java.util.Arrays;
+import java.util.List;
+
+import org.junit.Before;
+import org.junit.Test;
+
+import com.google.common.primitives.Longs;
+
+public class LongListToLongArrayConversionUnitTest {
+
+    private List<Long> list;
+
+    @Before
+    public void setUp() {
+        list = Arrays.asList(1L, 2L, 3L, 4L, 5L);
+    }
+
+    @Test
+    public void givenALongList_whenConvertWithListToArray_thenReturnLongArray() {
+
+        // Init an array with the same size as the list to convert
+        Long[] arrayWithSettedSize = new Long[list.size()];
+        arrayWithSettedSize = list.toArray(arrayWithSettedSize);
+        assertTrue(list.size() == arrayWithSettedSize.length);
+
+        // Init an empty array
+        Long[] arrayWithNoSettedSize = new Long[0];
+        arrayWithNoSettedSize = list.toArray(arrayWithNoSettedSize);
+        assertTrue(list.size() == arrayWithNoSettedSize.length);
+    }
+
+    @Test
+    public void givenALongList_whenConvertWithLongsToArray_thenReturnLongArray() {
+        // Convertion using Guava library Longs.toArray() method
+        long[] array = Longs.toArray(list);
+        assertTrue(compareListWithArray(list, array));
+    }
+
+    @Test
+    public void givenALongList_whenConvertWithStreamMapToLong_thenReturnLongArray() {
+        // Using mapToLong() - lambda expression
+        long[] arrayUsingLambda = list.stream()
+          .mapToLong(l -> l)
+          .toArray();
+        assertTrue(compareListWithArray(list, arrayUsingLambda));
+
+        // Using mapToLong() - method reference
+        long[] arrayUsingMethodReference = list.stream()
+          .mapToLong(Long::longValue)
+          .toArray();
+        assertTrue(compareListWithArray(list, arrayUsingMethodReference));
+    }
+
+    public static boolean compareListWithArray(List<Long> list, long[] array) {
+        // Check if the sizes of the array and list are equal
+        if (array.length != list.size()) {
+            return false;
+        }
+
+        // Compare each element of the array with the corresponding element in the list
+        for (int i = 0; i < array.length; i++) {
+            // Convert Long to long for comparison
+            if (array[i] != list.get(i)) {
+                return false;
+            }
+        }
+
+        return true;
+    }
+}