[join-str-comma-and] Joining Strings follows natural language grammar (#14941)

core-java-modules/core-java-string-conversions-3/src/test/java/com/baeldung/joinasnl/JoinStringsAsNaturalLangUnitTest.java

@@ -0,0 +1,36 @@
+package com.baeldung.joinasnl;
+
+import static java.util.Collections.emptyList;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import java.util.List;
+
+import org.junit.jupiter.api.Test;
+
+public class JoinStringsAsNaturalLangUnitTest {
+    String joinItemsAsNaturalLanguage(List<String> list, boolean oxfordComma) {
+        if (list.size() < 3) {
+            return String.join(" and ", list);
+        }
+        // list has at least three elements
+        int lastIdx = list.size() - 1;
+        StringBuilder sb = new StringBuilder();
+        return sb.append(String.join(", ", list.subList(0, lastIdx)))
+          .append(oxfordComma ? ", and " : " and ")
+          .append(list.get(lastIdx))
+          .toString();
+    }
+
+    @Test
+    void whenCallingJoinByGrammar_thenGetTheExpectedResult() {
+        assertEquals("", joinItemsAsNaturalLanguage(emptyList(), false));
+        assertEquals("A", joinItemsAsNaturalLanguage(List.of("A"), false));
+        assertEquals("A and B", joinItemsAsNaturalLanguage(List.of("A", "B"), false));
+        assertEquals("A, B, C, D and I have a comma (,)", joinItemsAsNaturalLanguage(List.of("A", "B", "C", "D", "I have a comma (,)"), false));
+        // with oxford comma = true
+        assertEquals("", joinItemsAsNaturalLanguage(emptyList(), true));
+        assertEquals("A", joinItemsAsNaturalLanguage(List.of("A"), true));
+        assertEquals("A and B", joinItemsAsNaturalLanguage(List.of("A", "B"), true));
+        assertEquals("A, B, C, D, and I have a comma (,)", joinItemsAsNaturalLanguage(List.of("A", "B", "C", "D", "I have a comma (,)"), true));
+    }
+}