[impr-list-to-str] string.join (#17751)

* [impr-list-to-str] string.join

* [impr-list-to-str] minor change

Author: sk1418

core-java-modules/core-java-collections-conversions/src/test/java/com/baeldung/java/lists/ListToStringUnitTest.java

@@ -15,17 +15,30 @@ public void whenListToString_thenPrintDefault() {
         System.out.println(intLIst);
     }
 
+    @Test
+    public void whenStringJoinWithStringList_thenPrintCustom() {
+        List<String> strList = Arrays.asList("one", "two", "three");
+        System.out.println(String.join(" : ", strList));
+    }
+
+    @Test
+    public void whenStringJoinWithNonStringList_thenPrintCustom() {
+        List<Integer> intList = Arrays.asList(1, 2, 3);
+        List<String> strList = intList.stream().map(String::valueOf).collect(Collectors.toList());
+        System.out.println(String.join(" : ", strList));
+    }
+
     @Test
     public void whenCollectorsJoining_thenPrintCustom() {
         List<Integer> intList = Arrays.asList(1, 2, 3);
         System.out.println(intList.stream()
-            .map(n -> String.valueOf(n))
+            .map(String::valueOf)
             .collect(Collectors.joining("-", "{", "}")));
     }
-    
+
     @Test
     public void whenStringUtilsJoin_thenPrintCustom() {
         List<Integer> intList = Arrays.asList(1, 2, 3);
-       System.out.println(StringUtils.join(intList, "|"));
+        System.out.println(StringUtils.join(intList, "|"));
     }
-}
+}