Fix Video.toString() output illegal json and refactor

partial-response/src/main/java/com/iluwatar/partialresponse/FieldJsonMapper.java

@@ -25,6 +25,7 @@
 package com.iluwatar.partialresponse;
 
 import java.lang.reflect.Field;
+import java.util.StringJoiner;
 
 /**
  * Map a video to json.
@@ -39,18 +40,15 @@ public class FieldJsonMapper {
    * @return json of required fields from video
    */
   public String toJson(Video video, String[] fields) throws Exception {
-    var json = new StringBuilder().append("{");
+    var json = new StringJoiner(",", "{", "}");
 
     var i = 0;
     var fieldsLength = fields.length;
     while (i < fieldsLength) {
-      json.append(getString(video, Video.class.getDeclaredField(fields[i])));
-      if (i != fieldsLength - 1) {
-        json.append(",");
-      }
+      json.add(getString(video, Video.class.getDeclaredField(fields[i])));
       i++;
     }
-    json.append("}");
+
     return json.toString();
   }
 

partial-response/src/main/java/com/iluwatar/partialresponse/Video.java

@@ -43,7 +43,7 @@ public String toString() {
             + "\"length\": " + length + ","
             + "\"description\": \"" + description + "\","
             + "\"director\": \"" + director + "\","
-            + "\"language\": \"" + language + "\","
+            + "\"language\": \"" + language + "\""
             + "}";
   }
 }

partial-response/src/test/java/com/iluwatar/partialresponse/VideoResourceTest.java

@@ -63,7 +63,7 @@ void shouldGiveVideoDetailsById() throws Exception {
     var actualDetails = resource.getDetails(1);
 
     var expectedDetails = "{\"id\": 1,\"title\": \"Avatar\",\"length\": 178,\"description\": "
-        + "\"epic science fiction film\",\"director\": \"James Cameron\",\"language\": \"English\",}";
+        + "\"epic science fiction film\",\"director\": \"James Cameron\",\"language\": \"English\"}";
     Assertions.assertEquals(expectedDetails, actualDetails);
   }
 
@@ -78,4 +78,17 @@ void shouldGiveSpecifiedFieldsInformationOfVideo() throws Exception {
 
     Assertions.assertEquals(expectedDetails, actualFieldsDetails);
   }
-}
+
+  @Test
+  void shouldAllSpecifiedFieldsInformationOfVideo() throws Exception {
+    var fields = new String[]{"id", "title", "length", "description", "director", "language"};
+
+    var expectedDetails = "{\"id\": 1,\"title\": \"Avatar\",\"length\": 178,\"description\": "
+        + "\"epic science fiction film\",\"director\": \"James Cameron\",\"language\": \"English\"}";
+    Mockito.when(fieldJsonMapper.toJson(any(Video.class), eq(fields))).thenReturn(expectedDetails);
+
+    var actualFieldsDetails = resource.getDetails(1, fields);
+
+    Assertions.assertEquals(expectedDetails, actualFieldsDetails);
+  }
+}