Fix logical error with offsets array in JsonFileWriter. Add broken string test case to simple.json

Change-Id: If62adb8cc209868e105f07f276c54140fe366df5

Author: wesm

integration/data/simple.json

@@ -22,6 +22,18 @@
             {"type": "DATA", "typeBitWidth": 64}
           ]
         }
+      },
+      {
+        "name": "baz",
+        "type": {"name": "utf8"},
+        "nullable": true, "children": [],
+        "typeLayout": {
+          "vectors": [
+            {"type": "VALIDITY", "typeBitWidth": 1},
+            {"type": "OFFSET", "typeBitWidth": 32},
+            {"type": "DATA", "typeBitWidth": 64}
+          ]
+        }
       }
     ]
   },
@@ -40,6 +52,13 @@
           "count": 5,
           "VALIDITY": [1, 0, 0, 1, 1],
           "DATA": [1.0, 2.0, 3.0, 4.0, 5.0]
+        },
+        {
+          "name": "baz",
+          "count": 5,
+          "VALIDITY": [1, 0, 0, 1, 1],
+          "OFFSET": [0, 2, 2, 2, 5, 9],
+          "DATA": ["aa", "", "", "bbb", "cccc"]
         }
       ]
     }

java/vector/src/main/java/org/apache/arrow/vector/file/json/JsonFileReader.java

@@ -127,8 +127,13 @@ private void readVector(Field field, FieldVector vector) throws JsonParseExcepti
         ValueVector valueVector = (ValueVector)innerVector;
         valueVector.allocateNew();
         Mutator mutator = valueVector.getMutator();
-        mutator.setValueCount(count);
-        for (int i = 0; i < count; i++) {
+
+        int innerVectorCount = count;
+        if (vectorType.getName() == "OFFSET") {
+          innerVectorCount++;
+        }
+        mutator.setValueCount(innerVectorCount);
+        for (int i = 0; i < innerVectorCount; i++) {
           parser.nextToken();
           setValueFromParser(valueVector, i);
         }

java/vector/src/main/java/org/apache/arrow/vector/file/json/JsonFileWriter.java

@@ -114,7 +114,7 @@ private void writeVector(Field field, FieldVector vector) throws IOException {
         BufferBacked innerVector = fieldInnerVectors.get(v);
         generator.writeArrayFieldStart(vectorType.getName());
         ValueVector valueVector = (ValueVector)innerVector;
-        for (int i = 0; i < valueCount; i++) {
+        for (int i = 0; i < valueVector.getAccessor().getValueCount(); i++) {
           writeValueToGenerator(valueVector, i);
         }
         generator.writeEndArray();