I’ve been trying to figure this out for a bit but couldn’t pin-point my mistake.

I have a JSON I’d like to read into Java objects, manipulate those objects, then write them to a new JSON.

Code looks something like:

POJO

@AllArgsConstructor
@NoArgsConstructor
@Data
@ToString
public class MyPojo {

    private String comment;
    private long pos;
    private int type;
}

Read

    public List<MyPojo> readMyPojos(File srcFile) {

        List<MyPojo> jsonObjects = new LinkedList<>();

        try {
            ObjectMapper objectMapper = new ObjectMapper();
            ObjectReader objectReader = objectMapper.readerFor(MyPojo.class);

            Iterator<MyPojo> iterator = objectReader.readValues(srcFile);
            while (iterator.hasNext()) {
                jsonObjects.add(iterator.next());
            }
        } catch (IOException e) {
            throw new RuntimeException(e);
        }

// manipulation operations ...

        return jsonObjects;
    }

Write

    public void writeNewMyPojos(File srcFile, List<MyPojo> newFileContents) {

        String srcFileName = srcFile.getName();

        int fileExtensionIndex = srcFileName.lastIndexOf('.');
        String destFileExtension = srcFileName.substring(fileExtensionIndex);
        String destFileName = srcFileName.substring(0, fileExtensionIndex) + "_updated" + destFileExtension;
        File destFile = new File(srcFile.getParentFile(), destFileName);

        try {
            ObjectMapper objectMapper = new ObjectMapper();
            objectMapper.enable(SerializationFeature.INDENT_OUTPUT);

            objectMapper.writeValue(destFile, newFileContents);
        } catch (IOException e) {
            throw new RuntimeException(e);
        }
    }

As for the results:

Example Input

        newFileContents = List.of(
                new MyPojo("Test1", 1, 2),
                new MyPojo("Test2", 1, 2),
                new MyPojo("Test3", 1, 2)
        );

Expected Output (roughly)

[ 
  {
    "comment" : "Test1",
    "pos" : 1,
    "type" : 2
  }, 
  {
    "comment" : "Test2",
    "pos" : 1,
    "type" : 2
  }, 
  {
    "comment" : "Test3",
    "pos" : 1,
    "type" : 2
  } 
]

Actual Output

[ {
  "comment" : "Test1",
  "pos" : 1,
  "type" : 2
}, {
  "comment" : "Test2",
  "pos" : 1,
  "type" : 2
}, {
  "comment" : "Test3",
  "pos" : 1,
  "type" : 2
} ]

I’ve tried jackson 2.17, 2.16 and even 2.13 (as ChatGPT seems to use that version) to no avail.