Trying to create json object in Java but getting nested array? How to do it in a better way?

Question

I am trying to get this output:

{
   "columnNames":[
      "fruit"
   ],
   "values":[
      [
         "Apple is green"
      ]
   ],
   "time":"2017-12-11T00:00:00Z"
}

But I am ending up with the following...

{
   "columnNames":[
      "fruit"
   ],
   "values":[
      [
         {
            "stringValue":"Apple is green",
            "intValue":0,
            "booleanValue":false
         }
      ]
   ],
   "time":null
}

Here is my code (so far) and I would appreciate if you could tell me where I am going wrong?

public static void main(String[] args) {
    List<String> columnNames = new ArrayList<>();
    columnNames.add("fruit");
    List<List<ResponseValues>> responseValues = new ArrayList<>();
    List<ResponseValues> values = Collections.singletonList(new ResponseValues("Apple is green"));
    responseValues.add(values);

    String expectedResult = createAJson(columnNames, responseValues);
    System.out.println(expectedResult);
}


public static String createAJson(List<String> columnNames, List<List<ResponseValues>> values) {
    String jsonBlock = "";
    repBuilder repBuilder = new repBuilder();
    ObjectMapper mapper = new ObjectMapper();

    repBuilder.setColumnNames(columnNames);
    repBuilder.setValues(values);

    try {
        jsonBlock = mapper.writeValueAsString(repBuilder);
    }
    catch (JsonProcessingException e) {
        e.printStackTrace();
    }
    return jsonBlock;
}

And some boiler plate code to create the response value section:

@JsonPropertyOrder({
"columnNames",
"values",
"time"
})

public class RepBuilder {

    public List<String> columnNames;
    public List<List<ResponseValues>> values;
    private String time;

    @JsonProperty("columnNames")
    public List<String> getColumnNames() {
        return columnNames;
    }

    @JsonProperty("columnNames")
    public void setColumnNames(List<String> columnNames) {
        this.columnNames = columnNames;
    }

    @JsonProperty("values")
    public List<List<ResponseValues>> getValues() {
        return values;
    }

    @JsonProperty("values")
    public void setValues(List<List<ResponseValues>> values) {
        this.values = values;
    }

    @JsonProperty("time")
    public String gettime() {
        return time;
    }

    @JsonProperty("time")
    public void settime(String time) {
        this.time = time;
    }

}

And response values: This was done this way so I can set STRING or INTEGER or BOOLEAN and add to list?

@JsonInclude(JsonInclude.Include.NON_NULL)
public class ResponseValues {

    private String stringValue;
    private int intValue;
    private boolean booleanValue;

    public ResponseValues(String stringValue) {
        this.stringValue = stringValue;
    }

    public ResponseValues(int intValue) {
        this.intValue = intValue;
    }

    public ResponseValues(boolean booleanValue) {
        this.booleanValue = booleanValue;
    }

    public String getStringValue() {
        return stringValue;
    }

    public void setStringValue(String stringValue) {
        this.stringValue = stringValue;
    }

    public int getIntValue() {
        return intValue;
    }

    public void setIntValue(int intValue) {
        this.intValue = intValue;
    }

    public boolean getBooleanValue() {
        return booleanValue;
    }

    public void setBooleanValue(boolean booleanValue) {
        this.booleanValue = booleanValue;
    }
}

Can anyone point out what I am missing? I also tried to use @JsonProperty(access = JsonProperty.Access.WRITE_ONLY) but that doesn't work for me, any other ideas? I would appreciate if you use my code in context of your answer (as im still learning java)

You're never setting the time, so it's null. And you have a List<List<ResponseValues>> although you want a List<List<String>>. So you end up in the JSON as an array containing an array of objects instead of an array containing an array of strings. — JB Nizet
– JB Nizet, Commented Dec 31, 2018 at 14:06

xingbin · Accepted Answer · 2018-12-31 15:56:18Z

1

Custom a serializer for ResponseValues, for example:

public class ResponseSerializer extends JsonSerializer<ResponseValues>{ 

    @Override
    public void serialize(ResponseValues responseValues, JsonGenerator gen,
            SerializerProvider provider) throws IOException,
            JsonProcessingException {

        gen.writeString(responseValues.getStringValue());
    }

}

and use it with annotation:

@JsonSerialize(using = ResponseSerializer.class)
public class ResponseValues

Update

If you need check them in order, you can do it like this:

@Override
public void serialize(ResponseValues responseValues, JsonGenerator gen,
        SerializerProvider provider) throws IOException,
        JsonProcessingException {

    String stringValue = responseValues.getStringValue();
    if (stringValue != null) {
        gen.writeString(responseValues.getStringValue());
        return; // if string value is valid, write it and return
    }

    int intValue = responseValues.getIntValue();
    // let's suppose 0 is not a valid status
    if (intValue != 0) { 
        gen.writeNumber(intValue);
        return;
        // also you can use gen.writeString(String.valueOf(intValue)); if you need a String
    }

    boolean booleanValue = responseValues.getBooleanValue();
    gen.writeBoolean(booleanValue);

}

edited Dec 31, 2018 at 15:56

answered Dec 31, 2018 at 14:14

xingbin

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8 Comments

Saffik Over a year ago

Did exactly this and get no values at all in them for columnNames and values?{ "columnNames":[ ], "values":[ ], "time":null }

xingbin Over a year ago

@Saffik Really? I get {"columnNames":["fruit"],"values":[["Apple is green"]],"time":null}.

Saffik Over a year ago

Yeah I get null, also would this work if I choose to not getStringValue but for getIntValue or getBooleanValue

xingbin Over a year ago

@Saffik The code you post does not compile. It seems you should replace values.add(values); with responseValues.add(values); and make createAJson static. So it compiles and output {"columnNames":["fruit"],"values":[["Apple is green"]],"time":null}.

Saffik Over a year ago

OK - with your suggestions it work. One more thing though.. If I do in main method the following: List<ResponseValues> values = Collections.singletonList(new ResponseValues(4)); - This won't work? as ResponseSerializer only writes getStringValue? Any idea how to make it work for the getIntValue and getBooleanValue?

|

Mohamed Anees A · Accepted Answer · 2018-12-31 14:10:34Z

1

private int intValue;
private boolean booleanValue;

These(int,boolean) are primitives, so these are by default initialised with default values of 0 and false respectively. You can use

private Integer intValue;
private Boolean booleanValue;

and configure writer to ignore null values. Also,I don't see where you are setting "time". Since it is of type String, it has null as default in your output.

Last coming to Suggestion since you are learning java,

Start your class names with a capital letter.Change repBuilder -> RepBuilder
I loved your Collections.singletonList() and your efforts are evident in your question. Keep growing!

answered Dec 31, 2018 at 14:10

Mohamed Anees A

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5 Comments

Saffik Over a year ago

I shall do, thanks for your recommendation. One thing though, how do i tell writer to ignore null values? do i use @JsonInclude(JsonInclude.Include.NON_NULL)?

Mohamed Anees A Over a year ago

Yes exactly! Also, And you have a List<List<ResponseValues>> although you want a List<List<String>> as mentioned by @JB Nizet in comments

Saffik Over a year ago

Ahan. With the above to ignore null values, I still get com.fasterxml.jackson.databind.JsonMappingException: (was java.lang.NullPointerException)

Mohamed Anees A Over a year ago

Please post the complete exception which you are getting

Saffik Over a year ago

OK - having a look now. Trying a few things... :)

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