parse JSON array/object using Gson in java

Question

I want to parse a JSON file in JAVA using GSON and the file format is as follows:

{
"parcel":[
    {
        "firstName": "ABC",
        "lastName": "LAST",
        "email": "[email protected]",
        "country": "United States",
        "dates": [
            "2013-05-01",
            "2014-05-07"
        ]
    },
    {
        "firstName": "CVS",
        "lastName": "Bad",
        "email": "[email protected]",
        "country": "Iceland",
        "dates": [
            "2010-04-30",
            "2011-01-01",

        ]
    }
    ]

}

And my main method in the public class goes like this:

    public static void main(String[] args) throws IOException {
       try(Reader reader = new InputStreamReader(JsonToJava.class.getResourceAsStream("input.json"), "UTF-8")){
        Gson gson = new GsonBuilder().create();
        Parcel p = gson.fromJson(reader, Parcel.class);
        System.out.println(p);
       }
}

And the following is the code for Parcel:

public class Parcel {

Data data;

@Override
public String toString() {
    return "Parcel [data=" + data + "]";
}

public Data getData() {
    return data;
}

public void setData(Data data) {
    this.data = data;
}
}
class Data{
public String getFirstName() {
    return firstName;
}

public void setFirstName(String firstName) {
    this.firstName = firstName;
}

public String getLastName() {
    return lastName;
}

public void setLastName(String lastName) {
    this.lastName = lastName;
}

public String getEmail() {
    return email;
}

public void setEmail(String email) {
    this.email = email;
}

public String getCountry() {
    return country;
}

public void setCountry(String country) {
    this.country = country;
}

public Arrays[] getAvailableDates() {
    return dates;
}

public void setAvailableDates(Arrays[] dates) {
    this.dates = dates;
}

@Override
public String toString() {
    return "Data [firstName=" + firstName + ", lastName=" + lastName + ", email=" + email + ", country=" + country
            + ", dates=" + Arrays.toString(dates) + "]";
}

private String firstName;

private String lastName;

private String email;

private String country;

private Arrays[] dates;

}

But I can't seem to be hitting the right way to store all values of the variables in a list. Right now All I get is Parcel[data = null].

I can't question the JSON format. Nonetheless, is there a way to straightaway jump to the keys and parse out the other garbage? — Niras
– Niras, Commented Apr 29, 2018 at 16:26

vstm · Accepted Answer · 2018-05-01 06:43:48Z

2

Well the JSON input and your class hierarchy have to be compatible. Now you have a class Parcel with one Data attribute named "data". Meanwhile your JSON is an object with an attribute named "class". So gson does not find a "class" property in the Parcel class and the "data" attribute get's ignored.

Secondly your JSON has an array of Data's instead of just one Data instance, so you'd have to make an array out of it.

So this should work better:

import com.google.gson.annotations.SerializedName;

public class Parcel {
    @SerializedName("class")
    Data[] data;

    @Override
    public String toString() {
        StringBuilder result = new StringBuilder();

        result.append("Parcel [data=");

        for(Data singleData : data) {
            if (singleData != data[0]) {
                result.append(", ");
            }

            result.append(singleData.toString());
        }

        result.append("]");

        return result.toString();
    }
}

This might still crash, since you omitted the "Arrays" class you used in the Data class. If you change the type from Arrays to String it should work though.

Also I used the @SerializedName annotation to map the data property to the class property, since class is a keyword in java.

Solution with skipping the root Element ("parcel"):

try(Reader reader = new InputStreamReader(new ByteArrayInputStream(jsonString1.getBytes()), "UTF-8")){
    Gson gson = new GsonBuilder().create();

    JsonElement rootElement = gson.fromJson(reader, JsonObject.class).get("parcel");
    Data[] parcels = gson.fromJson(rootElement, Data[].class);
    for (Data parcel : parcels) {
        System.out.println(parcel);
    }
}

edited May 1, 2018 at 6:43

answered Apr 29, 2018 at 17:28

vstm

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3 Comments

Niras Over a year ago

Thanks @vstm I fixed class to "parcel". I tried your suggestion, sorry a bit confused here. And hence wondering, if I can get away with the two java classes -> Parcel and data and instead have only one. I essentially want to capture only names, email, country and dates and work on that. I was wondering if there is a way to just parse first few characters in the JSON file and work with the content inside {},{}, and so on.

vstm Over a year ago

@Niras: Yes this is possible, you have to browse through the json document and get the array which contains the parcels, then you can just read the elements inside of the array. I have added an edit to the end of my answer to show how you could do that.

Niras Over a year ago

Thank You, I was able to finally get this working. I need to add here, that I used this link - pojo.sodhanalibrary.com to create my Java classes error-free.

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