In this example you’ll see how the Gson library handles the object fields. For object fields to be serialized into JSON string it doesn’t need to use any annotations, it can even read private fields. If you have a null value field it will not be serialized into JSON string. To exclude a field from serialization you can mark the field with the transient keyword.

In the snippet below we create a People object. This object has a null value field named age, this will not included in the serialization because we didn’t assign any value to it. The secret field is also not serialized because we mark it with transient keyword.

package org.kodejava.example.google.gson;

import com.google.gson.Gson;

import java.util.Calendar;

public class GsonFieldExample {
    public static void main(String[] args) {
        Calendar dob = Calendar.getInstance();
        dob.set(1980, Calendar.NOVEMBER, 11);
        People people = new People("John", "350 Banana St.", dob.getTime());
        people.setSecret("This is a secret!");

        Gson gson = new Gson();
        String json = gson.toJson(people);
        System.out.println("json = " + json);
    }
}

If your run the code above you’ll see the following line printed:

json = {"name":"John","address":"350 Banana St.","dateOfBirth":"Nov 11, 1980 8:47:04 AM"}

Here is the People class.

package org.kodejava.example.google.gson;

import java.util.Date;

public class People {
    private String name;
    private String address;
    private Date dateOfBirth;
    private Integer age;
    private transient String secret;

    public People(String name, String address, Date dateOfBirth) {
        this.name = name;
        this.address = address;
        this.dateOfBirth = dateOfBirth;
    }

    public String getSecret() {
        return secret;
    }

    public void setSecret(String secret) {
        this.secret = secret;
    }
}

Maven Dependencies

<!-- http://repo1.maven.org/maven2/com/google/code/gson/gson/2.8.0/gson-2.8.0.jar -->
<dependency>
    <groupId>com.google.code.gson</groupId>
    <artifactId>gson</artifactId>
    <version>2.8.0</version>
</dependency>

On the previous example, How do I convert object into JSON? we convert object into JSON string. In this example you will see how to do the opposite, converting JSON string back into object.

To convert JSON string to object use Gson.fromJson() method. This method takes the JSON string and the object type of the JSON string to be converted.

package org.kodejava.example.google.gson;

import com.google.gson.Gson;

public class JSONToStudent {
    public static void main(String[] args) {
        String json = "{" +
                "'name' : 'Duke'," +
                "'address' : 'Menlo Park'," +
                "'dateOfBirth' : 'Feb 1, 2000 12:00:00 AM'" +
                "}";

        Gson gson = new Gson();
        Student student = gson.fromJson(json, Student.class);

        System.out.println("s.getName()        = " + student.getName());
        System.out.println("s.getAddress()     = " + student.getAddress());
        System.out.println("s.getDateOfBirth() = " + student.getDateOfBirth());
    }
}

This example will print the following result:

s.getName()        = Duke
s.getAddress()     = Menlo Park
s.getDateOfBirth() = Tue Feb 01 00:00:00 WITA 2000

You can find the Student class on the previous example, How do I convert object into JSON?.

Maven Dependencies

<!-- http://repo1.maven.org/maven2/com/google/code/gson/gson/2.8.0/gson-2.8.0.jar -->
<dependency>
    <groupId>com.google.code.gson</groupId>
    <artifactId>gson</artifactId>
    <version>2.8.0</version>
</dependency>