Convert List of Objects to JSON Using Gson Library in Java

In this article, we will be discussing how to convert a list of objects to JSON using the Gson library in Java.

JSON is an interchangeable format that is used for data exchange. Values in JSON are represented as key-value pairs. To know more about JSON, refer JSON.

Java GSON Library: Converting a list of objects to JSON

Gson is a third-party Java library developed by Google. It is used for converting Java objects to JSON and vice versa. In the Gson library, we can convert a list of objects to JSON using the toJson() method.

We cannot use the Gson library without downloading or adding it to the project. Because it is not a part of the Java standard library. But we can still use the Gson library without downloading it. We can use it from the Maven repository.

Let's see how to add the Gson library to our project:

• First, we need to add the Gson library to our project. If you are using Maven, add this to your pom.xml file:

  <dependency>
      <groupId>com.google.code.gson</groupId>
      <artifactId>gson</artifactId>
      <version>2.8.9</version>
  </dependency>
  

• If you are not using Maven, add a jar file to your project. You can download the jar file from here.

Steps to convert a list of objects to JSON using the toJson() method:

• First, import the Gson library, as discussed above.
• Then, create a list of objects.
• Then we will create an instance of Gson class.
• Next, we will convert the list to JSON using the toJson() method of Gson.
• Finally, we will print the JSON string.

Example

The Following is the code to convert a list of objects to JSON using the Gson library:

import com.google.gson.Gson;
import com.google.gson.GsonBuilder;
import java.util.ArrayList;
import java.util.List;

public class ListToJson {
   public static void main(String[] args) {
      // Create a list of objects
      List<Person> personList = new ArrayList<>();
      personList.add(new Person("Ansh", 23));
      personList.add(new Person("Bam", 17));
      personList.add(new Person("Ace", 22));

      // Create a Gson instance
      Gson gson = new GsonBuilder().setPrettyPrinting().create();

      // Convert the list to JSON
      String json = gson.toJson(personList);

      // Print the JSON string
      System.out.println(json);
   }
}

Following is the output of the above code:

[
  {
    "name": "Ansh",
    "age": 23
  },
  {
    "name": "Bam",
    "age": 17
  },
  {
    "name": "Ace",
    "age": 22
  }
]

Using toJsonTree() method

We can also convert a list of objects to JSON using the toJsonTree() method of Gson.

• First, import the Gson library, as discussed above.
• Then, create a list of objects.
• Then we will create an instance of Gson class.
• Next, we will convert the list to JSON using the toJsonTree() method of Gson.
• Then we will convert the JSON array to a string using the toString() method of JsonArray.
• Finally, we will print the JSON string.

Let's see the code to convert a list of objects to JSON using toJsonTree() method:

import com.google.gson.Gson;
import com.google.gson.JsonArray;
import com.google.gson.JsonElement;
import com.google.gson.JsonParser;
import java.util.ArrayList;
import java.util.List;

public class ListToJsonTree {
   public static void main(String[] args) {
      // Create a list of objects
      List<Person> personList = new ArrayList<>();
      personList.add(new Person("Ansh", 23));
      personList.add(new Person("Bam", 17));
      personList.add(new Person("Ace", 22));

      // Create a Gson instance
      Gson gson = new Gson();

      // Convert the list to JSON array
      JsonElement jsonElement = gson.toJsonTree(personList);
      JsonArray jsonArray = jsonElement.getAsJsonArray();

      // Convert the JSON array to string
      String jsonString = jsonArray.toString();

      // Print the JSON string
      System.out.println(jsonString);
   }
}

Following is the output of the above code:

[
  {
    "name": "Ansh",
    "age": 23
  },
  {
    "name": "Bam",
    "age": 17
  },
  {
    "name": "Ace",
    "age": 22
  }
]