Gson – How to parse JSON Arrays, an array of arrays

https://mkyong.com/java/gson-how-to-parse-json-arrays-an-array-of-arrays/

In this article, we will show few JSON array examples and also how to use Gson to map it back to Java Object.

pom.xml

<!-- Gson JSON -> Object -->
  <dependency>
      <groupId>com.google.code.gson</groupId>
      <artifactId>gson</artifactId>
      <version>2.8.6</version>
  </dependency>

JSON basic.

• [ ] = array
• { } = object

1. JSON array of object.

1.1 Sample.

array1.json

[
  {
    "id": 1, "name": "a"
  },
  {
    "id": 2, "name": "b"
  }
]

1.2 Gson convert above JSON array into a List<Item>.

Item.java

package com.mkyong;

public class Item {

    private int id;
    private String name;

}

JsonArray1.java

package com.mkyong;

import com.google.gson.Gson;
import com.google.gson.reflect.TypeToken;
import org.apache.commons.io.IOUtils;

import java.io.IOException;
import java.io.InputStream;
import java.lang.reflect.Type;
import java.nio.charset.StandardCharsets;
import java.util.List;

public class JsonArray1 {

    public static void main(String[] args) throws IOException {

        JavaParseJsonArray main = new JavaParseJsonArray();

        Gson gson = new Gson();

        Type listType = new TypeToken<List<Item>>() {}.getType();

        List<Item> list = gson.fromJson(main.loadFileFromClasspath("array1.json"), listType);

        System.out.println(gson.toJson(list));

    }

    public String loadFileFromClasspath(String fileName) throws IOException {
        ClassLoader classLoader = getClass().getClassLoader();
        try (InputStream inputStream = classLoader.getResourceAsStream(fileName)) {
            // common-io
            return IOUtils.toString(inputStream, StandardCharsets.UTF_8);
        }
    }

}

P.S We use common-io to convert inputStream to String.

pom.xml

<dependency>
    <groupId>commons-io</groupId>
    <artifactId>commons-io</artifactId>
    <version>2.6</version>
</dependency>

2. JSON array of {object with an array of object}.

2.1 Sample.

array2.json

[
  {
    "id": 1,
    "name": "a",
    "types":
    [
      {"id": 1,"name": "a1"},
      {"id": 2,"name": "a2"}
    ]
  },
  {
    "id": 2,
    "name": "b",
    "types":
    [
      {"id": 1,"name": "b1"},
      {"id": 2,"name": "b2"}
    ]
  }
]

2.2 Gson convert above JSON array into a List<Item> containing List<ItemType> types.

Item.java

package com.mkyong;

import java.util.List;

public class Item {

    private int id;
    private String name;
    private List<ItemType> types;

}

ItemType.java

package com.mkyong;

public class ItemType {

    private int id;
    private String name;

}

2.3 Same code, no need to change.

Type listType = new TypeToken<List<Item>>() {}.getType();
  List<Item> list = gson.fromJson(main.loadFileFromClasspath("array2.json"), listType);

3. JSON array of {object with an array of an array of object}.

3.1 Sample.

array3.json

[
  {
    "id": 1,
    "name": "a",
    "types":
    [
      [
        {"id": 1,"name": "a1"},
        {"id": 2,"name": "a2"}
      ],
      [
        {"id": 3,"name": "a3"}
      ]
    ]
  },
  {
    "id": 2,
    "name": "b",
    "types":
    [
      [
        {"id": 1,"name": "b1"}
      ],
      [
        {"id": 2,"name": "b2"}
      ]
    ]
  }
]

3.2 Change the types to List<ItemType> types[];

Item.java

package com.mkyong;

import java.util.List;

public class Item {

    private int id;
    private String name;
    private List<ItemType> types[]; // change types -> types[]

}

ItemType.java

package com.mkyong;

public class ItemType {

    private int id;
    private String name;

}

3.3 Same code, no need to change.

Type listType = new TypeToken<List<Item>>() {}.getType();
  List<Item> list = gson.fromJson(main.loadFileFromClasspath("array3.json"), listType);

3.4 See the debugging mode.

4. JSON array of {object with an array of an array of String}.

4.1 This is a bit unusual JSON array sample.

array4.json

[
  {
    "id": 1,
    "name": "a",
    "types":
    [
      [
        "a1", 1
      ],
      [
        "a2", 2
      ]
    ]
  },
  {
    "id": 2,
    "name": "b",
    "types":
    [
      [
        "b1", 1
      ]
    ]
  }
]

4.2 Change the types to List<String> types[];

Item.java

package com.mkyong;

import java.util.List;

public class Item {

    private int id;
    private String name;
    private List<String> types[];

}

4.3 Same code, no need to change.

Type listType = new TypeToken<List<Item>>() {}.getType();
  List<Item> list = gson.fromJson(main.loadFileFromClasspath("array4.json"), listType);

References

• Wikipedia – JSON
• Gson – How to parse JSON
• Java JSON Tutorials

array gson json json array json parser