How do I convert Map to JSON and vice versa using Jackson?

In the following code snippet we will convert java.util.Map object into JSON string and convert back the JSON string into Java Map object. In this example we will be using the Jackson library.

To convert from Map to JSON string the steps are:

  • Create a map of string keys and values.
  • Create an instance of Jackson ObjectMapper.
  • To convert map to JSON string we call the writeValueAsString() method and pass the map as argument.
// Converting Map to JSON
String json = null;
try {
    json = mapper.writeValueAsString(colours);
    System.out.println("json = " + json);
} catch (JsonProcessingException e) {
    e.printStackTrace();
}

Now, to convert from JSON string back to map we can do it in the following steps:

  • Create a JSON string, in this case we use the one converted from the colours map.
  • Create an instance of Jackson ObjectMapper.
  • Call the mapper’s readValue() method with JSON string and an empty instance of TypeReference as arguments.
// Converting JSON to MAP
try {
    Map<String, String> newColours =
            mapper.readValue(json, new TypeReference<>() {});
} catch (JsonProcessingException e) {
    e.printStackTrace();
}

And here is the complete code snippet.

package org.kodejava.jackson;

import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.core.type.TypeReference;
import com.fasterxml.jackson.databind.ObjectMapper;

import java.util.HashMap;
import java.util.Map;

public class MapToJson {
    public static void main(String[] args) {
        Map<String, String> colours = new HashMap<>();
        colours.put("BLACK", "#000000");
        colours.put("RED", "#FF0000");
        colours.put("GREEN", "#008000");
        colours.put("BLUE", "#0000FF");
        colours.put("YELLOW", "#FFFF00");
        colours.put("WHITE", "#FFFFFF");

        ObjectMapper mapper = new ObjectMapper();

        // Converting Map to JSON
        String json = null;
        try {
            json = mapper.writeValueAsString(colours);
            System.out.println("json = " + json);
        } catch (JsonProcessingException e) {
            e.printStackTrace();
        }

        // Converting JSON to MAP
        try {
            Map<String, String> newColours =
                    mapper.readValue(json, new TypeReference<>() {});
            System.out.println("Map:");
            for (var entry : newColours.entrySet()) {
                System.out.println(entry.getKey() + " = " + entry.getValue());
            }
        } catch (JsonProcessingException e) {
            e.printStackTrace();
        }
    }
}

Running the code snippet above will print the following output:

json = {"RED":"#FF0000","WHITE":"#FFFFFF","BLUE":"#0000FF","BLACK":"#000000","YELLOW":"#FFFF00","GREEN":"#008000"}
Map:
RED = #FF0000
WHITE = #FFFFFF
BLUE = #0000FF
BLACK = #000000
YELLOW = #FFFF00
GREEN = #008000

Maven Dependencies

<dependencies>
    <!-- https://search.maven.org/remotecontent?filepath=com/fasterxml/jackson/core/jackson-core/2.13.3/jackson-core-2.13.3.jar -->
    <dependency>
        <groupId>com.fasterxml.jackson.core</groupId>
        <artifactId>jackson-core</artifactId>
        <version>2.13.3</version>
    </dependency>
    <!-- https://search.maven.org/remotecontent?filepath=com/fasterxml/jackson/core/jackson-databind/2.13.3/jackson-databind-2.13.3.jar -->
    <dependency>
        <groupId>com.fasterxml.jackson.core</groupId>
        <artifactId>jackson-databind</artifactId>
        <version>2.13.3</version>
    </dependency>
</dependencies>

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Jackson for Java. Is it more than JSON?

JSON has been a popular data-interchange format for quite some time now. It is not just simple, but also lightweight, and most programmers find it easy to use. However, JSON can be cumbersome to work with when you need more complex functionality.

That’s where Jackson for Java comes in. Jackson is a powerful JSON library that provides a wide range of features that make working with JSON much easier. In this blog post, we will discuss what Jackson for Java is, how it differs from regular JSON, and how you can use it in your own projects. We will also take a look at the pros and cons of using Jackson for Java so that you can decide if it is the right library for you.

What is Jackson for Java, and what are its Features?

Jackson is a Java library that provides a number of features that make working with JSON much easier. Some of the most notable features of Jackson for Java include:

  • The ability to annotate fields so that they are mapped to specific JSON keys: With Jackson, you can annotate fields in your Java objects so that they are mapped to specific keys in the JSON document. This makes it much easier to work with complex JSON documents. For example, if you have a field in your Java object that is mapped to a “name” key in the JSON document, you can access that field using the @JsonProperty("name") annotation.

  • Support for POJOs (Plain Old Java Objects) and JAXB beans (Java Architecture for XML Binding): Jackson supports both POJOs and JAXB beans. This means that you can serialize and deserialize objects without writing any boilerplate code.

  • A wide range of modules that provide additional functionality: Jackson comes with a number of modules that provide additional functionality. These modules include support for XML, YAML, and CSV formats.

In addition to these features, Jackson also has excellent performance thanks to its use of streaming data processing. This means that it can handle large amounts of data with ease.

JSON vs. Jackson for Java – what’s the difference?

The main difference between JSON and Jackson for Java is that Jackson is a library that provides additional functionality on top of JSON. This includes features such as the ability to annotate fields, support for POJOs and JAXB beans, and the ability to serialize and deserialize objects without writing any boilerplate code.

So, if you need additional functionality beyond what JSON provides, then Jackson is the library for you. However, if you only need the basic functionality that JSON provides, then JSON is a better choice.

How to use Jackson for Java in your Project?

If you want to use Jackson for Java in your project, the first step is to add the library to your project dependencies.

<dependency>
    <groupId>com.fasterxml.jackson.core</groupId>
    <artifactId>jackson-databind</artifactId>
    <version>2.9.8</version>
</dependency>

The easiest way to do this is using a dependency management tool such as Maven or Gradle. Once you have added the Jackson library to your project, you can start using it in your code. For example, if you have a field in your Java object that is mapped to a “name” key in the JSON document, you can access that field using the @JsonProperty("name") annotation. Here is an example of how you can convert List object to JSON. Here, we’ll be using the ObjectMapper.writeValueAsString() method.

package net.javaguides.jackson;

import java.util.ArrayList;
import java.util.List;

import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.SerializationFeature;

/**
* Using Jackson API for list serialization and deserialization
* @author ramesh fadatare
*
*/
public class JacksonListToJson {
    public static void main(String[] args) throws JsonProcessingException {

        // Create ObjectMapper object.
        ObjectMapper mapper = new ObjectMapper();
        mapper.enable(SerializationFeature.INDENT_OUTPUT);

        List < String > progLangs = new ArrayList < > ();
        progLangs.add("C");
        progLangs.add("C++");
        progLangs.add("Java");
        progLangs.add("Java EE");
        progLangs.add("Python");
        progLangs.add("Scala");
        progLangs.add("JavaScript");
        // Serialize Object to JSON.
        String json = mapper.writeValueAsString(progLangs);

        // Print json
        System.out.println(json);
    }
}

You can also use Jackson to serialize and deserialize objects without writing any boilerplate code. This is because Jackson automatically generates the necessary code for you. To do this, you simply need to add the @JsonSerialize and @JsonDeserialize annotations to your Java objects.

If you have some experience on Jackson for Java, you may want to include it in your resume. Now, a resume for a programmer should list programming languages, software, and tools the individual is proficient in. Additionally, project experience and technical skills should be highlighted. A résumé is essentially your career booster so you will need to ensure that it also showcases all your skills and experience. Take time to include every essential detail.

Pros and Cons of Using Jackson for Java

Jackson is a very powerful library that can make working with JSON much easier. There are pros and cons to using Jackson for Java. The main pro is that it’s a very fast and lightweight library, which makes it ideal for large-scale projects. It can also serialize and deserialize Java objects quickly.

However, one potential con is that it can be difficult to reverse certain operations performed by the library, such as converting back from JSON to Java. For example, if you have a Java object with a list of Cat objects, and you want to convert it back to JSON, reversing the process might not be as straightforward as you’d like. In cases like this, you may need to use a different library or write your own code to handle the conversion.

Another potential downside of using Jackson is that because it’s so popular, there may be less flexibility in how you use it. For example, if you want to use Jackson for XML parsing, you may need to use a third-party library such as JAXB.

Overall, Jackson is a very powerful library that can make working with JSON much easier. It has a few potential downsides, but its pros far outweigh its cons.

Comparison of Other Popular JSON Libraries

There are many different JSON libraries available for Java. Some of the most popular include Gson, org.json, and FastJSON. Gson is a library that can be used for converting between Java objects and JSON documents. It can also be used for serializing and deserializing objects. Gson has excellent performance thanks to its use of streaming data processing.

Org.json is a library that provides JSON parsing and generation in Java. It’s simple and easy to use, but it doesn’t provide as much functionality as some other libraries on this list. Here is an example of how to parse JSON string in Java with org.json library.

import org.json.*;

String jsonString = ... ; //assign your JSON String here
JSONObject obj = new JSONObject(jsonString);
String pageName = obj.getJSONObject("pageInfo").getString("pageName");

JSONArray arr = obj.getJSONArray("posts"); // notice that `"posts": [...]`
for (int i = 0; i < arr.length(); i++) {
    String post_id = arr.getJSONObject(i).getString("post_id");
    ......
}

See more examples on this page. FastJSON is a high-performance JSON library for Java. It’s simple to use and provides a wide range of features.

Wrapping Up

Overall, Jackson is the best choice for working with JSON in Java, thanks to its excellent performance and wide range of features. However, there are other great options available, so be sure to choose the library that’s right for your project.

How do I convert CSV to JSON string using Jackson?

In the following code snippet we will convert CSV into JSON string using Jackson JSON library. A comma-separated values is a delimited text, it uses comma to separate values. It starts with header on the first line, that will be the JSON key. Each subsequence lines is the data of the csv, which also contains several values separated by comma.

Let’s see the code how to do this in Jackson.

package org.kodejava.jackson;

import com.fasterxml.jackson.databind.MappingIterator;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.dataformat.csv.CsvMapper;
import com.fasterxml.jackson.dataformat.csv.CsvSchema;

import java.io.IOException;
import java.util.List;
import java.util.Map;

public class CsvToJson {
    public static void main(String[] args) {
        // Comma delimited text created using text blocks
        String countries = """
                ISO, CODE, NAME\s
                CZE, CZ, Czech Republic\s
                DNK, DK, Denmark\s
                DJI, DJ, Djibouti\s
                DMA, DM, Dominica\s
                ECU, EC, Ecuador
                """;

        CsvSchema csvSchema = CsvSchema.emptySchema().withHeader();
        CsvMapper csvMapper = new CsvMapper();

        try {
            List<Map<?, ?>> list;
            try (MappingIterator<Map<?, ?>> mappingIterator = csvMapper.reader()
                    .forType(Map.class)
                    .with(csvSchema)
                    .readValues(countries)) {
                list = mappingIterator.readAll();
            }

            ObjectMapper objectMapper = new ObjectMapper();
            String jsonPretty = objectMapper.writerWithDefaultPrettyPrinter()
                    .writeValueAsString(list);
            System.out.println(jsonPretty);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

Here are the explanation of the code above:

  • Define a csv string, in this case we have a list of countries.
  • Create an empty schema of CsvSchema to process csv with header line.
  • Create an instance of CsvMapper, a specialized type of ObjectMapper.
  • Read and parse csv values into List<Map<?, ?>>.
  • We use the ObjectMapper create a pretty-printed JSON from the list object.

Running the code produces the following output:

[ {
  "ISO" : "CZE",
  "CODE" : " CZ",
  "NAME" : " Czech Republic "
}, {
  "ISO" : "DNK",
  "CODE" : " DK",
  "NAME" : " Denmark "
}, {
  "ISO" : "DJI",
  "CODE" : " DJ",
  "NAME" : " Djibouti "
}, {
  "ISO" : "DMA",
  "CODE" : " DM",
  "NAME" : " Dominica "
}, {
  "ISO" : "ECU",
  "CODE" : " EC",
  "NAME" : " Ecuador"
} ]

Maven Dependencies

<dependencies>
    <!-- https://search.maven.org/remotecontent?filepath=com/fasterxml/jackson/core/jackson-core/2.13.3/jackson-core-2.13.3.jar -->
    <dependency>
        <groupId>com.fasterxml.jackson.core</groupId>
        <artifactId>jackson-core</artifactId>
        <version>2.13.3</version>
    </dependency>
    <!-- https://search.maven.org/remotecontent?filepath=com/fasterxml/jackson/core/jackson-annotations/2.13.3/jackson-annotations-2.13.3.jar -->
    <dependency>
        <groupId>com.fasterxml.jackson.core</groupId>
        <artifactId>jackson-annotations</artifactId>
        <version>2.13.3</version>
    </dependency>
    <!-- https://search.maven.org/remotecontent?filepath=com/fasterxml/jackson/core/jackson-databind/2.13.3/jackson-databind-2.13.3.jar -->
    <dependency>
        <groupId>com.fasterxml.jackson.core</groupId>
        <artifactId>jackson-databind</artifactId>
        <version>2.13.3</version>
    </dependency>
    <!-- https://search.maven.org/remotecontent?filepath=com/fasterxml/jackson/dataformat/jackson-dataformat-csv/2.13.3/jackson-dataformat-csv-2.13.3.jar -->
    <dependency>
        <groupId>com.fasterxml.jackson.dataformat</groupId>
        <artifactId>jackson-dataformat-csv</artifactId>
        <version>2.13.3</version>
    </dependency>    
</dependencies>

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How do I convert CSV file to or from JSON file?

In the following code snippet you will see how to convert a CSV file into JSON file and vice versa. We use the JSON-Java library CDL class to convert between CSV and JSON format. The CDL class provide the toJSONArray(String) and toString(JSONArray) methods that allows us to do the conversion between data format.

In the CSV file, the first line in the file will be use as the keys to the generated JSON string. On the other way around, the JSON string keys will be written on the first line of the CSV file as the column header.

Convert CSV file to JSON file.

package org.kodejava.json;

import org.json.CDL;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.nio.charset.StandardCharsets;
import java.nio.file.Files;
import java.nio.file.Path;
import java.util.Objects;
import java.util.stream.Collectors;

public class CsvFileToJsonFile {
    public static void main(String[] args) {
        // Read csv data file and store it in a string
        InputStream is = CsvFileToJsonFile.class.getResourceAsStream("/data.csv");
        String csv = new BufferedReader(
                new InputStreamReader(Objects.requireNonNull(is), StandardCharsets.UTF_8))
                .lines()
                .collect(Collectors.joining("\n"));

        try {
            // Convert csv text to JSON string, and save it 
            // to a data.json file.
            String json = CDL.toJSONArray(csv).toString(2);
            Files.write(Path.of("data.json"), json.getBytes());
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

What we do in the snippet above:

  • Get cvs data as InputStream from the resources directory.
  • We use the BufferedReader and InputStreamReader to iterate and read the InputStream and return it as a string.
  • Convert the csv string into JSON string using CDL.toJSONArray().
  • We can pretty-printed the JSON string by specifying an indentFactor to the toString() method of the JSONArray object.
  • Write the JSON string to a file.

Here is the data.csv file example.

id,first_name,last_name,email,gender,ip_address
1,Abe,Foord,afoord0@harvard.edu,Female,81.38.18.88
2,Editha,Castagnaro,ecastagnaro1@nih.gov,Genderqueer,181.63.39.199
3,Tildie,Furminger,tfurminger2@hud.gov,Male,0.199.18.3

Convert JSON file to CSV file.

package org.kodejava.json;

import org.json.CDL;
import org.json.JSONArray;
import org.json.JSONTokener;

import java.io.InputStream;
import java.nio.file.Files;
import java.nio.file.Path;
import java.util.Objects;

public class JsonFileToCsvFile {
    public static void main(String[] args) {
        // Get data.json resource as InputStream, create JSONTokener
        // and convert the tokener into JSONArray object.
        InputStream is = JsonFileToCsvFile.class.getResourceAsStream("/data.json");
        JSONTokener tokener = new JSONTokener(Objects.requireNonNull(is));
        JSONArray jsonArray = new JSONArray(tokener);

        try {
            // Convert JSONArray into csv and save to file
            String csv = CDL.toString(jsonArray);
            Files.write(Path.of("data.csv"), csv.getBytes());
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

What we do in the code snippet above:

  • Get data.json from the resources directory as InputStream.
  • Create a JSONTokener and provide the InputStream as argument to its constructor.
  • Create a JSONArray and pass the JSONTokener object as the constructor argument.
  • Using CDL.toString() we convert the JSONArray object to csv text.
  • Finally, save the csv into file using Files.write().

And here is the data.json JSON file example.

[
  {
    "id": "1",
    "first_name": "Abe",
    "last_name": "Foord",
    "email": "afoord0@harvard.edu",
    "gender": "Female",
    "ip_address": "81.38.18.88"
  },
  {
    "id": "2",
    "first_name": "Editha",
    "last_name": "Castagnaro",
    "email": "ecastagnaro1@nih.gov",
    "gender": "Genderqueer",
    "ip_address": "181.63.39.199"
  },
  {
    "id": "3",
    "first_name": "Tildie",
    "last_name": "Furminger",
    "email": "tfurminger2@hud.gov",
    "gender": "Male",
    "ip_address": "0.199.18.3"
  }
]

Maven Dependencies

<dependencies>
    <!-- https://search.maven.org/remotecontent?filepath=org/json/json/20220320/json-20220320.jar -->
    <dependency>
        <groupId>org.json</groupId>
        <artifactId>json</artifactId>
        <version>20220320</version>
        <type>bundle</type>
    </dependency>
</dependencies>

Maven Central

How do I convert CSV into JSON string using JSON-Java?

In this example we convert CSV or CDL into JSON string. We are going to use the JSON-Java CDL (Comma Delimited Text) class. This class provides static methods that will convert a CSV into JSONArray or to convert a JSONArray into comma separated values.

Here are what we do in the code snippet below:

  • Create a comma delimited text. The first line is the headers, this will be the keys in our JSON string. The couples lines is the values. We use the Java text blocks feature to define the string.
  • Create JSONArray object by calling CDL.toJSONArray() static method as pass the comma delimited string as argument.
  • Next we create a JSONArray object, but we separate the header and the body. We do this by calling the CDL.toJSONArray() and provides headers and countries as arguments.
  • Last, we call the CDL.toString() method with JSONArray as argument to convert it to comma delimited text.

Let’s see the code in action.

package org.kodejava.json;

import org.json.CDL;
import org.json.JSONArray;

public class CsvToJson {
    public static void main(String[] args) {
        // Comma delimited text created using text blocks
        String countries = """
                ISO, CODE, NAME\s
                CZE, CZ, CZECH REPUBLIC\s
                DNK, DK, DENMARK\s
                DJI, DJ, DJIBOUTI\s
                DMA, DM, DOMINICA\s
                ECU, EC, ECUADOR
                """;

        // Convert comma delimited text into JSONArray object.
        JSONArray jsonCountries = CDL.toJSONArray(countries);
        System.out.println(jsonCountries.toString(2));

        // Using a separate header and values to create JSONArray
        // from a comma delimited text
        JSONArray header = new JSONArray();
        header.put("ISO");
        header.put("CODE");
        header.put("NAME");

        countries = """
                CZE, CZ, CZECH REPUBLIC\s
                DNK, DK, DENMARK\s
                DJI, DJ, DJIBOUTI\s
                DMA, DM, DOMINICA\s
                ECU, EC, ECUADOR
                """;

        jsonCountries = CDL.toJSONArray(header, countries);
        System.out.println(jsonCountries.toString(2));

        // Convert back from JSONArray to comma delimited text
        countries = CDL.toString(jsonCountries);
        System.out.println(countries);
    }
}

Running the code produces the following results.

To JSON string:

[
  {
    "ISO": "CZE",
    "CODE": "CZ",
    "NAME": "CZECH REPUBLIC"
  },
  {
    "ISO": "DNK",
    "CODE": "DK",
    "NAME": "DENMARK"
  },
  {
    "ISO": "DJI",
    "CODE": "DJ",
    "NAME": "DJIBOUTI"
  },
  {
    "ISO": "DMA",
    "CODE": "DM",
    "NAME": "DOMINICA"
  },
  {
    "ISO": "ECU",
    "CODE": "EC",
    "NAME": "ECUADOR"
  }
]

Back to CSV

ISO,CODE,NAME
CZE,CZ,CZECH REPUBLIC
DNK,DK,DENMARK
DJI,DJ,DJIBOUTI
DMA,DM,DOMINICA
ECU,EC,ECUADOR

Maven Dependencies

<dependencies>
    <!-- https://search.maven.org/remotecontent?filepath=org/json/json/20220320/json-20220320.jar -->
    <dependency>
        <groupId>org.json</groupId>
        <artifactId>json</artifactId>
        <version>20220320</version>
        <type>bundle</type>
    </dependency>
</dependencies>

Maven Central

How do I pretty-print JSON string in Google Gson?

In the following example you’ll see how to format JSON string using the Google Gson library. Here are the steps:

  • We create a Map.
  • Put a couple key-value pairs to it. We put a string, a LocalDate object and an array of String[].
  • Create a Gson object using the GsonBuilder. This allows us to configure the Gson object.
  • We use the setPrettyPrinting() to configure Gson to output pretty print.
  • The registerTypeAdapter() allows us to register custom serializer, in this case we use it to serialize LocalDate object.

Here is our code snippet:

package org.kodejava.gson;

import com.google.gson.Gson;
import com.google.gson.GsonBuilder;
import com.google.gson.JsonElement;
import com.google.gson.JsonPrimitive;
import com.google.gson.JsonSerializationContext;
import com.google.gson.JsonSerializer;

import java.lang.reflect.Type;
import java.time.LocalDate;
import java.time.Month;
import java.time.format.DateTimeFormatter;
import java.util.HashMap;
import java.util.Map;

public class GsonPrettyPrint {
    public static void main(String[] args) {
        Map<String, Object> map = new HashMap<>();
        map.put("name", "Duke");
        map.put("address", "Menlo Park");
        map.put("dateOfBirth", LocalDate.of(2000, Month.FEBRUARY, 1));
        map.put("languages", new String[]{"Java", "Kotlin", "JavaScript"});

        Gson gson = new GsonBuilder()
                .setPrettyPrinting()
                .registerTypeAdapter(LocalDate.class, new LocaleDateAdapter())
                .create();
        String json = gson.toJson(map);
        System.out.println(json);
    }

    static class LocaleDateAdapter implements JsonSerializer<LocalDate> {
        @Override
        public JsonElement serialize(LocalDate date, Type type, JsonSerializationContext jsonSerializationContext) {
            return new JsonPrimitive(date.format(DateTimeFormatter.ISO_DATE));
        }
    }
}

Running this code produces the following result:

{
  "address": "Menlo Park",
  "languages": [
    "Java",
    "Kotlin",
    "JavaScript"
  ],
  "name": "Duke",
  "dateOfBirth": "2000-02-01"
}

Maven Dependencies

<!-- https://repo1.maven.org/maven2/com/google/code/gson/gson/2.9.0/gson-2.9.0.jar -->
<dependency>
    <groupId>com.google.code.gson</groupId>
    <artifactId>gson</artifactId>
    <version>2.9.0</version>
</dependency>

Maven Central

How do I pretty print JSON string in JSON-Java?

In JSON-Java we can make a pretty-printed JSON string by specifying an indentFactor to the JSONObject‘s toString() method. The indent factor is the number of spaces to add to each level of indentation.

If indentFactor > 0 and the JSONObject has only one key, the JSON string will be printed a single line, and if the JSONObject has 2 or more keys, the JSON string will be printed in multiple lines.

Let’s create a pretty-printed JSONObject text using the code below.

package org.kodejava.json;

import org.json.JSONArray;
import org.json.JSONObject;

public class PrettyPrintJSON {
    public static void main(String[] args) {
        JSONObject jsonObject = new JSONObject();
        jsonObject.put("id", 1L);
        jsonObject.put("name", "Alice");
        jsonObject.put("age", 20);
        JSONArray courses = new JSONArray(
                new String[]{"Engineering", "Finance"});
        jsonObject.put("courses", courses);

        // Default print without indent factor
        System.out.println(jsonObject);

        // Pretty print with 2 indent factor
        System.out.println(jsonObject.toString(2));
    }
}

Running this code produces the following output:

{"courses":["Engineering","Finance"],"name":"Alice","id":1,"age":20}
{
  "courses": [
    "Engineering",
    "Finance"
  ],
  "name": "Alice",
  "id": 1,
  "age": 20
}

Maven Dependencies

<dependencies>
    <!-- https://search.maven.org/remotecontent?filepath=org/json/json/20220320/json-20220320.jar -->
    <dependency>
        <groupId>org.json</groupId>
        <artifactId>json</artifactId>
        <version>20220320</version>
        <type>bundle</type>
    </dependency>
</dependencies>

Maven Central

How do I create JSONArray object?

A JSONArray object represent an ordered sequence of values. We use the put() method to add or replace values in the JSONArray object. The value can be of the following types: Boolean, JSONArray, JSONObject, Number, String or the JSONObject.NULL object.

Beside using the put() method we can also create and add data into JSONArray in the following ways:

  • Using constructor to create JSONArray from string wrapped in square bracket [], where the values are separated by comma. JSONException maybe thrown if the string is not a valid JSON string.
  • Creating JSONArray by passing an array as an argument to its constructor, for example an array of String[].
  • Using constructor to create JSONArray from a Collection, such as an ArrayList object.

The following example show you how to create JSONArray object.

package org.kodejava.json;

import org.json.JSONArray;
import org.json.JSONObject;

import java.util.ArrayList;
import java.util.List;

public class CreateJSONArray {
    public static void main(String[] args) {
        // Create JSONArray and put some values
        JSONArray jsonArray = new JSONArray();
        jsonArray.put("Java");
        jsonArray.put("Kotlin");
        jsonArray.put("Go");
        System.out.println(jsonArray);

        // Create JSONArray from a string wrapped in bracket and
        // the values separated by comma. String value can be 
        // quoted with single quote.
        JSONArray colorArray = new JSONArray("[1, 'Apple', 2022-02-07]");
        System.out.println(colorArray);

        // Create JSONArray from an array of String.
        JSONArray stringArray =
                new JSONArray(new String[]{"January", "February", "March"});
        System.out.println(stringArray);

        // Create JSONArray by passing a List to its constructor
        List<String> list = new ArrayList<>();
        list.add("Red");
        list.add("Green");
        list.add("Blue");
        JSONArray listArray = new JSONArray(list);
        System.out.println(listArray);

        // Using for loop to get each value from the array
        for (int i = 0; i < listArray.length(); i++) {
            System.out.println("Color: " + listArray.get(i));
        }

        // Put JSONArray into a JSONObject.
        JSONObject jsonObject = new JSONObject();
        jsonObject.put("colors", listArray);
        System.out.println(jsonObject);
    }
}

Running the code above produces the following result:

["Java","Kotlin","Go"]
[1,"Apple","2022-02-07"]
["January","February","March"]
["Red","Green","Blue"]
Color: Red
Color: Green
Color: Blue
{"colors":["Red","Green","Blue"]}

Maven Dependencies

<dependencies>
    <!-- https://search.maven.org/remotecontent?filepath=org/json/json/20220320/json-20220320.jar -->
    <dependency>
        <groupId>org.json</groupId>
        <artifactId>json</artifactId>
        <version>20220320</version>
        <type>bundle</type>
    </dependency>
</dependencies>

Maven Central

How do I read values from JSONObject?

A JSONObject is an unordered collection of key-value pairs. To read values from the JSONObject we can use the get(String key) and opt(String key) methods. These generic methods return an Object, which you can cast to a certain type.

There are also typed get and opt methods to read value in specific type such as getString(), getInt(), getDouble(), optString(), optFloat(), optBigInteger(), optBoolean(), optEnum(), etc. For more detail, you can check the JSONObject API documentation.

The get methods throws JSONException when the key is not found in the JSONObject, while the opt methods does not throw exception but return null, and we can also pass a default value argument that will be returned when the key is not found.

The code below give you a simple example to read values from JSONObject.

package org.kodejava.json;

import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;

public class ReadJSONValue {
    public static void main(String[] args) {
        JSONObject jsonObject = new JSONObject();
        jsonObject.put("id", 1L);
        jsonObject.put("name", "Alice");
        jsonObject.put("age", 20);
        jsonObject.put("courses",
                new JSONArray(new String[] {"Engineering", "Finance"}));
        System.out.println(jsonObject);

        // Using get() and opt() methods we need to cast
        // the returned value to the type its store.
        long id1 = (long) jsonObject.get("id");
        String name1 = (String) jsonObject.get("name");
        int age1 = (int) jsonObject.get("age");
        JSONArray courses1 = (JSONArray) jsonObject.get("courses");

        // This will throw exception because JSONObject
        // does not have the address key in it.
        String address1;
        try {
            address1 = (String) jsonObject.get("address");
        } catch (JSONException e) {
            e.printStackTrace();
        }

        // Using opt() method to read address, does not throw
        // exception
        address1 = (String) jsonObject.opt("address");

        System.out.println("id1      = " + id1);
        System.out.println("name1    = " + name1);
        System.out.println("age1     = " + age1);
        System.out.println("address1 = " + address1);
        System.out.println("courses1 = " + courses1);
        System.out.println();

        // Using data type specific get() and opt() methods.
        // We don't have to cast the return from the getXXX()
        // and optXXX() methods.
        long id2 = jsonObject.getLong("id");
        String name2 = jsonObject.getString("name");
        int age2 = jsonObject.optInt("age");

        // Using optString() to read address and provide default
        // value when address is not found.
        String address2 = jsonObject.optString("address", "No Address");
        JSONArray courses2 = jsonObject.optJSONArray("courses");

        System.out.println("id2      = " + id2);
        System.out.println("name2    = " + name2);
        System.out.println("age2     = " + age2);
        System.out.println("address2 = " + address2);
        System.out.println("courses2 = " + courses2);
    }
}

Running this code produces the following results:

{"courses":["Engineering","Finance"],"name":"Alice","id":1,"age":20}
org.json.JSONException: JSONObject["address"] not found.
    at org.json.JSONObject.get(JSONObject.java:580)
    at org.kodejava.json.ReadJSONValue.main(ReadJSONValue.java:28)

id1      = 1
name1    = Alice
age1     = 20
address1 = null
courses1 = ["Engineering","Finance"]

id2      = 1
name2    = Alice
age2     = 20
address2 = No Address
courses2 = ["Engineering","Finance"]

Maven Dependencies

<dependencies>
    <!-- https://search.maven.org/remotecontent?filepath=org/json/json/20220320/json-20220320.jar -->
    <dependency>
        <groupId>org.json</groupId>
        <artifactId>json</artifactId>
        <version>20220320</version>
        <type>bundle</type>
    </dependency>
</dependencies>

Maven Central

How do I convert Java Object to JSON?

To convert Java objects or POJOs (Plain Old Java Objects) to JSON we can use one of JSONObject constructor that takes an object as its argument. In the following example we will convert Student POJO into JSON string. Student class must provide the getter methods, JSONObject creates JSON string by calling these methods.

In this code snippet we do as follows:

  • Creates Student object and set its properties using the setter methods.
  • Create JSONObject called object and use the Student object as argument to its constructor.
  • JSONObject use getter methods to produces JSON string.
  • Call object.toString() method to get the JSON string.
package org.kodejava.json;

import org.json.JSONObject;
import org.kodejava.json.support.Student;

import java.util.Arrays;

public class PojoToJSON {
    public static void main(String[] args) {
        Student student = new Student();
        student.setId(1L);
        student.setName("Alice");
        student.setAge(20);
        student.setCourses(Arrays.asList("Engineering", "Finance", "Chemistry"));

        JSONObject object = new JSONObject(student);
        String json = object.toString();
        System.out.println(json);
    }
}

Running this code produces the following result:

{"courses":["Engineering","Finance","Chemistry"],"name":"Alice","id":1,"age":20}

The Student class use in the code above:

package org.kodejava.json.support;

import java.util.List;

public class Student {
    private Long id;
    private String name;
    private int age;
    private List<String> courses;

    // Getters and Setters removed for simplicity
}

Maven Dependencies

<dependencies>
    <!-- https://search.maven.org/remotecontent?filepath=org/json/json/20220320/json-20220320.jar -->
    <dependency>
        <groupId>org.json</groupId>
        <artifactId>json</artifactId>
        <version>20220320</version>
        <type>bundle</type>
    </dependency>
</dependencies>

Maven Central