In this short tutorial, you'll learn how to convert a JSON string to a Map in Java and vice versa by using the Gson library.

Dependencies

To add Gson to your Gradle project, add the following dependency to build.gradle file:

implementation 'com.google.code.gson:gson:2.8.6'

For Maven, include the below dependency to your pom.xml file:

<dependency>
  <groupId>com.google.code.gson</groupId>
  <artifactId>gson</artifactId>
  <version>2.8.6</version>
</dependency>

Convert JSON String to Map

To convert a JSON string to a Java Map with the same properties and keys, you can use the fromJson() method from Gson:

try {
    // JSON string
    String json = "{\"name\":\"John Doe\",\"email\":\"john.doe@example.com\"," +
            "\"roles\":[\"Member\",\"Admin\"],\"admin\":true}";

    // convert JSON string to Java Map
    Map<String, Object> map = new Gson().fromJson(json, Map.class);

    // print map keys and values
    for (Map.Entry<String, Object> entry : map.entrySet()) {
        System.out.println(entry.getKey() + "=" + entry.getValue());
    }

} catch (Exception ex) {
    ex.printStackTrace();
}

You should see the following output printed on the console:

name=John Doe
email=john.doe@example.com
roles=[Member, Admin]
admin=true

To convert a JSON string stored in a file into a Java map, you can pass an instance of Reader to fromJson(), as shown below:

Map<String, Object> map = new Gson().fromJson(
        new FileReader("user.json"), 
        Map.class);

Convert Map to JSON String

To convert a Java Map to a JSON string, you can use the toJson() method from the Gson class:

try {
    // create a map
    Map<String, Object> map = new HashMap<>();
    map.put("name", "John Deo");
    map.put("email", "john.doe@example.com");
    map.put("roles", new String[]{"Member", "Admin"});
    map.put("admin", true);

    // convert map to JSON string
    String json = new Gson().toJson(map);

    // print JSON string
    System.out.println(json);

} catch (Exception ex) {
    ex.printStackTrace();
}

The above code will produce the following JSON:

{"roles":["Member","Admin"],"name":"John Deo","admin":true,"email":"john.doe@example.com"}

If you want to write the converted Map directly to a JSON file, just pass an instance of Writer to the toJson() method:

try {
    // create a map
    Map<String, Object> map = new HashMap<>();
    map.put("name", "John Deo");
    map.put("email", "john.doe@example.com");
    map.put("roles", new String[]{"Member", "Admin"});
    map.put("admin", true);

    // create a writer
    Writer writer = new FileWriter("user.json");

    // convert map to JSON File
    new Gson().toJson(map, writer);

    // close the writer
    writer.close();
    
} catch (Exception ex) {
    ex.printStackTrace();
}

For more Gson examples, check out the How to read and write JSON using Gson in Java tutorial.

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