Methods of Gson in Java

In Java, Gson is defined as a library which was developed by the Google for serialization and deserialization of JSON (JavaScript Object Notation). The main purpose of Gson in Java is converting objects of Java into formats of JSON and vice versa.

Let's understand in detail about Gson and its methods in Java.

Gson in Java

Gson is considered as one of the powerful library that was created by the Google for the main purpose of maintenance serialization and deserialization of JSON. The process of conversion of objects of Java into the formats of JSON and vice versa are made simple with the help of Gson. This is the reason it is a popular option for users when dealing with data related to JSON in the applications of Java.

The main class in Gson can be labelled as the "Gson" class. This class is responsible for providing methods which can be utilized by users for serialization and deserialization. Let us understand about about Gson in Java in detail with the help of a Java example program.

Example 1:

Filename: GsonClassExample.java

import com.google.gson.Gson;
import com.google.gson.JsonElement;
import com.google.gson.JsonObject;
class Person {
    private String name;
    private int age;
    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    }
    @Override
    public String toString() {
        return "Person{name='" + name + "', age=" + age + '}';
    }
}
public class GsonClassExample {
    public static void main(String[] args) {
        // Creating a Gson object
        Gson gson = new Gson();
        // Serialization: Object to JSON
        Person person = new Person("John Doe", 25);
        String jsonPerson = gson.toJson(person);
        System.out.println("Serialized Person: " + jsonPerson);
        // Deserialization: JSON to Object
        String jsonStudent = "{ \"name\": \"Alice\", \"age\": 22 }";
        Person student = gson.fromJson(jsonStudent, Person.class);
        System.out.println("Deserialized Person: " + student);
        // Working with JsonElement
        JsonElement jsonElement = gson.toJsonTree(person);
        JsonObject jsonObject = jsonElement.getAsJsonObject();
        jsonObject.addProperty("additionalInfo", "Some extra data");
        System.out.println("Modified JsonObject: " + jsonObject);
        // Working with JsonObject directly
        JsonObject customObject = new JsonObject();
        customObject.addProperty("customField", "Custom Value");
        customObject.add("nestedObject", jsonObject);
        System.out.println("Custom JsonObject: " + customObject);
        // Complex example: Object with nested JsonObjects
        JsonObject complexObject = new JsonObject();
        complexObject.addProperty("name", "Complex Object");
        complexObject.add("nestedPerson", gson.toJsonTree(person));
        complexObject.addProperty("flag", true);
        String complexJson = gson.toJson(complexObject);
        System.out.println("Complex Json: " + complexJson);
        // Deserialization of complex JSON
        JsonObject deserializedComplexObject = gson.fromJson(complexJson, JsonObject.class);
        String nestedPersonJson = deserializedComplexObject.getAsJsonObject("nestedPerson").toString();
        Person deserializedPerson = gson.fromJson(nestedPersonJson, Person.class);
        System.out.println("Deserialized Complex Object: " + deserializedComplexObject);
        System.out.println("Deserialized Nested Person: " + deserializedPerson);
    }
}

Example 2:

Filename: GsonClass.java

import com.google.gson.Gson;
import com.google.gson.GsonBuilder;
import com.google.gson.JsonArray;
import com.google.gson.JsonElement;
import com.google.gson.JsonObject;
import com.google.gson.JsonSerializationContext;
import com.google.gson.JsonSerializer;
import java.lang.reflect.Type;
import java.util.ArrayList;
import java.util.List;
class Book {
    private String title;
    private String author;
    private int year;
    public Book(String title, String author, int year) {
        this.title = title;
        this.author = author;
        this.year = year;
    }
    @Override
    public String toString() {
        return "Book{title='" + title + "', author='" + author + "', year=" + year + '}';
    }
}
class Library {
    private List<Book> books;
    public Library(List<Book> books) {
        this.books = books;
    }
    public List<Book> getBooks() {
        return books;
    }
    @Override
    public String toString() {
        return "Library{books=" + books + '}';
    }
}
class BookSerializer implements JsonSerializer<Book> {
    @Override
    public JsonElement serialize(Book book, Type typeOfSrc, JsonSerializationContext context) {
        JsonObject jsonObject = new JsonObject();
        jsonObject.addProperty("customTitle", book.getTitle());
        jsonObject.addProperty("customAuthor", book.getAuthor());
        jsonObject.addProperty("customYear", book.getYear());
        return jsonObject;
    }
}
public class GsonClass{
    public static void main(String[] args) {
        // Creating a Gson object with custom serializer
        GsonBuilder gsonBuilder = new GsonBuilder();
        gsonBuilder.registerTypeAdapter(Book.class, new BookSerializer());
        Gson gson = gsonBuilder.create();
        // Serialization: Object to JSON with custom serialization
        Book book = new Book("The Catcher in the Rye", "J.D. Salinger", 1951);
        String jsonBook = gson.toJson(book);
        System.out.println("Serialized Book with Custom Serialization: " + jsonBook);
        // Deserialization: JSON to Object
        String jsonLibrary = "{ \"books\": [{\"customTitle\":\"1984\",\"customAuthor\":\"George Orwell\",\"customYear\":1949}] }";
        Library library = gson.fromJson(jsonLibrary, Library.class);
        System.out.println("Deserialized Library: " + library);
        // Working with JsonArray
        List<Book> bookList = new ArrayList<>();
        bookList.add(new Book("To Kill a Mockingbird", "Harper Lee", 1960));
        bookList.add(new Book("Brave New World", "Aldous Huxley", 1932));
        JsonArray jsonArray = gson.toJsonTree(bookList).getAsJsonArray();
        System.out.println("JsonArray of Books: " + jsonArray);
        // Excluding fields from serialization
        Gson gsonExclusion = new GsonBuilder()
                .excludeFieldsWithoutExposeAnnotation()
                .create();
        String jsonExcluded = gsonExclusion.toJson(book);
        System.out.println("Serialized Book with Exclusion: " + jsonExcluded);
    }
}

Now let us look at few methods of Gson in Java and their functionalities.

Methods of Gson in Java

There are several methods available in the Gson class of the Java. Let us understand about each method in detail and their functionality.

Method	Functionality
toJson( Object o)	This method serializes and converts specific object into the similar representation of JSON.
toJson( JsonElement j)	This method is used for serializing a JSON element and convert it into a similar representation of JSON.
fromJson( String s, Class C)	This method deserializes the specific string of JSON and converts it into an object of specific class.
fromJson( JsonElement j, Class c)	This method deserializes a Json element and converts it into an object of the specific class.
fromJson( Reader r, Class C)	This method deserializes a string of JSON from the Reader and converts it into an object of the specific class.
toJsonTree( Object o)	This method is used for converting the specific object and converting it into an element of JSON.
toJson( JsonElement j, Appendable w)	This method is used for serializing an Json element and converting it into a specific Appendable.
fromJson( String S, Type t)	This method deserializes the required string of JSON and converts into an object of the required type.
fromJson( JsonElement j , Type t)	This method deserializes the Json element and converts it into an object of the required type.
fromJson( Reader r, Type t)	This method deserializes the string from a Reader and converts it into an object of the required type.
fromJson( Reader r, TypeToken TT)	This method deserializes a string of Json from the Reader and converts it into an object with the help of a TypeToken.
fromJson( JsonElement j, TypeToken TT)	This method deserializes a Json element and converts it into an object with the help of TypeToken.
fromJson( String s, TypeToken TT)	This method deserializes the string of the Json and converts it into an object with the help of TypeToken.
fromJson( JsonElement j, ParameterizedType PT)	This method deserializes an Json element and converts it into an object of the required parameterized type.

These are few of the methods of Gson in Java and their purpose or functionalities have been explained in detail. It was all the required explanation about Gson and methods of Gson in Java with examples.

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