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Google Gson Interview Questions and Answers

Ques. How to custom Serialization and De-serialization in Gson?
Ans.
Many times, we need to write/read the JSON values which are not default representation of java object. In that case, we need to write custom serializer and deserializer of that java type.

In our example, I am writing serializer and deserializer for java.util.Date class, which will help writing the Date format in "dd/MM/yyyy" format.

DateSerializer.java
import java.lang.reflect.Type;
import java.text.SimpleDateFormat;
import java.util.Date;
 
import com.google.gson.JsonElement;
import com.google.gson.JsonPrimitive;
import com.google.gson.JsonSerializationContext;
import com.google.gson.JsonSerializer;
 
public class DateSerializer implements JsonSerializer<Date>
{
   private static final SimpleDateFormat dateFormat = new SimpleDateFormat("dd/MM/yyyy");
   public JsonElement serialize(Date date, Type typeOfSrc, JsonSerializationContext context)
   {
      return new JsonPrimitive(dateFormat.format(date));
   }
}

DateDeserializer.java
import java.lang.reflect.Type;
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;
 
import com.google.gson.JsonDeserializationContext;
import com.google.gson.JsonDeserializer;
import com.google.gson.JsonElement;
 
public class DateDeserializer implements JsonDeserializer<Date>
{
   private static final SimpleDateFormat dateFormat = new SimpleDateFormat("dd/MM/yyyy");
   public Date deserialize(JsonElement dateStr, Type typeOfSrc, JsonDeserializationContext context)
   {
      try
      {
         return dateFormat.parse(dateStr.getAsString());
      } 
      catch (ParseException e)
      {
         e.printStackTrace();
      }
      return null;
   }
}

Now you can register these serializer and deserializer with GsonBuilder as below:

GsonBuilder gsonBuilder = new GsonBuilder();
gsonBuilder.registerTypeAdapter(Date.class, new DateSerializer());
gsonBuilder.registerTypeAdapter(Date.class, new DateDeserializer());

Complete example of serializer and deserializer is as below:
Employee employee = new Employee();
employee.setId(1);
employee.setFirstName("Arindam");
employee.setLastName("Ghosh");
employee.setRoles(Arrays.asList("FINANCE", "MANAGER"));
employee.setBirthDate(new Date());
 
GsonBuilder gsonBuilder = new GsonBuilder();
gsonBuilder.registerTypeAdapter(Date.class, new DateSerializer());
gsonBuilder.registerTypeAdapter(Date.class, new DateDeserializer());
Gson gson = gsonBuilder.create();
 
//Convert to JSON
System.out.println(gson.toJson(employee));
 
//Convert to java objects
System.out.println(gson.fromJson("{'id':1,'firstName':'Arindam','lastName':'Ghosh','roles':['FINANCE','MANAGER'],'birthDate':'17/06/2014'}", Employee.class));
                             
Output:
{"id":1,"firstName":"Arindam","lastName":"Ghosh","roles":["FINANCE","MANAGER"],"birthDate":"17/06/2014"}
Employee [id=1, firstName=Arindam, lastName=Ghosh, roles=[FINANCE, MANAGER], birthDate=Tue Jun 17 00:00:00 IST 2014]
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