Flat Buffers - Default Values

---

---

Overview

We've seen in our previous examples, how to serialize and deserialize various types in flat buffers. In case, we do not specify any value then a default value is stored. If we've specify the same default value for the variable, then no extra space is allocated by flatbuffers.

Flat Buffers supports default values of its data types as per given table below −

So, if one does not specify the data for these data types, then they would take the above default values. Now, let's continue with our theater example to demonstrate how it works.

In this example, we will let all the fields default. The only field which would be specified would be the name of the theater.

Continuing with our theater example from Flat Buffers - String chapter, following is the syntax that we need to have to instruct FlatBuffers that we will be creating various data types −

theater.fbs

namespace com.tutorialspoint.theater;

enum PAYMENT_SYSTEM: int { CASH = 0, CREDIT_CARD = 1, DEBIT_CARD, APP = 3 }

table Theater {
   name:string;
   address:string;
   
   total_capacity:short;
   mobile:int;
   base_ticket_price:float;
   
   drive_in:bool;
   
   payment:PAYMENT_SYSTEM;
   
   snacks:[string];
   
   owner: TheaterOwner;
}

table TheaterOwner {
	name:string;
	address:string;
}
root_type Theater;

Now our Theater table contains multiple attributes.

Creating Java Classes from fbs File

To use FlatBuffers, we will now have to use flatc binary to create the required classes from this ".fbs" file. Let us see how to do that −

flatc --java theater.fbs

This will create a Theater, TheaterOwner and PAYMENT_SYSTEM classes in com > tutorialspoint > theater folder in current directory. We're using this class in our application similar to as done in Flat Buffers - Schema chapter.

Using Java Classes created from fbs File

TheaterWriter.java

package com.tutorialspoint.theater;

import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;

import com.google.flatbuffers.FlatBufferBuilder;

public class TheaterWriter {
   public static void main(String[] args) throws FileNotFoundException, IOException {
      // create a flat buffer builder
      // it will be used to create Theater FlatBuffer
      FlatBufferBuilder builder = new FlatBufferBuilder(1024);
      
      // create offset for name
      int name = builder.createString("Mery");
      
      // create theater FlatBuffers using startTheater() method
      Theater.startTheater(builder);
      // add details to the Theater FlatBuffer
      Theater.addName(builder, name);      
      
      // mark end of data being entered in Greet FlatBuffer
      int theater = Theater.endTheater(builder);

      // finish the builder
      builder.finish(theater);

      // get the bytes to be stored
      byte[] data = builder.sizedByteArray();

      String filename = "theater_flatbuffers_output";
      System.out.println("Saving theater to file: " + filename);
      // write the builder content to the file named theater_flatbuffers_output
      try(FileOutputStream output = new FileOutputStream(filename)){
         output.write(data);
      }
      System.out.println("Saved theater with following data to disk: \n" + theater);
   }
}	

Next, we will have a reader to read the theater information −

TheaterReader.java

package com.tutorialspoint.theater;

import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.nio.ByteBuffer;

public class TheaterReader {
   public static void main(String[] args) throws FileNotFoundException, IOException {

      String filename = "theater_flatbuffers_output";
      System.out.println("Reading from file " + filename);
      try(FileInputStream input = new FileInputStream(filename)) {
         // get the serialized data
         byte[] data = input.readAllBytes();
         ByteBuffer buf = ByteBuffer.wrap(data);
         // read the root object in serialized data
         Theater theater = Theater.getRootAsTheater(buf);

         // print theater values 
         System.out.println("Name: " + theater.name());
         System.out.println("Address: " + theater.address());
         System.out.println("Total Capacity: " + theater.totalCapacity());
         System.out.println("Mobile: " + theater.mobile());
         System.out.println("Base Ticket Price: " + theater.baseTicketPrice());
         System.out.println("Drive In: " + theater.driveIn());
         System.out.println("Snacks: ");
         if(theater.snacksLength() != 0) {
            for(int i = 0; i < theater.snacksLength(); i++ ) {
               System.out.print(" " + theater.snacks(i));
            } 
         }else {
            System.out.println("Snacks are empty.");
         }

         System.out.println("Payment Method: " + PAYMENT_SYSTEM.name(theater.payment()));        
         System.out.println("Owner Details: ");
         TheaterOwner owner = theater.owner();
         if(owner != null) {
            System.out.println("Name: " + owner.name());
            System.out.println("Address: " + owner.address());        	 
         }else {
            System.out.println("Owner " + owner);
         }        
      }
   }
}

Compile the project

Now that we have set up the reader and the writer, let us compile the project.

mvn clean install

Serialize the Java Object

Now, post compilation, let us execute the writer first −

java -cp .\target\flatbuffers-tutorial-1.0.jar com.tutorialspoint.theater.TheaterWriter

Saving theater to file: theater_flatbuffers_output
Saved theater with following data to disk:
20

Deserialize the Serialized Object

Now, let us execute the reader to read from the same file −

java -cp .\target\flatbuffers-tutorial-1.0.jar com.tutorialspoint.theater.TheaterReader

Reading from file theater_flatbuffers_output
Name: Mery
Address: null
Total Capacity: 0
Mobile: 0
Base Ticket Price: 0.0
Drive In: false
Snacks:
Snacks are empty.
Payment Method: CASH
Owner Details:
Owner null

So, as we see, we are able to read the default values by deserializing the binary data to the Theater object.