Flat Buffers - enum

Overview

The enum data type is one of the composite datatypes of Flat Buffers. It is equivalent to an enum in the languages that we use, for example, Java etc.

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 an enum −

theater.fbs

namespace com.tutorialspoint.theater;

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

table Theater {
   payment:PAYMENT_SYSTEM;
}
root_type Theater;

Now our table contains an enum attribute. We've assigned a value to each enum constant except one DEBIT_CARD which by default take the incremental value as 2.

We define the enum and use it below as the data type along with "payment" attribute.

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.java and PAYMENT_SYSTEM class 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 theater FlatBuffers using startTheater() method
      Theater.startTheater(builder);
      // add details to the Theater FlatBuffer
      Theater.addPayment(builder, PAYMENT_SYSTEM.DEBIT_CARD);

      // 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("Payment Method: " + theater.payment());
      }
   }
}

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 information to file: theater_flatbuffers_output
Saved theater information with following data to disk:
8

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
Payment Method: 2

So, as we see, we are able to read the serialized enum by deserializing the binary data to Theater object. In the next chapter Flat Buffers - Vector, we will look at the vector, a composite type.