Flat Buffers - Vector

Quiz

Overview

The Vector data type is one of the composite datatypes of Flat Buffers. It is equivalent to an array or List 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 a vector −

theater.fbs

namespace com.tutorialspoint.theater;

table Theater {
   snacks:[string];  // vector of strings
   tickets:[float];    // vector of floats 	
}
root_type Theater;

Now our table contains vector attributes of string and float.

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 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

Creating and Writing Vector

In order to create a Vector, we need to first prepare the offset of scalar type array and then we can add the vector to the flat buffer.

// create data for an array of strings
int popcorn = builder.createString("Popcorn");
int coke = builder.createString("Coke");
int chips = builder.createString("Chips");
int soda = builder.createString("Soda");

// create array for snacks
int[] snacks = {popcorn, coke, chips, soda};

// create offset for snacks vector
int snacksVector = Theater.createSnacksVector(builder, snacks);

// add details to the Theater FlatBuffer
Theater.addSnacks(builder, snacksVector);

Following example code is showing the process of creating a Vector of String and ints.

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 data for an array of strings
      int popcorn = builder.createString("Popcorn");
      int coke = builder.createString("Coke");
      int chips = builder.createString("Chips");
      int soda = builder.createString("Soda");
      
      // create array for snacks
      int[] snacks = {popcorn, coke, chips, soda};
      
      // create array for tickets
      float[] tickets = {100.0f, 100.f, 200.f};
      
      // create offset for snacks vector
      int snacksVector = Theater.createSnacksVector(builder, snacks);
      
      // create offset for tickets vector
      int ticketsVector = Theater.createTicketsVector(builder, tickets);
      
      // create theater FlatBuffers using startTheater() method
      Theater.startTheater(builder);
      // add details to the Theater FlatBuffer
      Theater.addSnacks(builder, snacksVector);
      Theater.addTickets(builder, ticketsVector);       

      // 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);
   }
}

Reading Vector

In order to read a Vector, we have methods to get the length of vector and to get an entry by an index as shown below.

// iterate snacks vector of length determined by snacksLength() method
for(int i = 0; i < theater.snacksLength(); i++ ) {
   // get a snack by its index
   System.out.print(" " + theater.snacks(i));
}

Following example code is showing the process of reading a Vector of String and ints.

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("Snacks: ");
         for(int i = 0; i < theater.snacksLength(); i++ ) {
            System.out.print(" " + theater.snacks(i));
         }
         System.out.println("\nTickets: ");
         for(int i = 0; i < theater.ticketsLength(); i++ ) {
            System.out.print(" " + theater.tickets(i));
         }         
      }
   }
}

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:
96

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
Snacks:
 Popcorn Coke Chips Soda
Tickets:
 100.0 100.0 200.0

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

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