Protobuf - Map

Map is one of the composite datatypes of Protobuf. Protobuf translates this to a java.util.Map interface in Java.

Continuing with our theater example, following is the syntax that we need to have to instruct Protobuf that we will be creating a map −

syntax = "proto3";
package theater;
option java_package = "com.tutorialspoint.theater";

message Theater {
   map<string, int32> movieTicketPrice = 9;
}

Now our class/message contains a map of movie and their ticket price. Note that although we have "string -> int" map, we can as well have number, Boolean, and custom data types. However, note that we cannot have a nested map.

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

protoc  --java_out=java/src/main/java proto_files\theater.proto

The above command will create the required files and now we can use it in our Java code. First, we will create a writer to write the theater information −

package com.tutorialspoint.theater;

import java.util.List;
import java.util.Map;
import java.io.FileOutputStream;
import java.io.IOException;
import java.util.ArrayList;
import java.util.HashMap;
import com.tutorialspoint.theater.TheaterOuterClass.Theater;

public class TheaterWriter{
   public static void main(String[] args) throws IOException {
      Map<String, Integer> ticketPrice = new HashMap<>();
	    
      ticketPrice.put("Avengers Endgame", 700);
      ticketPrice.put("Captain America", 200);
      ticketPrice.put("Wonder Woman 1984", 400);
	    
      Theater theater = Theater.newBuilder()
         .putAllMovieTicketPrice(ticketPrice)
         .build();
		
      String filename = "theater_protobuf_output";
      System.out.println("Saving theater information to file: " + filename);
		
      try(FileOutputStream output = new FileOutputStream(filename)){
         theater.writeTo(output);
      }
      System.out.println("Saved theater information with following data to disk: \n" + theater);
   }
}

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

package com.tutorialspoint.theater;

import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import com.tutorialspoint.greeting.Greeting.Greet;
import com.tutorialspoint.theater.TheaterOuterClass.Theater;
import com.tutorialspoint.theater.TheaterOuterClass.Theater.Builder;

public class TheaterReader{
   public static void main(String[] args) throws IOException {
	   Builder theaterBuilder = Theater.newBuilder();

      String filename = "theater_protobuf_output";
      System.out.println("Reading from file " + filename);
        
      try(FileInputStream input = new FileInputStream(filename)) {
         Theater theater = theaterBuilder.mergeFrom(input).build();
         System.out.println(theater);
      }
   }
}

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

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

Saving theater information to file: theater_protobuf_output
Saved theater information with following data to disk:
movieTicketPrice {
   key: "Avengers Endgame"
   value: 700
}
movieTicketPrice {
   key: "Captain America"
   value: 200
}
movieTicketPrice {
   key: "Wonder Woman 1984"
   value: 400
}

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

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

Reading from file theater_protobuf_output
movieTicketPrice {
   key: "Avengers Endgame"
   value: 700
}
movieTicketPrice {
   key: "Captain America"
   value: 200
}
movieTicketPrice {
   key: "Wonder Woman 1984"
   value: 400
}

So, as we see, we are able to read the serialized map by deserializing the binary data to Theater object. In the next chapter, we will see how to create a nested class in Protobuf.

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