Protobuf - Enums

enum is one of the composite datatypes of Protobuf. It translates to an enum in the languages that we use, for example, Java.
Continuing with our theater example, following is the syntax that we need to have to instruct Protobuf that we will be creating an enum −
syntax = "proto3";
package theater;
option java_package = "com.tutorialspoint.theater";
      
message Theater {
   enum PAYMENT_SYSTEM{
      CASH = 0;
      CREDIT_CARD = 1;
      DEBIT_CARD = 2;
      APP = 3;  
   }
   PAYMENT_SYSTEM payment = 7;
}

Now our message class contains an Enum for payment. Each of them also has a position which is what Protobuf uses while serialization and deserialization. Each attribute of a member needs to have a unique number assigned.
We define the enum and use it below as the data type along with "payment" attribute. Note that although we have defined enum inside the message class, it can also reside outside of it.
To use Protobuf, we will now have to use 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 should 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.io.FileOutputStream;
import java.io.IOException;
import com.tutorialspoint.theater.TheaterOuterClass.Theater;
import com.tutorialspoint.theater.TheaterOuterClass.Theater.PAYMENT_SYSTEM;

public class TheaterWriter{
   public static void main(String[] args) throws IOException {
      Theater theater = Theater.newBuilder()
         .setPayment(PAYMENT_SYSTEM.CREDIT_CARD)
         .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 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.getBaseTicketPrice());
         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:
payment: CREDIT_CARD

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
payment: CREDIT_CARD

So, as we see, we are able to read the serialized enum by deserializing the binary data to Theater object. In the next chapter, we will take a look at Protobuf lists.
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