Adding a new branch to an Avro union

TestUnionEvolution.java

import java.io.ByteArrayOutputStream;
import java.io.IOException;
import org.apache.avro.Schema;
import org.apache.avro.generic.GenericData;
import org.apache.avro.generic.GenericDatumReader;
import org.apache.avro.generic.GenericDatumWriter;
import org.apache.avro.generic.GenericRecordBuilder;
import org.apache.avro.io.BinaryDecoder;
import org.apache.avro.io.BinaryEncoder;
import org.apache.avro.io.DatumReader;
import org.apache.avro.io.DatumWriter;
import org.apache.avro.io.DecoderFactory;
import org.apache.avro.io.EncoderFactory;

public class TestUnionEvolution {
    public static final Schema WRITER_SCHEMA = new Schema.Parser().parse(
        "{\"type\": \"record\", \"name\": \"Test\", \"fields\": [" +
            "{\"name\": \"myField\", \"default\": null, \"type\": [\"null\", \"string\", \"long\"]}" +
        "]}"
    );

    public static final Schema READER_SCHEMA = new Schema.Parser().parse(
        "{\"type\": \"record\", \"name\": \"Test\", \"fields\": [" +
            "{\"name\": \"myField\", \"default\": null, \"type\": [\"null\", \"string\"]}" +
        "]}"
    );

    public static byte[] encode(Object datum) throws IOException {
        ByteArrayOutputStream stream = new ByteArrayOutputStream();
        try {
            BinaryEncoder encoder = EncoderFactory.get().binaryEncoder(stream, null);
            DatumWriter<Object> writer = new GenericDatumWriter<Object>(WRITER_SCHEMA);
            writer.write(datum, encoder);
            encoder.flush();
            return stream.toByteArray();
        } finally {
            stream.close();
        }
    }

    public static Object decode(byte[] data) throws IOException {
        BinaryDecoder decoder = DecoderFactory.get().binaryDecoder(data, null);
        DatumReader<Object> reader = new GenericDatumReader<Object>(WRITER_SCHEMA, READER_SCHEMA);
        return reader.read(null, decoder);
    }

    public static void main(String[] args) throws IOException {
        GenericData.Record record = new GenericRecordBuilder(WRITER_SCHEMA)
                .set("myField", 42L)
                .build();
        System.out.println(decode(encode(record)));
    }
}