How to convert Json fields with dashes to Avro fields with underscores?

I have a string with json with attributes contained dashes. I want to convert it to an Avro object with field with underscore. I tried to use Gson library but got a null instead of value.

Avro model class (target):

@AvroGenerated
public class Contract extends SpecificRecordBase implements SpecificRecord {
    public static final Schema SCHEMA$ = (new Parser()).parse("....."});

    @Deprecated
    public String contract_id;

    public static Schema getClassSchema() {
        return SCHEMA$;
    }

JSON (source):

{"contract-id": "1372b63a_7c6a_4929_a99b_199fa870edc0"}

Converting:

final Gson gson  = new GsonBuilder()
        .setFieldNamingPolicy(FieldNamingPolicy.LOWER_CASE_WITH_DASHES)
        .setFieldNamingStrategy(new PackageFieldNamingStrategy())
        .registerTypeAdapter(Double.class, new DoubleSerializer())
        .create();

Contract avroObject = GSON_SERIALIZER.fromJson(data);

// printing avroObject.contract_id 

I get null for contract_id.

If the dashes are replaced with underscores and .setFieldNamingPolicy(FieldNamingPolicy.LOWER_CASE_WITH_DASHES) removed
the code returns the correct value.

Policy FieldNamingPolicy.LOWER_CASE_WITH_DASHES would work if the field was named as contractId. Because it is not then you need to define a FieldNamingStrategy of your own, for example like:

public static class MyStrategy implements FieldNamingStrategy {
    @Override
    public String translateName(Field f) {
        return f.getName().replace('_', '-');
    }        
}

and use it like:

Gson gson = new GsonBuilder()
    .setFieldNamingStrategy(new MyStrategy())
    .create();

or simply when building Gson, inline without extra classes:

.setFieldNamingStrategy( f ->  f.getName().replace('_', '-'))