@Override
    public String toString() {
        StringBuilder sb = new StringBuilder();
        sb.append("{");
        if (getCsv() != null)
            sb.append("CSV: ").append(getCsv());
        if (getJson() != null)
            sb.append("JSON: ").append(getJson());
        sb.append("}");
        return sb.toString();
    }
Specifies JSON as request's output serialization format.
aws-java-sdk-s3/src/main/java/com/amazonaws/services/s3/model/OutputSerialization.java