The signature for a JSON decode method is as follows:

    public void decode(Asn1JsonDecodeBuffer buffer)
           throws java.io.IOException

The buffer argument is an Asn1JsonDecodeBuffer object which provides the message to be decoded. This must be created and initialized before calling any decode method. See the description of this class in the Java Run-Time Classes section for details on how this class is used.

As you can see, the decode method returns void; the data is decoded into the instance on which decode is invoked and an exception is thrown if an error occurs. All ASN1C exceptions are derived from Asn1Exception. See the section on exceptions for a complete list and description of the various ASN1C exceptions that can be thrown.