...

1. Introduce a new java interface org.apache.kafka.tools.api.Decoder to replace scala trait kafka.serializer.Decoder
   
2. deprecate all "class names" of kafka.serializer.Decoder implementations. That includes kafka.serializer.DefaultDecoder, kafka.serializer.StringDecoder, kafka.serializer.LongDecoder, and kafka.serializer.IntegerDecoder 
3. remove the VerifiableProperties from constructor. We "always" pass empty VerifiableProperties before (known issue:

   Jira
   server ASF JIRA serverId 5aa69414-a9e9-3523-82ec-879b028fb15b key KAFKA-12311

   ), and hence using the VerifiableProperties to create new decoder does not make sense. If users require configurable decoder, we can make decoder extend Configurable in the future.

Compatibility, Deprecation, and Migration Plan

• before 4.0, users can keep using the deprecated kafka.serializer.Decoder
• after 4.0, all users need to update is the superclass is changed from kafka.serializer.Decoder to org.apache.kafka.tools.api.Decoder. They have same methods, so users don't need to change any code of method implementationmethod, but the new constructor does not accept VerifiableProperties now. Hence, users' custom decoders have to offer default constructor
  
• before 4.0, users can access both interfaces by depending on core module.
• after 4.0, users need to import tools-api module if core module is gone

...