How do I use filter EVM log events by an indexed field in web3j?

Question

I've looked through the documentation, but there doesn't seem to be this scenario listed. What I want to do is filter the Ethereum event log of a specific contract by an indexed field with Web3j.

Example solidity code: event someEvent(bytes32 indexed id, bytes value);

While we can get all the events within a specific time period using EthFilter filter = new EthFilter(startBlock, endBlock, contractAddress);, and filter for a specific event type by using the Event class, how do I get all events of a specific type that match an indexed field (for example, where id == {0, 0, 0 ... 0})?

Answer 1

From the docs https://docs.web3j.io/filters.html:

Where topics are indexed parameters of a contract event. Hope this helps.

Answer 2

you just listen for the topic named to be a value you're interested in.

For example, I'm listening for a integer uint16 tokenId event Emitted(uint16 indexed tokenId, address owner, uint32 timestamp);

using this code

String topic = String.format("0x%064X", tokenId);

EthFilter filter = new EthFilter(DefaultBlockParameterName.LATEST, 
DefaultBlockParameterName.LATEST, ADDRESS).addSingleTopic(topic);