pragma solidity ^0.4.24;

contract MyContract {

  string message;
  event MyEvent(address contractAddress, string message);


  constructor() public {
    message = "I'm ready!";
  }

  function setGreetings(string _message) public {
    message = _message;
  }

  function getGreetings() public view returns (string) {
    return message;
  }

  function triggetEvent() public {
    MyEvent(address(this), message);
  }

}

web3 = Web3j.build(new HttpService("http://localhost:9545/"));
Web3ClientVersion web3ClientVersion = web3.web3ClientVersion().send();
MyContract myContract = MyContract.load(CONTRACT_ADDRESS, web3, credentials, GAS_PRICE, GAS_LIMIT);
EthFilter filter = new EthFilter(DefaultBlockParameterName.EARLIEST, DefaultBlockParameterName.LATEST, myContract.getContractAddress());
String encodedEventSignature = EventEncoder.encode(MyContract.MYEVENT_EVENT);
filter.addSingleTopic(encodedEventSignature);
log.info("subscribing to event with filter");
web3.ethLogObservable(filter).subscribe(eventString -> log.info("event string={}", eventString.toString()));

public static final Event MYEVENT_EVENT = new Event("MyEvent",
            Arrays.<TypeReference<?>>asList(),
            Arrays.<TypeReference<?>>asList(new TypeReference<Address>() {}, new TypeReference<Utf8String>() {}));