web3j - Unable to call a function that returns tuple[]

Question

I am relatively new to interacting with smart contracts in java and I am facing a problem while trying to retrieve a tuple[] returned by the smart contract function. Here is the ABI definition of the function I want to call:

{
  "inputs":[{"internalType":"address","name":"account","type":"address"}],
  "name":"claimableRewards",
  "outputs":
    [{"components":
      [
        {"internalType":"address","name":"token","type":"address"},
        {"internalType":"uint256","name":"amount","type":"uint256"}
      ],
      "internalType":"struct MultiFeeDistribution.RewardData[]",
      "name":"rewards",
      "type":"tuple[]"
    }],
  "stateMutability":"view",
  "type":"function"
}

Here is the link to the smart contract code: https://polygonscan.com/address/0x920f22e1e5da04504b765f8110ab96a20e6408bd#code

And here is the java code that I wrote to call the function (I have removed the errors checking to make the code easier to read) :

        List<Type> claimableRewardsParams = Arrays.<Type>asList(new Address(credentials.getAddress()));
        List<TypeReference<?>> claimableRewardsReturnTypes = Arrays.<TypeReference<?>>asList(new TypeReference<DynamicArray<DynamicStruct>>() {});

        final Function claimableRewardsFunction = new Function(
                "claimableRewards",
                claimableRewardsParams,
                claimableRewardsReturnTypes);

        String claimableRewardsEncodedFunction = FunctionEncoder
                .encode(claimableRewardsFunction);          

        EthCall claimableRewardsResponse = web3.ethCall(
                Transaction.createEthCallTransaction(walletAddress, adamantRewardsContractAddress, claimableRewardsEncodedFunction),
                DefaultBlockParameterName.LATEST)
                .sendAsync().get();

        List<Type> claimableRewardsSomeTypes = FunctionReturnDecoder.decode(
                claimableRewardsResponse.getValue(), claimableRewardsFunction.getOutputParameters());

When I run the program I get the following exception:

Exception in thread "main" java.lang.RuntimeException: TypeReferenced struct must contain a constructor with types that extend Type

I have tried several other definitions for claimableRewardsReturnTypes but I can't get it to work. Can someone please help me ?

Thanks in advance.

Answer 1

Dealing with web3j internal types is not the best starting point.

What you can do is generate the Java contract wrappers for your ABI. The wrappers are Java classes that abstract the internal Web3j logic and provide you with functions you can use to interact with your contract using native Java types.

You can do that, for example, using the web3j-cli and use the following:

$ web3j generate solidity -a <abi_file> -o <output_dir> -p <package_name> 

Another easier way is to use the web3j-gradle-plugin or web3j-maven-plugin to generate the wrappers from your projects via the provided generateContractWrappers and use the wrappers from there.