web3j.ethCall returns null when retrieven contract variable

Question

Environment:

Geth node Geth/v1.9.0-unstable/windows-amd64/go1.12

Commands used to start the node to mine:

• geth --datadir "." --networkid 101 -verbosity 6 --port 30301 --rpcport 8101 --nodiscover -rpc --rpcapi "db,eth,net,web3,personal" --vmdebug --mine console 2>>"node01.log"
• miner.start(2)
• personal.unlockAccount(eth.accounts[0])

HelloWorld.sol contract:

pragma solidity ^0.5;    
contract HelloWorld {      
    uint256 counter = 5;

    function add() public {  //increases counter by 1
        counter++;
    }   
    function subtract() public { //decreases counter by 1
        counter--;
    }   
    function getCounter() public view returns (uint256) {
        return counter;
    } 
}

Java Wrapper Class of the contract

The generated function is:

public RemoteCall<BigInteger> getCounter() {
    final Function function = new Function(FUNC_GETCOUNTER, 
            Arrays.<Type>asList(), 
            Arrays.<TypeReference<?>>asList(new TypeReference<Uint256>() {}));
    return executeRemoteCallSingleValueReturn(function, BigInteger.class);
}

Command used to generate the java class:

web3j truffle generate HelloWorld.json 
      -o java/classes -p com.web3j

Java interaction with the contract

HelloWorld contract = HelloWorld.load(
                    contractAddress,
                    web3j,
                    credentials,
                    GAS_PRICE,
                    GAS_LIMIT);

        System.out.println("Valid: " + contract.isValid());

       // call method getCounter which retrieves contract data
        Function function = new Function(
                "getCounter",
                Arrays.asList(),
                Arrays.asList(new TypeReference<Uint256>() {})
        );

        String encodedFunction = FunctionEncoder.encode(function);

        //unlock account
        PersonalUnlockAccount personalUnlockAccount = admin.personalUnlockAccount(account,password).send();
        if (personalUnlockAccount.accountUnlocked()) {
            System.out.println("Account " + account + " is UNLOCKED.");
        }

        EthCall ethCall = web3j.ethCall(
                Transaction.createEthCallTransaction(credentials.getAddress(), contractAddress, encodedFunction),
                DefaultBlockParameterName.LATEST)
                .sendAsync().get();

        String value = ethCall.getValue();
        List<Type> list = FunctionReturnDecoder.decode(value, function.getOutputParameters());

        System.out.println(list.size());

Error:

I have tried both with contract deployed with truffle, and with the contract deployed in web3j with .deploy(). In both cases the contract gets deployed and I can check the transaction hash on the geth private note created by me. But in both cases I cannot successfully call the getCounter() function. The deployed / loaded contract is valid. I called the method add() with Transaction.createFunctionCallTransactionand I managed to mine the transaction and obtain the transaction hash. When I call the getCounter() method in the debugger ethCall.result = "0x".

Additional Question:

I have read that this method call should also work because it only retrieves contract data:

contract.getCounter().send();

But it throws the exception:

org.web3j.tx.exceptions.ContractCallException: Empty value (0x) returned from contract

Please help with any hints related to what I might be doing wrong. Thank you.

Answer 1

This line:

contract.getCounter().send();

should be

contract.getCounter().call({from:yourAccount});

Reason

When you request state from Ethereum, it doesn't cost gas because you're just reading a node. If the node is on your computer, you're just reading your hard drive. On the other hand, when you change state, it requires miners to pick up the change and add it to the blockchain. With Web3, you signify whether you're invoking a state changing function or just a read by using either send({from:yourAccount) or call({from:yourAccount}) respectively.