Checking balance in ERC20 contract using web3j

4
votes

I want to check user's balance of a couple of ERC20 compliant tokens using web3j.

Is there a generic way of doing that (generic for every ERC20 contract) or should I get ABI for each of the contracts and generate java classes from it?

1
javaethereumsmartcontractsweb3-java
Refer to this answer of Send ERC20 token with web3 – LF00

1 Answers

1
votes

I have never used web3j, but I have used web3js quite a bit. I will link you to relevant information.

Here is an interface that is already created in the tests of the web3j library, so the best place to start.

Extra notes (which might well be basic for you)

Checking the balance is something that you don't want to generate a transaction for (since it doesn't change the state of the blockchain) and so you should use a 'call', as explained here.

Also, it may be useful to understand how Ethereum creates the ABI in the first place. Every transaction or call can contain data with it, and the network then uses this data to determine which function is being called and it's parameters. The logic for this function is sitting at the address of the first 4 bytes of the kekak hash of the functions name/parameters (some info), which is one reason why it is so important that this hash is collision free (imagine 2 different functions hashing to the same address). But the take home of this is that all erc20 tokens (if they follow the standard) have common ABIs for those functions.

PS. For next time I think this question is better suited for Ethereum Stackexchange.