ERC20 Tokens – How Metamask Sends ERC20 Tokens Without ABI

I assume you are using Web3j and a deployed smart contract implementing ERC20 interface, where transfer method looks like this:

function transfer(address _to, uint256 _value) public returns (bool success) {
    require(balances[msg.sender] >= _value && _value > 0);
    balances[msg.sender] -= _value;
    balances[_to] += _value;
    return true;
}

You can use web3j wrapper for your contract to call contract methods on a blockahain. Suppose your Contract filname is MyContract.sol, then you'll need solc compiler and web3j tool and do the following in your console:

$ solc {contract}.sol --bin --abi --optimize -o {output-dir}/

this will generate .abi and .bin files from which you generate a java wrapper class:

$ web3j solidity generate /path/to/<smart-contract>.bin /path/to/<smart-contract>.abi -o /path/to/src/main/java -p com.your.organisation.name

This will produce a java wrapper class for your contract like MyContract.java on which you can call all the methods available on the smart contract:

import static org.web3j.tx.Contract.GAS_LIMIT;
import static org.web3j.tx.ManagedTransaction.GAS_PRICE;

//web3j initialization code goes here

//Load the deployed contract:
MyContract contract = MyContract.load(contractAddress, web3j, credentials,     GAS_PRICE, GAS_LIMIT);

//Call methods on the contract:
Future<> result = contract.transfer(_to, _value);

You can find more info on how to work with smart contracts wrappers here. Hope that helps.

edit. abi and bin for this particular contract can be obtained from etherscan.io You can transfer tokens even if you are not the owner of the contract. But you are only abe to transfer tokens available to your own account as seen in the contract:

 function transfer(address _to, uint256 _amount) returns (bool success) {
     if (balances[msg.sender] >= _amount 
        && _amount > 0
         && balances[_to] + _amount > balances[_to]) {
         balances[msg.sender] -= _amount;
         balances[_to] += _amount;
         Transfer(msg.sender, _to, _amount);
        return true;
     } else {
         return false;
     }
 }

Donot use value field if you dont want to send ether. Value field is only used for ether. Now you have to construct the data for the transaction. if using web3@0.2.0 then you can do

contractData = contractInstance.transfer.getData(address, amount)
//contractData = '0x12345643213456000000000023434234'

this will give you the data for the transaction. After that the transaction object will look like this

{
    nonce: nonceHex,
    gasPrice: gasPriceHex,
    gasLimit: gasLimitHex,
    data: contractData,
    to: contractAddress,
    chainId: 1(main net), 2(expanse), 4(rinkeby)
    from: fromAddress
}

But it wont say "Pay 100 ABC tokens", but there will be a popup for confirmation to send transaction