[Ethereum] How to get the raw private key from a WalletFile object

To generate the Credentials if you have a plain public key and private key, you need to convert them in to the hex representation and then pass them in to the constructor for Credentials

String hexPrivateKey = String.format("%040x", new BigInteger(1, privateKey.getBytes()));
String hexPublicKey = String.format("%040x", new BigInteger(1, publicKey.getBytes()));  
Credentials creds = Credentials.create(hexPrivateKey, hexPublicKey);

Where publicKey and privateKey are Strings containing your plain pub and priv keys

Here is a more complete program than the one you provided. The only undefined part is Contract_sol_Contract, which you did not define. Notice how privateKey and publicKey are computed.

import org.web3j.crypto.Credentials;
import org.web3j.crypto.ECKeyPair;
import org.web3j.protocol.Web3j;
import org.web3j.protocol.http.HttpService;

import java.io.IOException;
import java.math.BigInteger;

public class SO3 {
    public SO3() throws IOException {
        Web3j web3 = Web3j.build(new HttpService("http://localhost:8545"));
        String account = web3.ethAccounts().send().getAccounts().get(0);
        Credentials credentials = Credentials.create(account);
        ECKeyPair keyPair = credentials.getEcKeyPair();
        BigInteger privateKey = keyPair.getPrivateKey();
        BigInteger publicKey = keyPair.getPublicKey();
        Contract_sol_Contract contract = Contract_sol_Contract.deploy(web3, credentials, new BigInteger("240000"), new BigInteger("2400000"), props).send();
    }
}

I think that the reason that web3j deviated from web3 in requiring just the public or private key is because providing less information is a better security practice. The web3 accounts docs has this warning: The accounts private key. This should never be shared or stored unencrypted in localstorage! Also make sure to null the memory after usage. This means passing around the entire accounts object is discouraged.

Seems the web3 authors knew they created a security issue because earlier on that same page they give this big fat warning: This package has NOT been audited and might potentially be unsafe. Take precautions to clear memory properly, store the private keys safely, and test transaction receiving and sending functionality properly before using in production!.