HIP-213: Application Signs for End User

//Java

private AccountId getAccountId() throws TimeoutException, PrecheckStatusException, ReceiptStatusException {
// Retrieve the Operator Id
AccountId OPERATOR_ID = AccountId.fromString(Objects.requireNonNull(Dotenv.load().get("OPERATOR_ID")));

// need a placeholder key to satisfy the client.setOperator;
PrivateKey dummyKey = PrivateKey.generate();

//Create a testnet Client
Client client = Client.forTestnet();

// Set the operator (and default signer) to the new private key
client.setOperator(OPERATOR_ID, dummyKey);

// We need the public key of the operator id for the signWith operation below
PublicKey OPERATOR_PUBLIC_KEY = PrivateKey.fromString(Dotenv.load().get("OPERATOR_KEY")).getPublicKey();

// Creates a function that is invoked during the signWith method
Function<byte[], byte[]> transactionSigner = generateSignatureFunction();

// Create a TransferTransaction that transfers 1 tinyBar from the operator to account 0.0.3
TransactionResponse response = new TransferTransaction()
        .addHbarTransfer(OPERATOR_ID, Hbar.fromTinybars(1).negated())
        .addHbarTransfer(AccountId.fromString("0.0.3"), Hbar.fromTinybars(1))
        .freezeWith(client)
        .signWith(OPERATOR_PUBLIC_KEY, transactionSigner)
        .execute(client);
TransactionReceipt receipt = response.getReceipt(client);

return receipt.accountId;
}

private Function<byte[],byte[]> generateSignatureFunction() {
// Retrieve the Private Key from the .env file
PrivateKey OPERATOR_KEY = PrivateKey.fromString(Objects.requireNonNull(Dotenv.load().get("OPERATOR_KEY")));

// return a function that will sign the byte array transaction during the signWith method
return (
        t -> OPERATOR_KEY.sign(t)
);
}