AccountUpdate.java

    private KlayTransactionReceipt.TransactionReceipt transfer(KlayCredentials from, KlayCredentials to, BigInteger value) throws IOException {
        log.info("transfer {} KLAY [{} -> {}]", value.toString(), from.getAddress(), to.getAddress());

        BigInteger nonce = caver.klay().getTransactionCount(from.getAddress(), BLK_PARAM).send().getValue();
        TxTypeValueTransfer tx = TxTypeValueTransfer.createTransaction(
                nonce,
                GAS_PRICE,
                GAS_LIMIT,
                to.getAddress(),
                value,
                from.getAddress()
        );

        TransactionManager manager = new TransactionManager.Builder(caver, from)
                .setTransactionReceiptProcessor(new PollingTransactionReceiptProcessor(caver, 1000, 10))
                .setChaindId(CHAIN_ID)
                .build();

        return manager.executeTransaction(tx);
    }

    @Test
    public void AccountUpdate() throws InvalidAlgorithmParameterException, NoSuchAlgorithmException, NoSuchProviderException, IOException {
        BigInteger privKey = Keys.createEcKeyPair().getPrivateKey();
        KlayCredentials credential = KlayCredentials.create(privKey.toString(16)); // newly created credential
        // At this point the account represented by acct does not exist as there hasn't been a single transaction regarding this account.

        log.info("address = {}, private_key = {}", credential.getAddress(), privKey.toString(16));

        KlayTransactionReceipt.TransactionReceipt receipt;

        // Let's send small amount of KLAY to give birth to our new account
        receipt = transfer(senderCredential, credential, Convert.toPeb("0.5", Convert.Unit.KLAY).toBigInteger());
        Assert.assertNotNull(receipt);

        // Let's see if the account has been created by retrieving it
        KlayAccount account;

        account = caver.klay().getAccount(credential.getAddress(), DefaultBlockParameterName.LATEST).send();
        Assert.assertNotNull(account);
        log.info("Account Type = {}", account.getResult().getAccType());
        log.info("Balance = {}", caver.klay().getBalance(credential.getAddress(), BLK_PARAM).send().getValue());


        // Now you can change your private key
        ECKeyPair newKeyPair = Keys.createEcKeyPair();

        AccountUpdateTransaction accountUpdateTx = AccountUpdateTransaction.create(
                credential.getAddress(),
                AccountKeyPublic.create(newKeyPair.getPublicKey()),
                GAS_LIMIT
        );

        // New transaction manager
        TransactionManager manager = new TransactionManager.Builder(caver, credential)
                .setTransactionReceiptProcessor(new PollingTransactionReceiptProcessor(caver, 1000, 10))
                .setChaindId(CHAIN_ID)
                .build();

        receipt = manager.executeTransaction(accountUpdateTx);
        Assert.assertNotNull(receipt);

        log.info("Receipt -> status = {}, tx_hash = {}", receipt.getStatus(), receipt.getTransactionHash());
        log.info("Balance = {}", caver.klay().getBalance(credential.getAddress(), BLK_PARAM).send().getValue());

        account = caver.klay().getAccount(credential.getAddress(), DefaultBlockParameterName.LATEST).send();
        Assert.assertNotNull(account);
        log.info("Account Type = {}", account.getResult().getAccType());
        log.info("Balance = {}", caver.klay().getBalance(credential.getAddress(), BLK_PARAM).send().getValue());

        // Confirming the updated account is working
        KlayCredentials updatedCredential = KlayCredentials.create(
                newKeyPair.getPrivateKey().toString(16),
                credential.getAddress()
        );
        receipt = transfer(updatedCredential, senderCredential, Convert.toPeb("0.2", Convert.Unit.KLAY).toBigInteger());
        Assert.assertNotNull(receipt);

        log.info("Balance = {}", caver.klay().getBalance(credential.getAddress(), BLK_PARAM).send().getValue());
    }