feat: implement TransactionProcessor with type-based dispatch for all transaction operations

src/main/java/dev/banking/model/BankAccount.java

@@ -5,6 +5,11 @@ public class BankAccount {
     private final int accountId;
     private long balance;
 
+    /*
+     * Students may introduce additional fields
+     * if required by their synchronization strategy.
+     */
+
     public BankAccount(int accountId, long initialBalance) {
         this.accountId = accountId;
         this.balance = initialBalance;

src/main/java/dev/banking/processor/TransactionProcessor.java

@@ -16,29 +16,36 @@ public class TransactionProcessor {
 
     public void process(Transaction tx) {
 
-        BankAccount source =
+        if (tx instanceof DepositTransaction deposit) {
-                accounts.get(tx.getSourceAccountId());
 
-        switch (tx.getType()) {
+            BankAccount account = accounts.get(deposit.getAccountId());
 
-            case DEPOSIT -> {
+            account.deposit(deposit.getAmount());
-                source.deposit(tx.getAmount());
         }
 
-            case WITHDRAW -> {
+        else if (tx instanceof WithdrawTransaction withdraw) {
-                source.withdraw(tx.getAmount());
+
+            BankAccount account = accounts.get(withdraw.getAccountId());
+
+            account.withdraw(withdraw.getAmount());
         }
 
-            case TRANSFER -> {
+        else if (tx instanceof TransferTransaction transfer) {
 
-                BankAccount target =
+            BankAccount source = accounts.get(transfer.getSourceAccountId());
-                        accounts.get(tx.getTargetAccountId());
+
+            BankAccount target = accounts.get(transfer.getTargetAccountId());
 
             source.transfer(
                     target,
-                        tx.getAmount()
+                    transfer.getAmount()
+            );
+        }
+
+        else {
+            throw new IllegalArgumentException(
+                    "Unknown transaction type: " + tx.getClass()
             );
         }
     }
-    }
 }

src/main/java/dev/banking/service/BankingSystem.java

@@ -6,6 +6,18 @@ import dev.banking.processor.TransactionProcessor;
 import java.util.List;
 import java.util.concurrent.ExecutorService;
 
+/**
+ * Dispatches a list of transactions to a shared ExecutorService
+ * for concurrent (asynchronous) processing.
+ *
+ * Each transaction is submitted as an independent task and may
+ * be executed in parallel depending on thread availability.
+ *
+ * No ordering guarantees are provided between transactions.
+ *
+ * Lifecycle management of the ExecutorService (creation,
+ * shutdown, termination) is handled outside this class.
+ */
 public class BankingSystem {
 
     private final ExecutorService executor;