feat(model): introduce transaction hierarchy with deposit, withdraw, and transfer types

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

@@ -0,0 +1,36 @@
+package dev.banking.model;
+
+/**
+ * Represents a deposit operation.
+ */
+public final class DepositTransaction extends Transaction {
+
+    private final int accountId;
+
+    public DepositTransaction(
+            int accountId,
+            int amount
+    ) {
+        super(amount);
+
+        if (accountId < 0) {
+            throw new IllegalArgumentException(
+                    "Invalid account id."
+            );
+        }
+
+        this.accountId = accountId;
+    }
+
+    public int getAccountId() {
+        return accountId;
+    }
+
+    @Override
+    public String toString() {
+        return "DepositTransaction{" +
+                "accountId=" + accountId +
+                ", amount=" + getAmount() +
+                '}';
+    }
+}

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

@@ -1,38 +1,25 @@
 package dev.banking.model;
 
-public class Transaction {
-
-    private final TransactionType type;
-
-    private final int sourceAccountId;
-    private final int targetAccountId;
+/**
+ * Base class for all transaction types.
+ *
+ * Transactions are immutable and thread-safe.
+ */
+public abstract class Transaction {
 
     private final int amount;
 
-    public Transaction(
-            TransactionType type,
-            int sourceAccountId,
-            int targetAccountId,
-            int amount
-    ) {
-        this.type = type;
-        this.sourceAccountId = sourceAccountId;
-        this.targetAccountId = targetAccountId;
+    protected Transaction(int amount) {
+
+        if (amount <= 0) {
+            throw new IllegalArgumentException(
+                    "Transaction amount must be positive."
+            );
+        }
+
         this.amount = amount;
     }
 
-    public TransactionType getType() {
-        return type;
-    }
-
-    public int getSourceAccountId() {
-        return sourceAccountId;
-    }
-
-    public int getTargetAccountId() {
-        return targetAccountId;
-    }
-
     public int getAmount() {
         return amount;
     }

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

@@ -0,0 +1,57 @@
+package dev.banking.model;
+
+/**
+ * Represents a transfer operation between two accounts.
+ */
+public final class TransferTransaction
+        extends Transaction {
+
+    private final int sourceAccountId;
+    private final int targetAccountId;
+
+    public TransferTransaction(
+            int sourceAccountId,
+            int targetAccountId,
+            int amount
+    ) {
+        super(amount);
+
+        if (sourceAccountId < 0) {
+            throw new IllegalArgumentException(
+                    "Invalid source account id."
+            );
+        }
+
+        if (targetAccountId < 0) {
+            throw new IllegalArgumentException(
+                    "Invalid target account id."
+            );
+        }
+
+        if (sourceAccountId == targetAccountId) {
+            throw new IllegalArgumentException(
+                    "Source and target accounts must be different."
+            );
+        }
+
+        this.sourceAccountId = sourceAccountId;
+        this.targetAccountId = targetAccountId;
+    }
+
+    public int getSourceAccountId() {
+        return sourceAccountId;
+    }
+
+    public int getTargetAccountId() {
+        return targetAccountId;
+    }
+
+    @Override
+    public String toString() {
+        return "TransferTransaction{" +
+                "sourceAccountId=" + sourceAccountId +
+                ", targetAccountId=" + targetAccountId +
+                ", amount=" + getAmount() +
+                '}';
+    }
+}

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

@@ -0,0 +1,36 @@
+package dev.banking.model;
+
+/**
+ * Represents a withdrawal operation.
+ */
+public final class WithdrawTransaction extends Transaction {
+
+    private final int accountId;
+
+    public WithdrawTransaction(
+            int accountId,
+            int amount
+    ) {
+        super(amount);
+
+        if (accountId < 0) {
+            throw new IllegalArgumentException(
+                    "Invalid account id."
+            );
+        }
+
+        this.accountId = accountId;
+    }
+
+    public int getAccountId() {
+        return accountId;
+    }
+
+    @Override
+    public String toString() {
+        return "WithdrawTransaction{" +
+                "accountId=" + accountId +
+                ", amount=" + getAmount() +
+                '}';
+    }
+}