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

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;
     }