implement BankAccount.java

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

@@ -1,9 +1,13 @@
 package dev.banking.model;
 
+import java.util.concurrent.locks.Lock;
+import java.util.concurrent.locks.ReentrantLock;
+
 public class BankAccount {
 
     private final int accountId;
     private long balance;
+    private Lock lock = new ReentrantLock();
 
     /*
      * Students may introduce additional fields
@@ -23,44 +27,44 @@ public class BankAccount {
         return accountId;
     }
 
-    /*
+    /* Return the current balance in a thread-safe way.
-     * TODO:
-     * Return the current balance in a thread-safe way.
      *
      * Requirements:
      * - Must be safe under concurrent reads/writes
      * - Should not block unnecessarily if using read/write locks
      */
     public long getBalance() {
-        throw new UnsupportedOperationException("TODO: implement thread-safe balance read");
+
+        lock.lock();
+        try{
+            return balance;
+        }
+        finally {
+            lock.unlock();
+        }
     }
 
-    /*
+    /* Increase balance atomically.
-     * TODO:
-     * Increase balance atomically.
      *
      * Requirements:
      * - Must not lose updates under concurrency
      */
-    public void deposit(long amount) {
+    public synchronized void deposit(long amount) {
-        throw new UnsupportedOperationException("TODO: implement thread-safe deposit");
+        balance += amount;
     }
 
-    /*
+    /* Decrease balance atomically.
-     * TODO:
-     * Decrease balance atomically.
      *
      * Requirements:
      * - Must not cause race conditions
      * - Negative balance handling is NOT required unless you decide
      *   to extend the system (optional)
      */
-    public void withdraw(long amount) {
+    public synchronized void withdraw(long amount) {
-        throw new UnsupportedOperationException("TODO: implement thread-safe withdraw");
+        balance -= amount;
     }
 
     /*
-     * TODO:
      * Transfer money between two accounts atomically.
      *
      * IMPORTANT REQUIREMENTS:
@@ -73,6 +77,33 @@ public class BankAccount {
      * - Or tryLock with retry strategy
      */
     public void transfer(BankAccount target, long amount) {
-        throw new UnsupportedOperationException("TODO: implement atomic deadlock-free transfer");
+
+        BankAccount first, second;
+
+        if(target == this) return;
+
+        if (this.getAccountId() < target.getAccountId()) {
+            first = this;
+            second = target;
+        }
+        else {
+            first = target;
+            second = this;
+        }
+
+        first.lock.lock();
+        try {
+            second.lock.lock();
+            try {
+                this.withdraw(amount);
+                target.deposit(amount);
+            }
+            finally {
+                second.lock.unlock();
+            }
+        }
+        finally {
+            first.lock.unlock();
+        }
     }
 }