HW 09

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

@@ -1,9 +1,14 @@
 package dev.banking.model;
 
+import java.util.concurrent.atomic.AtomicInteger;
+import java.util.concurrent.atomic.AtomicLong;
+import java.util.concurrent.locks.ReentrantLock;
+
 public class BankAccount {
 
     private final int accountId;
     private long balance;
+    private ReentrantLock lock = new ReentrantLock();
 
     /*
      * Students may introduce additional fields
@@ -31,8 +36,8 @@ public class BankAccount {
      * - 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");
+    public synchronized long getBalance() { //??????????????????????????/?
+        return balance;
     }
 
     /*
@@ -42,8 +47,8 @@ public class BankAccount {
      * Requirements:
      * - Must not lose updates under concurrency
      */
-    public void deposit(long amount) {
-        throw new UnsupportedOperationException("TODO: implement thread-safe deposit");
+    public synchronized void deposit(long amount) {
+        balance += amount;
     }
 
     /*
@@ -55,8 +60,8 @@ public class BankAccount {
      * - Negative balance handling is NOT required unless you decide
      *   to extend the system (optional)
      */
-    public void withdraw(long amount) {
-        throw new UnsupportedOperationException("TODO: implement thread-safe withdraw");
+    public synchronized void withdraw(long amount) {
+        balance -= amount;
     }
 
     /*
@@ -73,6 +78,14 @@ 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 = target.getAccountId() < accountId ? target : this ;
+        BankAccount second = target.getAccountId() < accountId ? this : target ;
+
+        synchronized (first) {
+            synchronized (second) {
+                target.deposit(amount);
+                this.withdraw(amount);
+            }
+        }
     }
 }