Add approach for Bank Account

exercises/practice/bank-account/.approaches/config.json

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+{
+  "introduction": {
+    "authors": [
+      "kahgoh"
+    ]
+  },
+  "approaches": [
+    {
+      "uuid": "78d753b0-aa58-43dc-83c0-9ea41496de84",
+      "slug": "synchronized-methods",
+      "title": "Synchronized methods",
+      "blurb": "Use synchronized methods to prevent methods from running simultaneously in different threads",
+      "authors": [
+        "kahgoh"
+      ]
+    },
+    {
+      "uuid": "5f3b0152-02eb-40d5-9104-5edc30b4447e",
+      "slug": "synchronized-statements",
+      "title": "Synchronized statements",
+      "blurb": "Use an object to prevent threads from running blocks of code at the same time",
+      "authors": [
+        "kahgoh"
+      ]
+    },
+    {
+      "uuid": "0acd6f2b-27d0-4ae6-9c22-22b0b2047039",
+      "slug": "reentrant-lock",
+      "title": "Reentrant lock",
+      "blurb": "Use ",
+      "authors": [
+        "kahgoh"
+      ]
+    },
+    {
+      "uuid": "4ad42f88-f750-4af9-bbbd-d8b2dc5e8078",
+      "slug": "readwrite-lock",
+      "title": "Readwrite lock",
+      "blurb": "Use separate read and write locks to achieve greater concurrency",
+      "authors": [
+        "kahgoh"
+      ]
+    }
+  ]
+}

exercises/practice/bank-account/.approaches/introduction.md

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+# Introduction
+
+In Bank Account, you are tasked with implementing a number of operations that can be performed on a bank account.
+However, these operations may be performed by multiple threads at the same time.
+
+## General guidance
+
+The key to solving Bank Account is to prevent an account from being updated from multiple threads at the same time.
+For example, consider an account that begins with $0.
+A $10 deposit is made twice.
+Each transaction starts a thread.
+If the threads happen to start simultaneously, they might both see the account starting $0.
+They each add $10 to this amount and update the account accordingly.
+In this case, each thread sets the amount to $10 even though the transaction was made twice.
+
+The problem here is that both threads saw that there was $0 in the account prior to adding the deposit.
+Instead, each thread needs to take turns to process the deposit for the account so that they can process the transaction one at a time.
+This way, the later thread can "see" the deposited amount from the first thread.
+
+## Approach: Synchronized methods
+
+```java
+class BankAccount {
+    private int balance = 0;
+    private boolean isClosed = true;
+
+    synchronized void open() throws BankAccountActionInvalidException {
+        if (!isClosed) {
+            throw new BankAccountActionInvalidException("Account already open");
+        }
+        isClosed = false;
+        balance = 0;
+    }
+
+    synchronized void close() throws BankAccountActionInvalidException {
+        if (isClosed) {
+            throw new BankAccountActionInvalidException("Account not open");
+        }
+        isClosed = true;
+    }
+
+    synchronized int getBalance() throws BankAccountActionInvalidException {
+        checkIfClosed();
+        return balance;
+    }
+
+    synchronized void deposit(int amount) throws BankAccountActionInvalidException {
+        checkIfClosed();
+        checkIfValidAmount(amount);
+
+        balance += amount;
+    }
+
+    synchronized void withdraw(int amount) throws BankAccountActionInvalidException {
+        checkIfClosed();
+        checkIfValidAmount(amount);
+        checkIfEnoughMoneyInAccount(amount);
+
+        balance -= amount;
+    }
+
+    private void checkIfValidAmount(int amount) throws BankAccountActionInvalidException {
+        if (amount < 0) {
+            throw new BankAccountActionInvalidException("Cannot deposit or withdraw negative amount");
+        }
+    }
+
+    private void checkIfEnoughMoneyInAccount(int amount) throws BankAccountActionInvalidException {
+        if (balance == 0) {
+            throw new BankAccountActionInvalidException("Cannot withdraw money from an empty account");
+        }
+        if (balance - amount < 0) {
+            throw new BankAccountActionInvalidException("Cannot withdraw more money than is currently in the account");
+        }
+    }
+
+    private void checkIfClosed() throws BankAccountActionInvalidException {
+        if (isClosed) {
+            throw new BankAccountActionInvalidException("Account closed");
+        }
+    }
+}
+```
+
+For more information, check the [Synchronized methods approach][approach-synchronized-methods].
+
+## Approach: Synchronized statements
+
+```java
+class BankAccount {
+    private final Object lock = new Object();
+    private int balance = 0;
+    private boolean isClosed = true;
+
+    void open() throws BankAccountActionInvalidException {
+        synchronized(lock) {
+            if (!isClosed) {
+                throw new BankAccountActionInvalidException("Account already open");
+            }
+            isClosed = false;
+            balance = 0;
+        }
+    }
+
+    void close() throws BankAccountActionInvalidException {
+        synchronized(lock) {
+            if (isClosed) {
+                throw new BankAccountActionInvalidException("Account not open");
+            }
+            isClosed = true;
+        }
+    }
+
+    int getBalance() throws BankAccountActionInvalidException {
+        synchronized(lock) {
+            checkIfClosed();
+            return balance;
+        }
+    }
+
+    void deposit(int amount) throws BankAccountActionInvalidException {
+        synchronized(lock) {
+            checkIfClosed();
+            checkIfValidAmount(amount);
+
+            balance += amount;
+        }
+    }
+
+    void withdraw(int amount) throws BankAccountActionInvalidException {
+        synchronized(lock) {
+            checkIfClosed();
+            checkIfValidAmount(amount);
+            checkIfEnoughMoneyInAccount(amount);
+
+            balance -= amount;
+        }
+    }
+
+    private void checkIfValidAmount(int amount) throws BankAccountActionInvalidException {
+        if (amount < 0) {
+            throw new BankAccountActionInvalidException("Cannot deposit or withdraw negative amount");
+        }
+    }
+
+    private void checkIfEnoughMoneyInAccount(int amount) throws BankAccountActionInvalidException {
+        if (balance == 0) {
+            throw new BankAccountActionInvalidException("Cannot withdraw money from an empty account");
+        }
+        if (balance - amount < 0) {
+            throw new BankAccountActionInvalidException("Cannot withdraw more money than is currently in the account");
+        }
+    }
+
+    private void checkIfClosed() throws BankAccountActionInvalidException {
+        if (isClosed) {
+            throw new BankAccountActionInvalidException("Account closed");
+        }
+    }
+}
+```
+
+For more information, check the [Synchronized statements approach][approach-synchronized-statements].
+
+## Approach: Reentrant lock
+
+```java
+import java.util.concurrent.locks.ReentrantLock;
+
+class BankAccount {
+
+    private final ReentrantLock lock = new ReentrantLock();
+
+    private boolean isOpen = false;
+    private int balance = 0;
+
+    void open() throws BankAccountActionInvalidException {
+        lock.lock();
+        try {
+            if (isOpen) {
+                throw new BankAccountActionInvalidException("Account already open");
+            }
+            isOpen = true;
+            balance = 0;
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    void close() throws BankAccountActionInvalidException {
+        lock.lock();
+        try {
+            if (!isOpen) {
+                throw new BankAccountActionInvalidException("Account not open");
+            }
+            isOpen = false;
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    int getBalance() throws BankAccountActionInvalidException {
+        lock.lock();
+        try {
+            if (!isOpen) {
+                throw new BankAccountActionInvalidException("Account closed");
+            }
+            return balance;
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    void deposit(int amount) throws BankAccountActionInvalidException {
+        lock.lock();
+        try {
+            if (!isOpen) {
+                throw new BankAccountActionInvalidException("Account closed");
+            }
+            if (amount < 0) {
+                throw new BankAccountActionInvalidException("Cannot deposit or withdraw negative amount");
+            }
+            balance += amount;
+        } finally {
+            lock.unlock();
+        }
+    }
+
+    void withdraw(int amount) throws BankAccountActionInvalidException {
+        lock.lock();
+        try {
+            if (!isOpen) {
+                throw new BankAccountActionInvalidException("Account closed");
+            }
+            if (amount > balance) {
+                throw new BankAccountActionInvalidException("Cannot withdraw more money than is currently in the account");
+            }
+            if (amount < 0) {
+                throw new BankAccountActionInvalidException("Cannot deposit or withdraw negative amount");
+            }
+            balance -= amount;
+        } finally {
+            lock.unlock();
+        }
+    }
+
+}
+```
+
+For more information, check the [Reentrant lock approach][approach-reentrant-lock].
+
+## Approach: Read write lock
+
+```java
+import java.util.concurrent.locks.ReadWriteLock;
+import java.util.concurrent.locks.ReentrantLock;
+import java.util.concurrent.locks.ReentrantReadWriteLock;
+
+class BankAccount {
+
+    private final ReadWriteLock lock = new ReentrantReadWriteLock();
+
+    private boolean isOpen = false;
+
+    private int balance = 0;
+
+    void open() throws BankAccountActionInvalidException {
+        lock.writeLock().lock();
+        try {
+            if (isOpen) {
+                throw new BankAccountActionInvalidException("Account already open");
+            }
+            isOpen = true;
+            balance = 0;
+        } finally {
+            lock.writeLock().unlock();
+        }
+    }
+
+    void close() throws BankAccountActionInvalidException {
+        lock.writeLock().lock();
+        try {
+            if (!isOpen) {
+                throw new BankAccountActionInvalidException("Account not open");
+            }
+            isOpen = false;
+        } finally {
+            lock.writeLock().unlock();
+        }
+    }
+
+    int getBalance() throws BankAccountActionInvalidException {
+        lock.readLock().lock();
+        try {
+            if (!isOpen) {
+                throw new BankAccountActionInvalidException("Account closed");
+            }
+            return balance;
+        } finally {
+            lock.readLock().unlock();
+        }
+    }
+
+    void deposit(int amount) throws BankAccountActionInvalidException {
+        lock.writeLock().lock();
+        try {
+            if (!isOpen) {
+                throw new BankAccountActionInvalidException("Account closed");
+            }
+            if (amount < 0) {
+                throw new BankAccountActionInvalidException("Cannot deposit or withdraw negative amount");
+            }
+            balance += amount;
+        } finally {
+            lock.writeLock().unlock();
+        }
+    }
+
+    void withdraw(int amount) throws BankAccountActionInvalidException {
+        lock.writeLock().lock();
+        try {
+            if (!isOpen) {
+                throw new BankAccountActionInvalidException("Account closed");
+            }
+            if (amount > balance) {
+                throw new BankAccountActionInvalidException("Cannot withdraw more money than is currently in the account");
+            }
+            if (amount < 0) {
+                throw new BankAccountActionInvalidException("Cannot deposit or withdraw negative amount");
+            }
+            balance -= amount;
+        } finally {
+            lock.writeLock().unlock();
+        }
+    }
+}
+```
+
+For more information, check the [Read write lock approach][approach-read-write-lock].
+
+## Which approach to use?
+
+- The synchronized methods is the simplest, requiring no extra objects to be created.
+- Synchronized statements provide greater control over which code statements are performed with a lock and which object is to be used as the lock.
+- The read write lock allows greater concurrency by letting multiple read operations, such as `getBalance`, run in parallel.
+  However, it requires the lock to be explicitly released.
+
+[approach-read-write-lock]: https://exercism.org/tracks/java/exercises/bank-account/approaches/readwrite-lock
+[approach-reentrant-lock]: https://exercism.org/tracks/java/exercises/bank-acconuunt/approaches/reentrant-lock
+[approach-synchronized-methods]: https://exercism.org/tracks/java/exercises/bank-account/approaches/synchronized-methods
+[approach-synchronized-statements]: https://exercism.org/tracks/java/exercises/bank-account/approaches/synchronzied-statements