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diff --git a/bank_extension_class_diagram.png b/bank_extension_class_diagram.png
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diff --git a/src/main/java/com/booleanuk/core/BankAccount.java b/src/main/java/com/booleanuk/core/BankAccount.java
new file mode 100644
index 000000000..a7c818bf8
--- /dev/null
+++ b/src/main/java/com/booleanuk/core/BankAccount.java
@@ -0,0 +1,96 @@
+package com.booleanuk.core;
+
+import java.time.LocalDateTime;
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Random;
+
+public abstract class BankAccount {
+    private final String accountNumber;
+    private final String branchNumber;
+    private final List<Transaction> transactions;
+
+    public BankAccount(int branchNumber) {
+        Random random = new Random();
+        int randomAccountNumber = random.nextInt(99999999);
+        this.accountNumber = "0".repeat((int) (8 - Math.ceil(Math.log10(randomAccountNumber)))) + randomAccountNumber;
+        if (branchNumber > 999999) {
+            System.out.println("Invalid branch code (must be 6 digit number). Branch code will be set to 0.");
+            branchNumber = 0;
+        }
+        this.branchNumber = "0".repeat((int) (6 - Math.ceil(Math.log10(branchNumber)))) + branchNumber;
+        this.transactions = new ArrayList<>();
+    }
+
+    public BankAccount(String accountNumber, String branchNumber) {
+        // todo: make sure numbers are OK length
+        this.accountNumber = accountNumber;
+        this.branchNumber = branchNumber;
+        this.transactions = new ArrayList<>();
+    }
+
+    public boolean withdraw(double amount, LocalDateTime dateTime) {
+        double balance = this.getBalance();
+        if (amount <= balance) {
+            balance -= amount;
+            transactions.add(new Transaction(-amount, balance, dateTime));
+        }
+        return amount <= balance;
+    }
+
+    public boolean withdraw(double amount) {
+        return this.withdraw(amount, LocalDateTime.now());
+    }
+
+    // maybe can be void... or maybe consider overflow?
+    public boolean deposit(double amount, LocalDateTime dateTime) {
+        double balance = this.getBalance();
+        balance += amount;
+        transactions.add(new Transaction(amount, balance, dateTime));
+        return true;
+    }
+
+    public boolean deposit(double amount) {
+        return this.deposit(amount, LocalDateTime.now());
+    }
+
+    // TODO Fix formatting
+    public String generateBankStatement() {
+        StringBuilder sb = new StringBuilder();
+        sb.append("date\t||credit\t||debit\t||balance\n");
+
+        for (Transaction t : transactions.reversed()) {
+            sb.append(t.getDateTime().getDayOfMonth()).append("/")
+                    .append(t.getDateTime().getMonthValue()).append("/")
+                    .append(t.getDateTime().getYear()).append("||");
+            if (t.getAmount() > 0) {
+                sb.append(String.format("%.2f", Math.abs(t.getAmount()))).append("||\t");
+            } else {
+                sb.append("\t||").append(String.format("%.2f", Math.abs(t.getAmount())));
+            }
+            sb.append("||").append(String.format("%.2f", t.getCurrentBalance())).append("\n");
+        }
+
+        return sb.toString();
+    }
+
+    public String getAccountNumber() {
+        return accountNumber;
+    }
+
+    public double getBalance() {
+        double balance = 0;
+        for (Transaction t : transactions) {
+            balance += t.getAmount();
+        }
+        return balance;
+    }
+
+    public List<Transaction> getTransactions() {
+        return transactions;
+    }
+
+    public String getBranchNumber() {
+        return branchNumber;
+    }
+}
diff --git a/src/main/java/com/booleanuk/core/BankSystem.java b/src/main/java/com/booleanuk/core/BankSystem.java
new file mode 100644
index 000000000..34fd687dd
--- /dev/null
+++ b/src/main/java/com/booleanuk/core/BankSystem.java
@@ -0,0 +1,41 @@
+package com.booleanuk.core;
+
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+
+public class BankSystem {
+    private List<OverdraftRequest> overdraftRequests;
+    private Map<String, SavingsAccount> savingsAccounts;
+    private Map<String, CurrentAccount> currentAccounts;
+
+    public BankSystem() {
+        overdraftRequests = new ArrayList<>();
+        savingsAccounts = new HashMap<>();
+        currentAccounts = new HashMap<>();
+    }
+
+    // maybe void?
+    public boolean approve(OverdraftRequest or) {
+        return savingsAccounts.get(or.accountNumber + or.branchNumber).setMaxOverdraft(or.getAmount());
+    }
+
+    public void addCurrentAccount(CurrentAccount b) {
+        currentAccounts.put(b.getAccountNumber() + b.getBranchNumber(), b);
+    }
+
+    public void addSavingsAccount(SavingsAccount b) {
+        savingsAccounts.put(b.getAccountNumber() + b.getBranchNumber(), b);
+    }
+
+    public Map<String, BankAccount> getBankAccounts() {
+        Map<String, BankAccount> bankAccounts = new HashMap<>(currentAccounts);
+        bankAccounts.putAll(savingsAccounts);
+        return bankAccounts;
+    }
+
+    public List<OverdraftRequest> getOverdraftRequests() {
+        return overdraftRequests;
+    }
+}
diff --git a/src/main/java/com/booleanuk/core/CurrentAccount.java b/src/main/java/com/booleanuk/core/CurrentAccount.java
new file mode 100644
index 000000000..46fa2f528
--- /dev/null
+++ b/src/main/java/com/booleanuk/core/CurrentAccount.java
@@ -0,0 +1,11 @@
+package com.booleanuk.core;
+
+public class CurrentAccount extends BankAccount {
+    public CurrentAccount() {
+        super(1);
+    }
+
+    public CurrentAccount(int branchNumber) {
+        super(branchNumber);
+    }
+}
diff --git a/src/main/java/com/booleanuk/core/OverdraftRequest.java b/src/main/java/com/booleanuk/core/OverdraftRequest.java
new file mode 100644
index 000000000..3d6c915d4
--- /dev/null
+++ b/src/main/java/com/booleanuk/core/OverdraftRequest.java
@@ -0,0 +1,25 @@
+package com.booleanuk.core;
+
+public class OverdraftRequest {
+    String accountNumber;
+    String branchNumber;
+    double amount;
+
+    public OverdraftRequest(String accountNumber, String branchNumber, double amount) {
+        this.accountNumber = accountNumber;
+        this.branchNumber = branchNumber;
+        this.amount = amount;
+    }
+
+    public String getAccountNumber() {
+        return accountNumber;
+    }
+
+    public String getBranchNumber() {
+        return branchNumber;
+    }
+
+    public double getAmount() {
+        return amount;
+    }
+}
diff --git a/src/main/java/com/booleanuk/core/SavingsAccount.java b/src/main/java/com/booleanuk/core/SavingsAccount.java
new file mode 100644
index 000000000..e26dace9c
--- /dev/null
+++ b/src/main/java/com/booleanuk/core/SavingsAccount.java
@@ -0,0 +1,49 @@
+package com.booleanuk.core;
+
+import java.time.LocalDateTime;
+
+public class SavingsAccount extends BankAccount {
+    private double maxOverdraft;
+
+    public SavingsAccount(String accountNumber, String branchNumber) {
+        super(accountNumber, branchNumber);
+        this.maxOverdraft = 0;
+    }
+
+    public SavingsAccount(int branchNumber) {
+        super(branchNumber);
+        this.maxOverdraft = 0;
+    }
+
+    @Override
+    public boolean withdraw(double amount, LocalDateTime dateTime) {
+        if (getBalance() - amount < - maxOverdraft) {
+            return false;
+        }
+        getTransactions().add(new Transaction(-amount, getBalance() - amount, dateTime));
+        return true;
+    }
+
+    @Override
+    public boolean withdraw(double amount) {
+        return this.withdraw(amount, LocalDateTime.now());
+    }
+
+    // probably won't be used, but useful in theory
+    public boolean hasOverdraft() {
+        return maxOverdraft > 0;
+    }
+
+    public double getMaxOverdraft() {
+        return maxOverdraft;
+    }
+
+    public boolean setMaxOverdraft(double maxOverdraft) {
+        if (maxOverdraft < 0) {
+            System.out.println("Cannot set negative overdraft.");
+            return false;
+        }
+        this.maxOverdraft = maxOverdraft;
+        return true;
+    }
+}
diff --git a/src/main/java/com/booleanuk/core/Transaction.java b/src/main/java/com/booleanuk/core/Transaction.java
new file mode 100644
index 000000000..3f19c6e62
--- /dev/null
+++ b/src/main/java/com/booleanuk/core/Transaction.java
@@ -0,0 +1,27 @@
+package com.booleanuk.core;
+
+import java.time.LocalDateTime;
+
+public class Transaction {
+    private final double amount;
+    private final double currentBalance;
+    private final LocalDateTime dateTime;
+
+    public Transaction(double amount, double currentBalance, LocalDateTime dateTime) {
+        this.amount = amount;
+        this.currentBalance = currentBalance;
+        this.dateTime = dateTime;
+    }
+
+    public double getAmount() {
+        return amount;
+    }
+
+    public double getCurrentBalance() {
+        return currentBalance;
+    }
+
+    public LocalDateTime getDateTime() {
+        return dateTime;
+    }
+}
diff --git a/src/test/java/com/booleanuk/core/TestBankAccount.java b/src/test/java/com/booleanuk/core/TestBankAccount.java
new file mode 100644
index 000000000..16debf23b
--- /dev/null
+++ b/src/test/java/com/booleanuk/core/TestBankAccount.java
@@ -0,0 +1,142 @@
+package com.booleanuk.core;
+
+import static org.junit.jupiter.api.Assertions.*;
+import org.junit.jupiter.api.Test;
+
+import java.time.LocalDateTime;
+
+public class TestBankAccount {
+    @Test
+    public void newAccountInstancesAreValid() {
+        BankAccount currentAccount = new CurrentAccount(999999);
+        assertEquals(8, currentAccount.getAccountNumber().length());
+        assertEquals(6, currentAccount.getBranchNumber().length());
+        assertEquals(0, currentAccount.getBalance());
+        assertEquals(0, currentAccount.getTransactions().size());
+    }
+    @Test
+    public void withdrawTooMuchShouldFail() {
+        BankAccount currentAccount = new CurrentAccount();
+        double amount = 1000;
+        assertFalse(currentAccount.withdraw(amount));
+    }
+
+    @Test
+    public void canWithdrawWithinReason() {
+        BankAccount account = new CurrentAccount();
+        account.deposit(1000);
+        assertTrue(account.withdraw(500));
+    }
+
+    @Test
+    public void withdrawingRemovesMoney() {
+        BankAccount account = new CurrentAccount();
+        account.deposit(1000);
+        assertTrue(account.withdraw(500));
+        assertEquals(500, account.getBalance());
+    }
+    @Test
+    public void withdrawingRemovesMoneyOnlyIfValidAmount() {
+        BankAccount account = new CurrentAccount();
+        account.deposit(1000);
+        assertFalse(account.withdraw(2000));
+        assertEquals(1000, account.getBalance());
+    }
+
+    @Test
+    public void depositAddsTheRightAmount() {
+        BankAccount account = new CurrentAccount();
+        assertTrue(account.deposit(1000));
+        assertEquals(1000, account.getBalance());
+    }
+
+    @Test
+    public void transactionsAreStored() {
+        BankAccount account = new CurrentAccount();
+        assertTrue(account.deposit(1000));
+        assertTrue(account.withdraw(100));
+        assertEquals(2, account.getTransactions().size());
+    }
+
+    @Test
+    public void transactionsAreStoredCorrectly() {
+        BankAccount account = new CurrentAccount();
+        LocalDateTime ldt = LocalDateTime.now();
+        assertTrue(account.deposit(1000));
+        assertTrue(account.withdraw(100));
+        assertEquals(2, account.getTransactions().size());
+        LocalDateTime depTime = account.getTransactions().getFirst().getDateTime();
+        LocalDateTime wdrTime = account.getTransactions().getLast().getDateTime();
+        // deposit time correct within 1 s
+        assertEquals(ldt.toLocalDate(), depTime.toLocalDate());
+        assertEquals(ldt.getHour(), depTime.getHour());
+        assertEquals(ldt.getMinute(), depTime.getMinute());
+        assertEquals(ldt.getSecond(), depTime.getSecond());
+        // withdraw time correct within 1 s
+        assertEquals(ldt.toLocalDate(), wdrTime.toLocalDate());
+        assertEquals(ldt.getHour(), wdrTime.getHour());
+        assertEquals(ldt.getMinute(), wdrTime.getMinute());
+        assertEquals(ldt.getSecond(), wdrTime.getSecond());
+        // deposit amounts correct
+        Transaction dep = account.getTransactions().getFirst();
+        assertEquals(1000, dep.getAmount());
+        assertEquals(1000, dep.getCurrentBalance());
+        // withdraw amounts correct
+        Transaction wdr = account.getTransactions().getLast();
+        assertEquals(-100, wdr.getAmount());
+        assertEquals(900, wdr.getCurrentBalance());
+    }
+
+    @Test
+    public void generateStatementOnNoTransactions() {
+        BankAccount account = new CurrentAccount();
+        assertEquals(
+                "date||credit||debit||balance",
+                account.generateBankStatement().replaceAll("\\s+",""));
+    }
+
+    @Test
+    public void generateStatementOnSomeTransactions() {
+        BankAccount account = new CurrentAccount();
+        LocalDateTime ldt1 = LocalDateTime.of(2012, 1, 14, 13, 54);
+        LocalDateTime ldt2 = LocalDateTime.of(2012, 1, 13, 22, 18);
+        LocalDateTime ldt3 = LocalDateTime.of(2012, 1, 10, 11, 27);
+        account.deposit(1000, ldt3);
+        account.withdraw(100, ldt2);
+        account.withdraw(200, ldt1);
+//        System.out.println(account.generateBankStatement());
+
+        String sb = "date||credit||debit||balance" +
+                "14/1/2012" +
+                "||||200.00||700.00" +
+                "13/1/2012" +
+                "||||100.00||900.00" +
+                "10/1/2012" +
+                "||1000.00||||1000.00".replaceAll("\\s+","");
+
+        assertEquals(sb, account.generateBankStatement().replaceAll("\\s+",""));
+    }
+
+    @Test
+    public void overdraftNeedsToBePositive() {
+        SavingsAccount account = new SavingsAccount(523611);
+        assertFalse(account.setMaxOverdraft(-50));
+    }
+
+    @Test
+    public void cannotGoBeyondOverdraftLimit() {
+        SavingsAccount account = new SavingsAccount(123456);
+        account.deposit(100);
+        assertFalse(account.withdraw(200));
+        assertEquals(100, account.getBalance());
+    }
+
+    @Test
+    public void canHaveNegativeBalanceWithinOverdraft() {
+        SavingsAccount account = new SavingsAccount(123456);
+        account.deposit(100);
+        account.setMaxOverdraft(200);
+        assertTrue(account.withdraw(200));
+        assertEquals(-100, account.getBalance());
+    }
+}
diff --git a/src/test/java/com/booleanuk/core/TestBankSystem.java b/src/test/java/com/booleanuk/core/TestBankSystem.java
new file mode 100644
index 000000000..2eba21a52
--- /dev/null
+++ b/src/test/java/com/booleanuk/core/TestBankSystem.java
@@ -0,0 +1,16 @@
+package com.booleanuk.core;
+
+import static org.junit.jupiter.api.Assertions.*;
+import org.junit.jupiter.api.Test;
+
+public class TestBankSystem {
+    @Test
+    public void approvingOverdraftRequestWorksCorrectly() {
+        BankSystem bs = new BankSystem();
+        SavingsAccount account = new SavingsAccount("12345678", "123456");
+        bs.addSavingsAccount(account);
+        OverdraftRequest or = new OverdraftRequest(account.getAccountNumber(), account.getBranchNumber(), 500);
+        bs.approve(or);
+        assertTrue(account.hasOverdraft());
+    }
+}
diff --git a/src/test/java/com/booleanuk/core/TestTransaction.java b/src/test/java/com/booleanuk/core/TestTransaction.java
new file mode 100644
index 000000000..1af8e03fc
--- /dev/null
+++ b/src/test/java/com/booleanuk/core/TestTransaction.java
@@ -0,0 +1,19 @@
+package com.booleanuk.core;
+
+import static org.junit.jupiter.api.Assertions.*;
+import org.junit.jupiter.api.Test;
+
+import java.time.LocalDateTime;
+
+public class TestTransaction {
+    @Test
+    public void canCreateTransaction() {
+        double amount = 1000;
+        double currentBalance = 100;
+        LocalDateTime dt = LocalDateTime.now();
+        Transaction t = new Transaction(amount, currentBalance + amount, dt);
+        assertEquals(1000, t.getAmount());
+        assertEquals(1100, t.getCurrentBalance());
+        assertEquals(dt, t.getDateTime());
+    }
+}