Comp 401 Assertions Practical Example and The Importance of Being Earnest

1. Comp 401Assertions Practical Example and The Importance of Being Earnest Instructor: Prasun Dewan

2. Prerequisite • Assertions (Preconditions and Postconditions)

3. Bank Account with Withdraw publicclassABankAccountimplementsBankAccount { intcurrentBalance = 0; publicstaticfinalint MIN_BALANCE = 100; publicABankAccount (intinitialBalance) { currentBalance= initialBalance; } publicintgetCurrentBalance () {returncurrentBalance; } publicvoid deposit (int amount) {currentBalance+= amount;} publicboolean withdraw (int amount) { intminNecessaryBalance = MIN_BALANCE + amount; if (minNecessaryBalance <= currentBalance) { currentBalance -= amount; returntrue; } elsereturnfalse; } }

4. Using Withdraw

5. Withdrawing one Dollar: Not Allowed Because of Min Balance requirement

6. Integer.MAX_INT Withdrawing a Large Amount: Allowed

7. Resulting Balance

8. Integer Overflow Most significant bit of positive (negative) numbers is 0(1)

9. Asserting Withdraw with Pre and Post Conditions publicbooleansafeWithdraw (int amount) { assert amount > 0: "amount < 0"; booleanretVal = withdraw(amount); assertcurrentBalance>= MIN_BALANCE: "currentBalance < MIN_BALANCE"); returnretVal; }

10. Using Safe Withdraw with MaxInt: The Importance of Being Earnest

11. Google for “integer overflow security” Integer Overflow in Real Life