Assertions & Invariants

Assertions & Invariants

Assertions are Boolean expressions that must be true at specific points in the code. They serve as a debugging aid by catching logic errors early, before they cause downstream problems.

Types of Assertions

  +---------------------------------------------------------+
  |              ASSERTION TYPES                             |
  +---------------------------------------------------------+
  |                                                         |
  |   PRECONDITIONS                                         |
  |   - Must be true BEFORE a method executes               |
  |   - Validates input assumptions                         |
  |   - Example: assert age >= 0                            |
  |                                                         |
  |   POSTCONDITIONS                                        |
  |   - Must be true AFTER a method executes                |
  |   - Validates the result                                |
  |   - Example: assert result >= 0                         |
  |                                                         |
  |   INVARIANTS                                            |
  |   - Must be true at all times during object lifetime    |
  |   - True after construction and after every method      |
  |   - Example: assert list.size() >= 0                   |
  |                                                         |
  +---------------------------------------------------------+

Using Assertions in Code

  public class BankAccount {
      private double balance;
      private boolean isActive;

      public void withdraw(double amount) {
          // Precondition
          assert amount > 0 : "Withdrawal amount must be positive";
          assert isActive : "Account must be active";
          assert amount <= balance : "Insufficient funds";

          balance -= amount;

          // Postcondition
          assert balance >= 0 : "Balance cannot go negative";

          // Invariant
          assert balance >= 0 : "Invariant: balance non-negative";
      }
  }

When to Use Assertions

• Use for programmer errors — not user input validation
• Use to document assumptions that should always be true
• Use to catch impossible states in production code
• Don't use for recoverable errors or input validation

Key Takeaways

• Assertions catch bugs early when they are cheapest to fix
• Preconditions, postconditions, and invariants form a contract
• Use assertions to document code assumptions
• Disable assertions in production for performance