Rewrite Legacy Code

Contributed by: Maximiliano Contieri

Difficulty level: Intermediate

Legacy code can be a significant challenge in any project. It often lacks tests, contains obsolete logic, and is difficult to maintain. The urge to rewrite such code from scratch can be strong, but it’s essential to take a structured approach to ensure that you don’t introduce new bugs or lose the original business logic.

Understand and Cover the Current Behavior

Before changing any line of legacy code, you need to understand its current behavior. This means thoroughly investigating what the system does. The first step is to write tests around the existing functionality.

Here’s a Java example on the banking domain:

public class TransactionService {
    public void move(Account first, Account second, double amount) {
        if (first.getBalance() < amount) {
            throw new InsufficientFundsException("Not enough funds in source account");
        }
        first.debit(amount);
        second.credit(amount);
    }
}

In this code, the logic transfers funds between two bank accounts. Although simple, imagine this is embedded within hundreds of other lines, making it hard to isolate.

You can start by writing tests for this behavior without actually changing the legacy code in the same way you do when addressing bugs (see “Fix Bugs in a Continuous Integration Pipeline” in this Shortcut series):

class TransactionServiceTest {

    @Test
    void shouldTransferFundsBetweenAccounts() {
        Account source = new Account(1000.0);
        Account target ...