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Refactoring should not change behavior, but legacy code often has no tests proving what its behavior even is. Characterization tests solve this.

A characterization test documents the code as it actually behaves, not as it should behave. It pins current output so any change becomes visible.

A seam is a place you can call into the code without running the whole app. Public methods on a class are the easiest seams.

Call the method with known inputs and record whatever it returns today, even if it looks wrong.

Turn the observed value into an assertion. This test now fails if behavior changes.

Add tests for empty input, zero tax, and large values. The goal is to surround the code so the refactor has a safety net.

If a method touches the network or disk, introduce a protocol seam and inject a fake so the test is fast and deterministic.

With tests green, refactor in small steps, re-running tests after each change. Any red means you changed behavior and must reconsider.

A characterization test may pin a bug. That is fine: first preserve it, then fix it as a separate, deliberate change with its own test.

Once proper unit tests describe the desired behavior, you can retire characterization tests that merely pinned legacy quirks.

You do not need full coverage at once. Add characterization tests only around the code you are about to touch; the net grows as you refactor.

Test your characterization testing knowledge.

You learned to find seams, capture and pin current behavior, break dependencies with protocols, refactor under green, and grow a test net incrementally around legacy Objective-C.