How does OCP help avoid bugs when making changes?

OCP helps avoid bugs when making changes because new behavior is added through extension rather than by changing already working code. This reduces the chance of unintentionally breaking existing logic.

In detail:

#1. Stable code is left untouched

Bugs most often appear when a developer changes a part of the system that has already been tested. If a module is closed for modification, its internal logic remains unchanged, so there is no risk of introducing an error there.

#2. New requirements are implemented in isolated modules

When extension happens through new classes or implementations, changes are concentrated only in the new area of code. Old behavior is not touched and is not put at risk.

#3. Fewer entry points where an error can occur

When OCP is violated, adding new behavior requires edits in several places, including the central code (for example, long if/else chains). When OCP is followed, only one zone changes - a new interface implementation or strategy.

#4. Testing becomes simpler

A new functional block can be tested separately, without retesting the whole system. Stable code remains covered by earlier tests, and the new code is covered separately, which reduces the chance of missing a bug.

#5. The cascade effect is eliminated

If extension forces existing code to be modified, errors can propagate further through the chain of dependencies. When OCP is followed, dependencies stay the same, and there is no cascade effect.

Summary: OCP reduces the number of bugs because new behavior is introduced without changing old code, and bugs mostly appear where old code gets rewritten.