Good question.

If you allow, let me answer it by giving a simple example. Say we have a class that represents date, for example 2003-11-07, and one of its property is month.

• If I make this property public, someone uses this class, can easily set month to 13. Well, this could be well valid for some calendars, but not for this calendar that most of us use. So we opened up a hole for error by make it public. To avoid this error, we have to implement the checking logic in every single piece of code uses this class.
• Now if we make it private, and in the setter, we check whether month is less than or equal to 12, we only need to implement this logic once for good.

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