Use Switches when you're testing on values of primitives. (ie. integers or characters).
Use polymorphism when you are choosing between different types.
Examples :
Testing whether a character the user has entered is one of 'a', 'b' or 'c' is a job for a switch.
Testing whether the object you're dealing with is a Dog or Cat is a job for polymorphic dispatch.
In many languages, if you have more complicated values you may not be able to use Switch anyway.
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