instanceof keyword usage

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Is using the instanceof keyword against the essence of object oriented programming? I mean is it a bad programming practice? I read somewhere that us

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心在旅途

2021-02-14 11:09

There are many good answers promoting virtual methods, but instanceof has its uses as well. Imagine that you iterate over List<Event>, to pick up all Urgent objects. You might do it using isUrgent() but I am not sure if it were necessarily more concise or readable. Also, isUrgent() would require making Event aware that its subclasses may possess the respective property, which might:
be regarded as something against modularity principles;
be even impossible, if Event belongs to some library that can not be modified.

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青春惊慌失措

2021-02-14 11:17

It's discouraged because people might use it to do something like this:

if( myAnimal instanceof Dog )
    ((Dog)myAnimal).bark();
else( myAnimal instanceof Cat )
    ((Cat)myAnimal).meow();

Instead, Animal should have a speak() method which Dog and Cat inherit. In proper OOP with polymorphism and dynamic binding, you would then simply do

myAnimal.speak();

However, there are some instances in which you must use instanceof to determine the specific type of an object. Perhaps you have a list of Animals in your house and the only ones you want to take out for a walk() are the Dogs. In that case you would iterate through your list and only walk() the dogs.

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