How does one use polymorphism instead of instanceof? (And why?)

Question

If we take the code below:

Shape p1 = new Square();
Square c1;
if(p1 instanceof Square) {
  c1 = (Square) p1;
}

What does it mean to prefer polymorphism to instanceof, and incidentally, why is it better?

Edit: I understand what polymorphism is; what I'm missing is how one would use it rather than instanceof.

Answer 1

The main difference between if...else... (or switch, or Visitor), and between polymorphism is modularity. There's so called open-closed principle, which basically means, that when you add a new feature to an existing program, the less changes you make in existing code the better (because every change requires some work, and may introduce bugs). So let's compare the amount of changes:

• adding a new method (eg. you have paint(), and getArea(), let's add getCircumference()): with if-else solution you only have to alter just one file - the file which will contain the new method. With polymorphism, you have to alter all your implementations of Shape class.

• adding a new kind of Shape (you have Square, Circle - let's add Triangle): with if-else solution you have to review all existing classes with if-else and add a new if branch for Triangle; with polymporphism all you have is to add a new class and implement all required methods in it.

So if...else... or polymorphism: it depends on modularity. If you expect that many new sublasses will be added later, use polymorphism; if you expect that many new methods will be added later, use if...else..., and in the class put only the most "basic" methods like accessors. Or in other words: when you expect to have many if...else... branches, you should rather use polymorphism, when you expect few such branches, just stay with if...else...

Additionally: when you expect few if...else... branches, but in lots of places, you should consider encapsulating this if...else... with Visitor pattern or just making an enum with a separate case for each branch.

Answer 2

The idea is that you shouldn't have to care what kind of shape you are dealing with. For example, if Shape defines an abstract draw() method, then Triangles, Squares and anything else that extends Shape will also have the same method.

A simple definition for polymorphism is "treating different types as if they are the same", i.e. using the same interface.

In an ideal world, we don't want to have to worry about what specific type of object we are dealing with, only the interface of a more general type that covers all usage scenarios in its interface.

Shape p1 = new Square();
Shape p2 = new Triangle();
p1.draw();
p2.draw();

In this code, we are directly calling Shape.draw() on p1 and p2. We don't care what the implementation class does, only what is defined by the interface (or abstract parent class).

Edit: Regarding the example in the question, It is generally recommended to avoid that kind of code pattern by encapsulating behaviour where possible. Using instanceof can be considered a code smell, as you would have to update all your conditionals whenever you add a new class.