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 t

Answer 1

That's not really a strong example, but here's what your code would look like.

Square c1 = new Square();
Shape p1 = c1;

(given that Square extends Shape of course)

Much better isn't it?

As for "why is it better", the other answers give out some important points.