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

if take code below:

shape p1 = new square(); square c1; if(p1 instanceof square) {   c1 = (square) p1; } 

what mean prefer polymorphism instanceof, , incidentally, why better?

edit: understand polymorphism is; i'm missing how 1 use rather instanceof.

the main difference between if...else... (or switch, or visitor), , between polymorphism modularity. there's called open-closed principle, means, when add new feature existing program, less changes make in existing code better (because every change requires work, , may introduce bugs). let's compare amount of changes:

• adding new method (eg. have paint(), , getarea(), let's add getcircumference()): if-else solution have alter 1 file - file contain new method. polymorphism, have alter implementations of shape class.

• adding new kind of shape (you have square, circle - let's add triangle): if-else solution have review existing classes if-else , add new if branch triangle; polymporphism have add new class , implement required methods in it.

so if...else... or polymorphism: depends on modularity. if expect many new sublasses added later, use polymorphism; if expect many new methods added later, use if...else..., , in class put "basic" methods accessors. or in other words: when expect have many if...else... branches, should rather use polymorphism, when expect few such branches, stay if...else...

additionally: when expect few if...else... branches, in lots of places, should consider encapsulating if...else... visitor pattern or making enum separate case each branch.