ExampleDifficulty: Intermediate1 min

Polymorphism Example: One Loop Drawing Three Shapes

A loop that draws a circle, a square and a line without knowing which is which.

Polymorphism means one call runs different code depending on the object: a loop over a list of a parent type calls each object's own override, its replacement for the parent's method, with no if and no type names. Three classes that each draw themselves, and a loop that draws all of them:

java

abstract class Shape { abstract void draw(); }

class Circle extends Shape { void draw() { System.out.println("circle"); } }
class Square extends Shape { void draw() { System.out.println("square"); } }
class Line   extends Shape { void draw() { System.out.println("line"); } }

java

List<Shape> shapes = List.of(new Circle(), new Square(), new Line());

for (Shape s : shapes) {
    s.draw();
}

text

circle
square
line

The loop variable is declared Shape and the objects are three different classes. Line 4 runs a different body on every pass, and the loop contains no if, no type name and no test.

The list is typed List<Shape>, which is what makes the loop legal, and the objects inside it keep their real classes. The declared type decided what could be called; the actual object decided what ran.

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