Runtime polymorphism

Figure 515. Meta info of an instance Slide presentation
  • Sd1/Inheritance/Fig/objectmeta-0
  • Sd1/Inheritance/Fig/objectmeta-1
  • Sd1/Inheritance/Fig/objectmeta-2
‹›


Figure 516. Getting instance meta info Slide presentation
Code 
final Object[] instances = {
  new String("Test"),
  new Date()
};
printMetaInfo(instances);
-----------------------------------------------------    
static void printMetaInfo(Object[] objects) {
  for (Object o: objects) {
    System.out.println(o.getClass().getName());
  }
}
Result 
java.lang.String
java.util.Date

Figure 517. Implementing getArea() Slide presentation 
public abstract class Shape {
...
    abstract public double getArea();
}
 
public class Rectangle extends Shape {
    ...
  private int width, height;
  @Override public double getArea() {
        return width * height; }
}
 
public class Circle extends Shape {
    ...
    private int radius;

    @Override public double getArea() {
        return Math.PI * radius * radius;}
}

Figure 518. Polymorphic getArea() call Slide presentation
Code 
final Shape[] shapes = {
  new Rectangle(0, 0, 2, 3),
  new Circle(0, 0, 1)};
}
printAreas(shapes);
----------------------------------------
static void printAreas(Shape[] shapes) {
  for (Shape shape : shapes) {
    System.out.println(shape.getArea());
  }
}
Result 
Area:6.0
Area:3.141592653589793

Figure 519. Runtime polymorphic dispatch Slide presentation
  • Sd1/Inheritance/Fig/polyDispatch-0
  • Sd1/Inheritance/Fig/polyDispatch-1
  • Sd1/Inheritance/Fig/polyDispatch-2
‹›


public abstract class Shape {
...
public final double getArea(){
  if (this instanceof Rectangle rectangle) {
    return rectangle.width * rectangle.height;
  } else if (this instanceof Circle circle) {
    return circle.radius * circle.radius * Math.PI;
  } else {
    return 0; // Unreachable
  }
}
 
public class Rectangle extends Shape {
    ...
  protected int width, height;
}
 
public class Circle extends Shape {
    ...
    protected int radius;
}
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