Review:   a subclass object can perform all actions of a superclass object

It is safe to use a superclass reference variable to reference to a subclass object:

Because any request (action) that you make using a superclass variable a can be satisfied

Upcasting: converting a subclass type to a superclass type

  • Upcasting = casting (= converting) a subclass reference into a superclass reference

    Example: 

      GeometricObject a;         // a is a superclass reference type

  Circle b = new Circle();   // b is a subclass reference type

  a = (GeometricObject) b;   // Performs an upcasting operation

  a = b;   // We can omit the cast operator because upcasting is safe

  • Recall from the last slide that:

    • Upcasting is a safe operation

    because:

      • We can make fewer requests using a superclass variable a than using a subclass variable b

DEMO: demo/04-inheritance/21-casting/Demo.java --- compile and show no error

Review:   a superclass object may perform fewer actions than a subclass object

It is unsafe to use a subclass reference variable to reference to a superclass object:

E.g.: user can make the illegal request to execute a.method3() (the superclass object cannot perform method3())

Downcasting: converting a superclass type to a subclass type

  • Downcasting = casting (= converting) a superclass reference into a subclass reference

    Example: 

     GeometricObject a = new GeometricObject("red"); // superclass reference

 Circle b;                                       // subclass reference

 b = (Circle) a;   // Performs an downcasting operation 

 b = a;            // Illegal to perform downcasting without cast operator

  • Recall from the previous slide that:

    • Downcasting is a unsafe operation

    because:

      • We may make an illegal request using a subclass variable b

DEMO: demo/04-inheritance/21-casting/Demo2.java --- compile and show compile error

Safe downcasting operations

  • However.... there are situations where downcasting is safe (and neccessary)

  • The most common situation is:

      1. We first performed an upcasting operation

      2. Later, we want to perform an action defined in the subclass

  • Example: 

          public static void main(String[] args)
    {
        GeometricObject a = new Circle("red", 1); 
                                           // (1) Upcasting

        System.out.println(a.getArea());
        
        System.out.println(a.getRadius()); // (2) Error ! 
    }

    We cannot invoke getRadius() using a GeometricObject variable a !!

DEMO: demo/04-inheritance/21-casting/Demo3.java

Safe downcasting operations

  • How to perform a safe downcasting operation: 

          public static void main(String[] args)
    {
        GeometricObject a = new Circle("red", 1); 
                                          // Upcasting

        System.out.println(a.getArea());
        
	Circle b;

	b = (Circle) a;

        System.out.println(a.getRadius()); // Error ! 
    }

    (0) The variable a ( GeometricObject type) does not have a getRadius() method

Safe downcasting operations

  • How to perform a safe downcasting operation: 

          public static void main(String[] args)
    {
        GeometricObject a = new Circle("red", 1); 
                                          // Upcasting

        System.out.println(a.getArea());
       
	Circle b;
 
	b = (Circle) a;

        System.out.println(b.getRadius()); // OK ! 
    }

    (1) We must use a Circle typed variable to invoke the getRadius() method

Safe downcasting operations

  • How to perform a safe downcasting operation for Circle objects 

          public static void main(String[] args)
    {
        GeometricObject a = new Circle("red", 1); 
                                          // Upcasting

        System.out.println(a.getArea());
       
	Circle b;
 
	b = (Circle) a;                  // Explicit upcasting

        System.out.println(b.getRadius()); // OK ! 
    }

    (2) Because a points to a Circle object, it is safe to downcast a to a Circle reference

DEMO: demo/04-inheritance/21-casting/Demo4.java

Another example of safe downcasting operation:   downcast a Rectangle

  • How to perform a safe downcasting operation for Rectangle objects: 

          public static void main(String[] args)
    {
        GeometricObject a = new Rectangle("red", 1, 2); 
                                          // Upcasting

        System.out.println(a.getArea());
       
	Rectangle b;
 
	b = (Rectangle) a;                // Downcasting

        System.out.println(b.getWidth()); 
        System.out.println(b.getHeight()); 
    }

  • Note: we must downcast the superclass type to the correct subclass type !!!

DEMO: demo/04-inheritance/21-casting/Demo5.java
DEMO: demo/04-inheritance/21-casting/Demo5b.java (what happens if you cast to the wrong subtype)

Downcasting an non-specific subtype reference

  • Problem:

    • Suppose the superclass variable references to a subclass object of non-specific subclass...

    Example: 

        public static void main(String[] args)
    {
        if ( ... )
            GeometricObject a = new Circle("red", 1);
        else
            GeometricObject a = new Rectangle("blue", 1, 2);

        // a can reference to a Circle or a Rectangle....

	print its radius if a is a Circle
	print its width and height if a is a Rectangle
    }

  • How can we write the above program ??

The instanceof boolean condition

  • The instanceof operator: 

        objectRefVar instanceof className

         Returns true if the object referenced to by
	 objectRefVar is a object of className type
         or a subclass type of className
  
         Otherwise, returns false

  • Example: 

        if ( a instanceof Circle )
        System.out.println("a is a Circle");

    if ( a instanceof Rectangle )
        System.out.println("a is a Rectangle");

    if ( a instanceof GeometricObject )
        System.out.println("a is a GeometricObject");

DEMO: demo/04-inheritance/21-casting/Demo6.java

How to downcast a unspecified subclass type

Skeletal code to downcast an unspecified superclass type to its correct subclass types: 

    public static void main(String[] args)
    {
        GeometricObject a = (can be  a Circle or Rectangle object)
                            // Upcasting

        if ( a is a Circle )
	{


	    print its radius
	}
	else if ( a is a Rectangle )
	{


	    print its width and height

	}
        else
	    print "invalid subclass type"
    }

How to downcast a unspecified subclass type

If a references to a Circle object, then downcast (convert) and assign to a Circle variable: 

    public static void main(String[] args)
    {
        GeometricObject a = (can be  a Circle or Rectangle object)
                            // Upcasting

        if ( a instanceof Circle )
	{
	    Circle b = (Circle) a; // Downcast to a Circle

	    System.out.println(b.getRadius());
	}
	else if ( a is a Rectangle )
	{


	    print its width and height

	}
        else
	    print "invalid subclass type"
    }

How to downcast a unspecified subclass type

If a references to a Rectangle object, then downcast (convert) and assign to a Rectangle variable: 

    public static void main(String[] args)
    {
        GeometricObject a = (can be  a Circle or Rectangle object)
                            // Upcasting

        if ( a instanceof Circle )
	{
	    Circle b = (Circle) a; // Downcast to a Circle

	    System.out.println(b.getRadius());
	}
	else if ( a instanceof Rectangle  )
	{
	    Rectangle b = (Rectangle) a; // Downcast to a Rectangle

	    System.out.println(b.getWidth());
	    System.out.println(b.getHeight());
	}
        else
	    print "invalid subclass type"
    }

How to downcast a unspecified subclass type

Otherwise (it can be a GeometricObject !), print the warning message: 

    public static void main(String[] args)
    {
        GeometricObject a = (can be  a Circle or Rectangle object)
                            // Upcasting

        if ( a instanceof Circle )
	{
	    Circle b = (Circle) a; // Downcast to a Circle

	    System.out.println(b.getRadius());
	}
	else if ( a instanceof Rectangle  )
	{
	    Rectangle b = (Rectangle) a; // Downcast to a Rectangle

	    System.out.println(b.getWidth());
	    System.out.println(b.getHeight());
	}
        else
	    System.out.println("Invalid subclass type");
    }

Demo demo/04-inheritance/21-casting/Demo7.java

More examples of the instanceof boolean condition

  • Consider the following inheritance hierachy: 

      Cat a;     a instanceof Cat    --> true    a instanceof Dog    --> false
             a instanceof Animal --> true    a instanceof Object --> true

  Animal b;  b instanceof Cat    --> false   a instanceof Dog    --> false
             b instanceof Animal --> true    b instanceof Object --> true