Unbounded parameter type

  • An unbounded generic type parameter T is specified as follows: 

        <T>           ( ≡ <T extends Object> )

    Meaning:

    • You can use any object (reference) type to make the parameter type T into a specific type

  • When an unbounded generic type parameter T is used in a generic class definition:

    • The type parameter <T> is replaced by: Object

  • However, sometimes the class Object is inappropriate as the parent class

    Example:

    • Object does not has certain required method(s) used in the code

Bounded parameter type

  • An bounded generic type parameter T is specified as follows: 

        <T extends SuperClass>

    Meaning:

    • You can only use a sub-type (is-a) of SuperClass to make T into a specific type

  • When a bounded generic type parameter T is used in a generic class definition:

    • The type parameter <T> is replaced by the bounding type !

      (Instead of Object)

  • The use of bounded type parameter is necessary when you have used a method in your code that is not defined in the Object class

    • We will see an example next

Example of defining a method with a bounded generic type

  • Consider the method smallerArea( ) that compares the area of 2 Circle objects: 

       public static boolean smallerArea( Circle o1, Circle o2 )
   {
       return o1.getArea() < o2.getArea();
   }

  • We will need another method to compares the area of 2 Rectangle objects: 

       public static boolean smallerArea( Rectangle o1, Rectangle o2 )
   {
       return o1.getArea() < o2.getArea();
   }

  • We will need another 2 method to compares the area of Circle objects and Rectangle objects: 

       public static boolean smallerArea( Circle o1, Rectangle o2 ) ...

   public static boolean smallerArea( Rectangle o1, Circle o2 ) ...

DEMO: demo/06-generics/04-bounded/Demo.java (comment a method out and recompile !)

Example of defining a method with a bounded generic type

  • We can try to use an unbounded generic method to define smallerArea( ): 

       public static <T> boolean smallerArea( T o1, T o2 )
   {
       return o1.getArea() < o2.getArea();
   }

    but it will produce a compile error: 

    Demo2.java:21: error: cannot find symbol
        return o1.getArea() < o2.getArea();
                 ^
  symbol:   method getArea()

  • Reason: the Java compiler has replaced <T> with: Object: 

       public static Object boolean smallerArea( Object o1, Object o2 )
   {
       return o1.getArea() < o2.getArea(); 
              // ***** Object does not have getArea( ) *****
   }

DEMO: demo/06-generics/04-bounded/Demo2.java

Example of defining a method with a bounded generic type

  • Solution: we limit the type parameter with a bounded generic parameter of the class GeometricObject: 

       public static <T extends GeometricObject> boolean smallerArea( T o1, T o2 )
   {
       return o1.getArea() < o2.getArea();
   }

  • This method will only accept arguments that are subclasses of GeometricObject (that has the getArea() method):

    Example: 

            Circle c1 = new Circle("red", 1);
        Rectangle r1 = new Rectangle("red", 1, 1);

        System.out.println( smallerArea( c1, c1 ) );
        System.out.println( smallerArea( r1, r1 ) );
        System.out.println( smallerArea( c1, r1 ) );

DEMO: demo/06-generics/04-bounded/Demo4.java

Looking under the hood of the Java compiler

  • The generic method with a bounded type parameter: 

       public static <T extends GeometricObject> boolean smallerArea( T o1, T o2 )
   {
       return o1.getArea() < o2.getArea();
   }

    is translated by the Java compiler by replacing the type paramter by the bounding class as follows: 

       public static boolean smallerArea( GeometricObject o1, GeometricObject o2 )
   {
       return o1.getArea() < o2.getArea();
   }

    There is no compile error because the GeometricObject class has the getArea() method defined !!!

  • For more deails, see:

  Quiz 1  

  • If the class MyClass is defined as follows: 

    public class MyClass <T extends Circle, E> implements ComparableThings
{
   ....
}

    which of the following instantiations are allowed?

    1. MyClass<Double, String> a = new MyClass<>(...);

    2. MyClass<String, Double> a = new MyClass<>(...);

    3. MyClass<Rectangle, Circle> a = new MyClass<>(...);

    4. MyClass<Circle, Rectangle> a = new MyClass<>(...);

    5. A and B

    6. C and D

  Quiz 1 - answer  

  • If the class MyClass is defined as follows: 

    public class MyClass <T extends Circle, E> implements ComparableThings
{
   ....
}

    which of the following instantiations are allowed?

    1. MyClass<Double, String> a = new MyClass<>(...);

    2. MyClass<String, Double> a = new MyClass<>(...);

    3. MyClass<Rectangle, Circle> a = new MyClass<>(...);

    4. MyClass<Circle, Rectangle> a = new MyClass<>(...);

    5. A and B

    6. C and D

  Quiz 2  

  • If the class MyClass is defined as follows: 

    public class MyClass <T extends GeometricObject, E> implements ComparableThings
{
   ....
}

    which of the following instantiations are allowed?

    1. MyClass<Double, String> a = new MyClass<>(...);

    2. MyClass<String, Double> a = new MyClass<>(...);

    3. MyClass<Rectangle, Circle> a = new MyClass<>(...);

    4. MyClass<Circle, Rectangle> a = new MyClass<>(...);

    5. A and B

    6. C and D

  Quiz 2 - answer  

  • If the class MyClass is defined as follows: 

    public class MyClass <T extends GeometricObject, E> implements ComparableThings
{
   ....
}

    which of the following instantiations are allowed?

    1. MyClass<Double, String> a = new MyClass<>(...);

    2. MyClass<String, Double> a = new MyClass<>(...);

    3. MyClass<Rectangle, Circle> a = new MyClass<>(...);

    4. MyClass<Circle, Rectangle> a = new MyClass<>(...);

    5. A and B

    6. C and D

Postscript

  • There is a lot more material on generic class in the text book

  • These topic is outside the scope of this course.

  • If interested in this topic, please study the remaining sections (19.5 - 19.10) on your own and ask me if you have questions during my office hours.