Lecture on use of Java generics

1. 1
Generics <? super Java>
By Brad Bazemore

2. 2
Create...

a box interface
− You can put into the box
− You can take out of box
− NOTE: needs to be able to handle ALL
possible types (not just primitives!)
GO!

3. 3
Generic Programing

generic programming is a style of
computer programming

written in terms of to-be-specified-later

instantiated when needed for specific
types provided as parameters

4. 4
Ex1

Given N data structures

Given M algorithms

If we want an algo for every data struct we
wind up with NM methods
− NOTE: this is BAD

Using generics we end up with only N+M
methods
− NODE: this is GOOD

5. 5
Java Generics

6. 6
What are they for?

"a type or method to operate on objects of
various types while providing compile-time type
safety." (Java Programming Language)

Basically...
− IT MAKES LIFE BETTER!!!

7. 7
QUIZ!

If my program has 3 types of data
structures and 5 types of algorithms...
1)How many methods will there be with NO
generics?
2)How many methods will there be WITH
generics?
GO!

8. 8
The Syntax

ObjectName <Type Variable>

Ex2
− ArrayList<String> L=new ArrayList<String>();

Attention Test
− Create a method signature for the put
operation for a box, and it takes a
generic of some kind
GO!

9. 9
Lo Pi Hi

Lets take it up a notch!

Ok so there is a catch, if I say <Number>
− Integer, Float, ect.

It will not work!!!

Why you ask?

Attention Test
QUIZ!

10. 10
Quiz

How do you implement a method with
generic type that accepts all subtypes of
Number?

11. 11
Now that I have you attention

The fancy word for this is “variance”

But so you can see the big picture

Covariant: converting from a specialized
type (Cats) to a more general type
(Animals): Every cat is an animal.

Contravariant: converting from a general
type (Shapes) to a more specialized type
(Rectangles): Is this shape a rectangle?

Invariant: not able to convert.

12. 12
So how to make this work...

Using the Java wild card “?”
− Yes it is the

The basic idea is this
− < ? extends T >
− < ? super T >

Extends means all subtypes of T

Super means all super types of T

13. 13
Ex3

14. 14
See?

Any Questions?

NOTE: there is a lab