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CMSC 131 Fall 2008 Jan Plane (Adapted from Bonnie Dorr)
Lecture Set #14:
Polymorphism
Introduction
1. Wrappers
2. Interfaces
CMSC 131 Fall 2008 Jan Plane (Adapted from Bonnie Dorr) 1
Wrappers
We may want to treat primitives as though they were objects For example, generic routines can be implemented using interfaces … but they are not usable on primitive types To overcome this problem, Java provides wrappers for primitive types Wrappers: classes whose objects contain single values of the “wrapped type” Wrappers also contain other useful conversion operations (to / from String, etc.) Wrappers included in java.lang: Byte Short Integer Long Float Double Character Boolean
CMSC 131 Fall 2008 Jan Plane (Adapted from Bonnie Dorr) 2
The Integer Wrapper
The documentation is on-line at
http://java.sun.com/j2se/1.5.0/docs/api/
Notes
Immutable
Constructors
Implements Comparable
Documentation says “Comparable”
Comparable in Java 5.0 is a interface
Has compareTo method.
CMSC 131 Fall 2008 Jan Plane (Adapted from Bonnie Dorr) 3
Code Re-use
Many operations recur in programming
sorting
max / min
(These operations may apply to strings, numbers, etc.)
Desirable: one implementation!
Less coding
Less likely to have typos
Easier maintenance of code
CMSC 131 Fall 2008 Jan Plane (Adapted from Bonnie Dorr) 6
Adapting Cat to Implement
Comparable
The Comparable Interface
insists that I must implement compareTo method which has the following prototype: int compareTo(Object o) it must return a negative if the current object is less, a positive if the current object is greater or a 0 if they are the same.
What is Object?
Type of all possible objects in any class Shortcoming of (earlier) Java: no good way to say “same type as this”
Instead: Implementation must take any object
CMSC 131 Fall 2008 Jan Plane (Adapted from Bonnie Dorr) 7
What about int? char?
Polymorphic findMin can be used on any
class implementing Comparable
What about primitive types (int, char,
double, etc.)?
They are not classes
They do not implement Comparable
Hence findMin cannot be used on them
That’s why we use wrappers!
