Chapter 15
Interfaces & Polymorphism
CMPU-101: Problem-Solving and Abstraction
Marc Smith
Vassar College
Spring 2007
2
Plan
Comparable interface
Talker interface
Polymorphism
Passing methods using polymorphism
Produce tables & bar graphs
3
The Two Meanings of "Interface"
1 — The set of public methods offered by a class. For
example, the classes listed in the Java API list their
public methods.
Should allow one to change the implementation
without changing the interface.
2 — A named set of public methods which may be offered
by any number of classes.
A class may implement an interface if it offers the
methods promised by that interface.
4
Musical Instruments
CanPlayNotes interface:
Xylophone — strike the key with a mallet
Violin — finger and then bow or pluck
Flute — finger and then blow air
Piano — strike the key with a finger
All instruments can play notes, but they do it in different
ways.
Their similarities are expressed in an interface and their
differences in the way they play notes.
5
The Comparable Interface
Requires only one method to be declared in classes that
implement this interface:
int compareTo(Object)
o1.compare To(o2) < 0
o1 < o 2
o1.compar eTo(o2) == 0
o1 = o 2
o1.compare To(o2) > 0
o1 > o 2
6
Prog1501
Compare Two Students
while ( true )
{
System.out.println();
Student s1 = Student.read(System.out,kb);
Student s2 = Student.read(System.out,kb);
int compare = s1.compareTo(s2);
System.out.println("\ns1.compareTo(s2) ==> " +
compare);
}
