CSC297.Java 2003F, Class 13: Polymorphism Continued

Admin:
* Bishop hasn't assigned anything this week.  
* What homework should I assign you?  
  * Write a procedure, center(screen,Object,width) that prints the object centered in width columns.
  * Think about how we might implement Scheme-like lists in Java.
    * Hint one: Create a Pair class.
    * Hint two: Use a vector backwards.

Overview:
* Interfaces, revisited
* Numerical example, continued
* Other benefits of interfaces
* Abstract classes

Intefaces contain only names of object methods
* No fields
* No constructors
* No static methods
* No bodies to the object methods

Classes "implement" interfaces
* Really a way of saying "this class *is* one of these things"
* The methods from the interface must be redefined and have a body.

Observations:
* Some simplification, but we still have to worry about how to deal with "non-matching" types
  * We often decide that throwing exceptions is okay in such cases.
  * The documentation should probably indicate that the type restriction is more severe
* We've been using this idea for awhile: Java assumes that classes provide a toString method and calls it whenever we add an object to a string.

Another use for interfaces:
* Separate "what my class should do" from "how my class does it"
* As clients of something like a list, we care not about how they are implemented, but what they can do.
* As implementers, we care about how they are implemented.
* When we design our own types, we will usually use interfaces.
* E.g., lists

public interface SchemeList
{
  /** Get the first element of the list. */
  public Object car();
  /** Get all but the first element of the list. */
  public Object cdr();
  // Can't be in an interface.  Whoops.
  /** Is the list empty? */
  public boolean nullP();
  // Add eomthing to the front.
  public SchemeList cons(Object newFront);
} // interface SchemeList

public class StudentRecords {
  SchemeList students;
  ...
  public Student enroll(Student s) {
    students = students.cons(s);
  }
  public Student lookup(Student s) {
    ...
  }
}