Suppose you have a set of two-letter words and their definitions. You want to be able to look up the definition of any word, very quickly. The two-letter word is the key that addresses the definition.

A module is a set of methods that work together as a whole to perform some task or set of related tasks. A module is encapsulated if its implementation is completely hidden, and it can be accessed only through a documented interface.

EXCEPTIONS

When a run-time error occurs in Java, the JVM throws an exception, prints an error message, and quits. Oddly, an exception is a Java object (named Exception), and you can prevent the error message from printing and the program from terminating by catching the Exception that Java threw.

Coming up Next: The Syntax Lectures

In the next three lectures, we’ll build an Array based implementation of a Map, and along the way, learn some new syntax.

1. Autoboxing, promotion, immutability, generics
2. Exceptions, Iterables/Iterators
3. Access control, equals, other loose ends
4. Wildcards, type upper bounds, covariance (not in the scope of the class).

JAVA PACKAGES

In Java, a package is a collection of classes and Java interfaces, and possibly subpackages, that trust each other. Packages have three benefits.

ABSTRACT CLASSES

An abstract class is a class whose sole purpose is to be extended.

Subtype Polymorphism

The biggest idea of the last couple of lectures: Subtype Polymorphism

• Polymorphism: “providing a single interface to entities of different types”

Complexity: The Enemy

When building large programs, our enemy is complexity.

Some tools for managing complexity:

• Hierarchical abstraction.
  • Create layers of abstraction, with clear abstraction barriers!
• “Design for change” (D. Parnas)
  • Organize program around objects.
  • Let objects decide how things are done.
  • Hide information others don’t need.

equals()

Every class has an equals() method. If you don’t define one explictly, you inherit Object.equals(), for which r1.equals(r2) returns the same boolean value as "r1 == r2", where r1 and r2 are references. However, many classes override equals() to compare the content of two objects.