Chapter 5 Part 2

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Chapter 5 Part 2

• COSI 11a
• 2006-11-08
• Prepared by Ross Shaull

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Objectives for Today

• Review encapsulation.
• Discuss the idea of a class interface, and how we
  use the tools we have learned to create a useful
  one.
• Learn about packages.
• Write more code in GuessWord.java.

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Encapsulation

• We sometimes call it information hiding.
• Who can give me some intuitions or definitions
  for encapsulation?
• Can the tools weve learned in chapters 4 and 5
  help us create a well encapsulated class? (yes!)

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Friendly Interface

• Put yourselves in the shoes of another programmer
  who is going to use the class youve written.
• You want to give this programmer an interface to
  your class that is
• Safe
• Easy to understand
• Easy to use

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Safe

• Constructors that either
• Provide parameters to set necessary values
  corresponding to internal state (i.e. instance
  variables) or,
• Set reasonable defaults for internal state not
  provided as argument to constructor.
• Private variables and final variables
• How do these improve safety?

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Easy to Understand

• Overloaded methods reduce the number of method
  names that programmers have to remember.
• Proper return values and parameters make it clear
  what a method does.
• Good comments that can be extracted using javadoc.

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Easy to Use

• Reduce keystrokes!
• Static methods help because you dont have to
  instantiate an object to use them.

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Why?

• Programs are most useful when they can be used
  multiple times, perhaps in contexts you didnt
  even think of.
• For example, the GuessGame class that we are
  writing might be useful as a component in
  another, more complicated program (who is
  familiar with Wheel of Fortune?).

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Why?
This makes your programs easier for you to
understand, too!

• This idea of reusing code is part of the core
  philosophy behind Object Oriented design
  (remember that term from the first lecture?).
• When you keep these principles in mind, your
  classes become more useful for other programmers.

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Packages

• Get organized!
• Keep code in multiple directories.
• Resolve name clashes.
• Works hand-in-hand with good OO design to make
  your classes easier to use for other programmers
  (and you).
• Page 384.

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Package Declaration
package general.utilities public class
DoubleOut // stuff here
This file must be stored in a directory
general/utilities
general/utilities can be put in a variety of
library paths!
Remember the CLASSPATH variable?
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Package Example (Windows)
C\myjavastuff\libraries\general\utilities\DoubleO
ut.java
CLASSPATH C\myjavastuff\libraries.
import general.utilities.DoubleOut public class
DoubleOutTest // use DoubleOut in here
The . is very important!
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Package Example (OS X)
/Users/ross/java/lib/general/utilities/DoubleOut.j
ava
CLASSPATH /Users/ross/java/lib/.
import general.utilities.DoubleOut public class
DoubleOutTest // use DoubleOut in here
The . is here again!
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Summary (Ask Questions!)

• Invoking methods
• Method parameters
• Static methods
• Constructors
• Information Leakage
• Testing techniques
• Packages