Question of the Day

On a game show you’re given the choice of three doors: Behind one door is a car; behind the others, goats. After you pick a door, the host, opens another door which has a goat. He then asks, "Do you change your selection?” Should you?

LECTURE 7:PUBLIC VS. PRIVATE &JAVADOC

javadoc

Program included with all Java compilers

Automatically generates web page for class Requires specially formatted comments Methods, constructors, & fields described

on page Makes using classes much, much simpler Very little work required to start Getting good at it requires lots of practice

javadoc Comments

Comments are located between /** & *//** This is a javadoc comment */

Two parts to any javadoc comments First is the description then any tags

Description of the member Descriptions can contain plain text and/or

HTML Features documented using tags in javadoc Example of features: parameters & return

value Tags start with @TagName then follow tag's

pattern

javadoc Descriptions

1st sentence summarizes the comment Ends at first period followed by whitespace Web pages prints summaries in upper area Should be concise, but complete

description Lower area of page has full description

Includes summary sentence Also lists information from tags

javadoc Tags

@author Author Name[, Author Name, …] Can only be used within class documentation Can use multiple @author tags

@return Description Use to describe value method returns Plain text and/or HTML can be used as

description

@param ParamName Description Should include a tag for each parameter ParamName identical to parameter name Description can be in plain text or HTML

What To Say In javadoc

Class What the class does How it should be used Use @author tags to list authors

Fields Value stored in the field How field will be used Assumptions about field’s value Important values field may have

What To Say In javadoc

Constructor/Methods How to use this method Results of calling this method Any other important details Use @param to state what inputs should be Use @return to explain the value is

returned Do NOT describe how it actually

works

More about comments

javadoc is MINIMAL set of comments needed Methods should also include normal

comments Normal comments describe how method

works Comments can help explain why method

works Makes your life easier when debugging!

Packages

All classes reside in a package Declared at start of class file: package pkgName;

Classes in default package if not specified Package part of full name of class

Within a package, class names must be unique Between packages can reuse class names Some classes usable without full name

Cannot import classes in default package These classes are nearly useless

Why Use Packages?

Simplify organization of classes Each package has small set of related

classes Can nest packages (e.g., java.util) Enables creating hierarchy of related

classes Limits knowledge needed by

programmer Java programs use 1000s of classes How many of these do you know?

Real-Life Debugging Story

System crashed when field was null 145 assignments in 100,000+ LOC

1 out of 145 did not check for null Took 2 weeks to find & fix bug

Remembered class & its field’s issues Saved time figuring out bug Ultimately, I got lucky to fix this

Preventing bug is clear goal

Visibility Modifiers

Specify usage of classes, methods, or fields

Each modifier has different purpose private – Access within same class (file)

only protected – Use anywhere in package or

subclass public – Use at anywhere & at any time package – Lazy developer did not type a

modifier

Common Nightmare

Prevent the Nightmare

Everyone tries keeping certain details private What is exposed is limited

Prevent the Nightmare

Everyone tries keeping certain details private What is exposed is limited Limit how & why changes occur

Prevent the Nightmare

Everyone tries keeping certain details private What is exposed is limited Limit how & why changes occur Stop others from seeing changes

Prevent the Nightmare

Exact same holds for objects Make fields private unless for very

important reason:private int fieldName;

Limits errors since field accesses entirely in 1 file

Improves modularity of your program Makes object-oriented programs easier to

write

Accessor Methods

Gets field’s value in another class Getter methods useful, but not special

Named getField or similar in most casespublic fieldType getField() {

Other classes must use accessor method Easily limit access by adding check for

password Searching for field’s uses is also simple

Mutator Methods

Sets field’s value from code in another class Like getters, just another method

Named setField normallypublic void setField(fieldType param) {

All field changes normally via mutator Updates can be checked to insure value is

legal Number of ways creating bugs is limited

Visibility of Methods

Methods normally act on or with object Use active verb as name Declare as either public or protected

Replace copies of code with single method May have common test, calculation, &c. Work is internal to how class performs tasks These methods should be private Simplifies coding & debugging this class

Access Protection Benefits

Enforce constraints on object's state Amount of javadoc limited

Can use any comments for private members

Provide simple client interface Abstraction:

Make available only what people must know

Encapsulation:Separate interface from

implementation

Your Turn

Get into your groups and complete activity

For Next Lecture

Read GT2.1 for Friday What do the terms mean? How is this related to access modifiers? Why would this be important?

Week #3 assignment on Angel for you to start Due Tuesday at 5PM (via Assignment

Submitter) During this lecture, we covered problem #1