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

Params & Locals vs. Fields

Locals “live” for method or block within method Name may be reused, but no relationship

exists Parameters pass data between

methods But receiving method must be called by

sender Fields store data between calls or even

longer Values available with instance (or longer if

static) Local always best choice to hold data

Easiest choice; use others only when needed

Params & Locals vs. Fields

Locals “live” for method or block within method Name may be reused, but no relationship

exists Parameters pass data between

methods But receiving method must be called by

sender Fields store data between calls or even

longer Values available with instance (or longer if

static) Local always best choice to hold data

Easiest choice; use others only when needed

Params & Locals vs. Fields

Locals “live” for method or block within method Name may be reused, but no relationship

exists Parameters pass data between

methods But receiving method must be called by

sender Fields store data between calls or even

longer Values available with instance (or longer if

static) Local always best choice to hold

data Easiest choice; use others only when

needed

Params & Locals vs. Fields

Locals “live” for method or block within method Name may be reused, but no relationship

exists Parameters pass data between

methods But receiving method must be called by

sender Fields store data between calls or even

longer Values available with instance (or longer if

static) LOCAL ALWAYS BEST CHOICE TO

HOLD DATA Easiest choice; use others only when

needed

Params & Locals vs. Fields

LOCAL ALWAYS

BEST CHOICE TO HOLD DATA

Use others only when needed

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

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 isField depending on type

public 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 but using actual name of fieldpublic 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 understanding required is

limited Private members hidden unless rewriting

code 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

Study for term’s 1st quiz on Friday Includes everything we have covered so far Focus on types, fields, methods, objects, variables… Problems like activities, weekly assignments, etc.

(Finish) Reading for tomorrow’s lab on web Provides good explanation about what we are doing

Week #3 assignment on Angel for you to start Due Tuesday at 5PM (via Assignment Submitter) During this lecture, we covered problem #1