Building Abstractions with Data (Part 3) CS 21a: Introduction to Computing I First Semester,...

transcript

Building Abstractions

with Data (Part 3)

CS 21a: Introduction to Computing I

First Semester, 2013-2014

Outline

►Assignment and State►Pass by Value versus Pass by Sharing►Message Passing and Final Remarks

on OOP

Two Different Kinds of Systems

Without State With State

(Re)Assignment Produces State

Without State

int money = 500;

int moneyAfterDeposit = deposit(money, 1000);

With State

int money = 500;

money = deposit(money, 1000);

Without State

int money = 500;

int moneyAfterDeposit = money + 1000;

With State

int money = 500;

money = money + 1000;

Without State

int money = 500;

int moneyAfterDeposit = money + 1000;

With State

int money = 500;

money = money + 1000;

Assignment operator (=) not to be confused with mathematical equals (==).

By The Way, Shortcuts!money += 1000; // money = money + 1000;

money++; // money += 1;

money -= 500; // there are also *= and /=

money--; // doesn’t make sense to have ** and //

Without State

► Input and output values have different names

► Make a new thing out of an existing thing

With State

► Same name for both “input” and “output”

► Changing an existing thing

Without State

► Variables are names attached to values

► Doesn’t make sense to change what something means

With State

► Variables are boxes whose contents can be changed

► Makes sense if what a name refers to is a changing, dynamic entity

Without State

► Applying the same operations to the same variables always produces the same results.

► Operation can be understood by plain substitution

int x = 5;

int y = x + 1; // 6

int z = x + 1; // still 6

With State

► Operations can produce side effects (aside from producing output, also changes state)

► Result of operation is dependent on time, history, environment

int x = 5;

int y = ++x; // now 6

int z = ++x; // now 7

Pre-increment versusPost-increment

►See example here►Difference in evaluation…► Is used a lot in older programs► Is useful for some accepted idioms►Should not be abused to make

ambiguous programs►Use these expressions often for side

effects, sparingly for the value

Changing Object State By Updating Fields

public class Diet

int calories;

public Diet() {

calories = 0;

public void recordBurgerEaten() {

calories += 400;

public class Diet

int calories;

public Diet() {

calories = 0;

public void recordBurgerEaten() {

calories += 400;

}Mutator method

Diet myDiet = new Diet(); // myDiet has 0 calories

myDiet.recordBurgerEaten();

println(myDiet.calories); // myDiet has 400 calories

myDiet.recordBurgerEaten();

println(myDiet.calories); // myDiet has 800 calories

When to Use Mutators?

►Only where it makes sense (updating bank account, calorie count, etc.)

►Example where it does not make sense: Fraction’s add

Outline

►Assignment and State►Pass by Value versus Pass by

Sharing►Message Passing and Final Remarks

on OOP

Two Kinds of Types in Java

►Primitives types► int, double, long, short, byte, float, char, boolean

►A variable holds a valid value►Object types► Java built-ins: String, Scanner►User-defined: Fraction, BankAccount► A variable holds a pointer to an object

Primitive Type Variablesversus Object Variables

►Primitive type variables directly contain valuesint x = 5;

►Object variables contain pointers to instancesBankAccount b = new BankAccount();

bbalance

Primitive Type Variablesand Assignment

int x = 1000;1000

int x = 1000;

int y = x;1000

int x = 1000;

int y = x;1000

Pass by value: y gets a copy of the value of x

int x = 1000;

int y = x;1000

Pass by value: y gets a copy of 1000

int x = 1000;

int y = x;

y = y - 100;

println( x );

println( y );

Prints:

1000900

Object Variablesand Assignment

BankAccount b = new BankAccount( 1000 );

bbalance

BankAccount c = b;

bbalance

BankAccount c = b;

bbalance

Pass by sharing: c gets a copy of the value of b

BankAccount c = b;

bbalance

Pass by sharing: c gets a copy of a pointer to the bank account. NO NEW OBJECT INSTANCE IS CREATED.

BankAccount b = new BankAccount( 1000 );BankAccount c = b;c.withdraw( 100 );println( b.getBalance() );println( c.getBalance() );

bbalance

c Prints:

900900

What’s a Pointer?

►It’s really a memory address!►Try this:BankAccount b = new BankAccount( 1000 );println(b);BankAccount c = new BankAccount( 1000 );println(c);println(c == b);c = b;println(c);println(c == b);

What’s a Pointer?

Doesn’t matter if the two bank accounts have the same balance. They’re different accounts!

What’s a Pointer?

Copies the memory address stored in b to c

What’s a Pointer?

c now points to the same object as b does

What’s a Pointer?

►So when you do…c.withdraw(100);►The balance is changed for the

instance that both b and c share

What Happens to the Second Bank Account?

►Garbage collection is a feature of a programming language, where objects that can no longer be accessed are automatically destroyed.

Method Calls

►A method is invoked on an object variable but acts on the object that the variable is pointing to

BankAccount b = new BankAccount(1000);

balance

Method Calls

►A method is invoked on an object variable but acts on the object that the variable is pointing to

BankAccount b = new BankAccount(1000);b.deposit( 250 );

balance

1250deposit(250)

The null Pointer

►An object variable that doesn’t point to an actual instance yet has a null pointer (points to nothing)

Method Calls on null

►Calling a method on an object variable whose value is null results in an error (NullPointerException)

BankAccount b = null;

b.deposit( 250 );

???deposit(250)

An Analogy to Clear That Up

Java World Balloon World

Object variable name Hand

Object variable value orPointer

String (not the CS kind)

Object instance Balloon

Call to constructor Make a new balloon

Return value from constructor

String that ties the new balloon

Assigning to object variable

Make the hand hold the string

Make the hand hold the stringnew BankAccount(500); balance

some memory address

Make the hand hold the stringBankAccount b = new BankAccount(500); balance

Using the dot operatorCalling a methodAccessing a field

Getting the balloon attached to the string and operating on it

b.deposit(500);

Three parts:1. What’s b?2. What’s the pointer pointing to?3. Apply operation to object

balance

b.deposit(500);

balance

b.deposit(500);

balance

b.deposit(500);

balance

Pass by sharing Share the balloon by attaching a new string (a copy of the old string) and making the new hand hold the new string

balance

BankAccount c = b;

Pass by sharing Share the balloon by attaching a new string (a copy of the old string) and making the new hand hold the new string

balance

c.deposit(500);

Null Pointer String with no balloon

balance

c = null;// c.deposit(500); -> NullPointerException

Unassigned variable (doesn’t even matter if Object or primitive)

Hand holding nothing

balance

BankAccount d;// d.deposit(500); -> compile error

Garbage Collection Unheld balloons fly away and eventually pop, releasing boxes which can be used again

balance

b = null;

Garbage Collection Unheld balloons fly away and eventually pop, releasing boxes which can be used again

bFree box cd

What Happens?

BankAccount b = new BankAccount( 1000 );println(b == null);println(b.getBalance());

What Happens?

BankAccount b = new BankAccount( 1000 );BankAccount c = new BankAccount( 500 );println(b.getBalance());println(c.getBalance());c = b;println(b.getBalance());println(c.getBalance());

What Happens?

BankAccount b = new BankAccount( 1000 );println(b.getBalance());b = null;println(b == null);println(b.getBalance());

What Happens?

public static void main(String args[]) {int x = 0;increment(x);println(x);

}public void increment(int x) {x++;

What Happens?

public static void main(String args[]) {int x = 0;x = increment(x);println(x);

}public int increment(int x) {x++;return x;

What Happens?public static void main(String args[]) {BankAccount x = new BankAccount(500);increment(x);println(x.getBalance());

}public void increment(BankAccount x) {x = new BankAccount(x.getBalance() + 1);

What Happens?public static void main(String args[]) {BankAccount x = new BankAccount(500);x = increment(x);println(x.getBalance());

}public BankAccount increment(BankAccount x) {x = new BankAccount(x.getBalance() + 1);return x;

What Happens?public static void main(String args[]) {BankAccount x = new BankAccount(500);increment(x);println(x.getBalance());

}public void increment(BankAccount x) {x.balance = x.getBalance() + 1;

A Better Versionpublic static void main(String args[]) {BankAccount x = new BankAccount(500);increment(x);println(x.getBalance());

}public void increment(BankAccount x) {x.setBalance(x.getBalance() + 1);

An Even Better Versionpublic static void main(String args[]) {BankAccount x = new BankAccount(500);x.increment();println(x.getBalance());

An Even Better and Useful Version

public static void main(String args[]) {BankAccount x = new BankAccount(500);x.setCompoundInterestRate(0.06);x.applyInterest();println(x.getBalance());

Some Technical Stuff

► Fields have default values. Local variables don’t.

► An object type field has a null pointer by default if the constructor doesn’t assign anything else to it.► Does not cause a compile error if you try to invoke

a method on it

► An unassigned object type local variable does not have even a null pointer.► Compile error if you try to invoke a method

on it.

What Happens?

BankAccount b;println(b.getBalance());

What Happens?

BankAccount b = null;println(b.getBalance());

What Happens?

static BankAccount b;public static void main(String args[]) {println(b.getBalance());

How to Avoid the Technical Stuff

►Always assign variables before using them!

►Never rely on default values!

Why the Trouble with Passing?

►Three sub-questions:►Why pass only copies of the value

(whether it’s a primitive or a pointer)?►Avoid accidental reassignment, unwanted

side effects

►Three sub-questions:►Why pass pointers for object types?

►To allow controlled side effects through mutator methods

►For efficiency’s sake: no new copy of the actual object (which may be big) is created

►If you really want a new copy, there are ways

►Three sub-questions:►Can it be otherwise?

►Yes! Java chooses to evaluate variables this way, but other languages have different evaluation strategies.

How to Avoid the Trouble with Passing?

►Most of the time, two strings share a balloon only because you have an object type parameter.

►Never reassign parameters!

Outline

►Assignment and State►Pass by Value versus Pass by Sharing►Message Passing and Final

Remarks on OOP

Message Passing

►Original view of OOP in the earlier languages like Smalltalk.

►Constructors: pass a message to tell an object to begin with a certain state.

►Accessors: pass a message to ask about an object’s state.

►Mutators: pass a message to tell an object to change its state.

Message Passing

►Each method invocation is a message for an object.

►The object responds in one of two ways:►Reply with a value►Act to change its state►Possibly both

Message Passing

►Big difference from functional view!►Functional view: same input, same

output►OO view: same input (message),

possibly different output (state) depending on time

Group Discussion

►Using the idea of message passing, explain how the web works (how you can access and modify data on the internet through your web browser).

Group Discussion

►Hint: There are only 5 crucial parts. Identify which is the object, the driver program, the input to the driver program, the accessor message, the mutator message.

►URL► Browser►GET request► POST request► Internet

OOP is Modeling

►Not the one with the catwalks and platforms!

►OOP is modeling real world objects?► Not really

►You don’t write classes for dogs, chairs, etc.

► OOP is modeling real world concepts.►You write classes for social network profiles,

geometric figures, and… bank accounts►Although you can argue that dog, chair, etc. are

concepts too

With Great Power Comes Great Responsibility

►Abstraction gives you power.►OOP > Procedural > imperative in

terms of abstraction.►Use OOP wisely.

►Abstraction is your tool for achieving a goal.

►Abstraction is not your goal!►Always focus on the problem, not on

the abstraction!►Appropriate level of abstraction – one

that allows you to efficiently solve the problem

►Remember:►Write for humans►Write for computers►Write generally, but not too generally

►If 10 layers of abstraction will allow you to do this, don’t go for 50.

►Don’t over-engineer.

Summary

►OOP is about coupling data and procedures.

►OOP is about coupling state and behavior.

►OOP is about changing state.►OOP is about message passing.►OOP is a powerful abstraction tool

that takes time to master and must be used with care.

Mega Summary

►What are algorithms?► Concerned with computers and

performance? ► Imperative: sequences of statements

► Concerned with science and generality? ►Declarative/Functional: compositions of

functions or expressions

► Concerned with humans and modularity? ►Procedural: sequences of chunks of statements►Object-oriented: system of interacting objects

Mega Summary

►Remember the fundamental distinctions to avoid confusion►Thinking the Algorithm►Writing the Program►Running the Computation►Use versus Definition► Instance versus Idea►Scope versus Environment

Mega Summary

►Benefits of Abstraction:►Readability

►Separating code into chunks►Reusability

►Allowing chunks to be reused►Robustness

►Allowing chunks to be changed easily without affecting other chunks

Mega Summary

►The history of programming is the history of abstraction.►Machine Language (1s and 0s)►Assembly Language (not discussed

here)► Imperative►Procedural and Functional►OOP►The next big thing?

Next Time…

►We’ve tackled the first beast: complexity

►Next, we’re going to tackle infinity►But before that, a short discussion of

two useful pre-defined objects and Lab

Building Abstractions with Data (Part 3) CS 21a: Introduction to Computing I First Semester,...

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