cse 501n fall ‘09 05: predicates and conditional statements

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CSE 501NFall ‘0905: Predicates and Conditional Statements

10 September 2009

Nick Leidenfrost

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Lecture Outline Review

Classes & Objects Access Modifiers Object Variables vs. Primitive Variables Logical Operands and Boolean Expressions

Zooming Out: Packages & Libraries Relational Operators

Combined Boolean Operators Conditional Statements

if / else Quiz #1

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Objects Objects are instances, or examples of a class

Each object has memory allocated for it to store all of the fields (instance variables) defined in the class

We declare object variables using the class name as the type:

public class BankAccount {

protected double balance;protected int accountNum;

…}

balance

accountNum

5360.90

104200121

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BankAccount account;

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ObjectsCreation

Objects are created and initialized by constructors Memory is allocated for object, constructor body is run for

programmer-defined initialization A default constructor is supplied by Java if you don’t define one

The constructor is invoked with the keyword newBankAccount account = new BankAccount();

public class BankAccount {

protected double balance;protected int accountNum;

public BankAccount () {}

}

Prior to this assignment, the

object variable holds the value null

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ObjectsIn Action!

Once created, we can use the dot operator on object variables to:Reference accessible fields of the object

Invoke (call) methods on the object

BankAccount account = new BankAccount();double money = account.balance; // Getaccount.balance = money+100000; // Set

account.withdraw(10000);

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ObjectsUsing Members (Fields / Methods)

Inside a class’ methods and constructor, members can be referenced or invoked directly, without the dot operator

public double withdraw (double amount) {balance -= amount;// - or –setBalance(balance – amount);

}

public void setBalance (double newBalance) {balance = newBalance;

}

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Variables and AssignmentPrimitive Variables vs. Object Variables

Assignment Affects Object and Primitive Variables Differently

Your Computer’s Memory(RAM)

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int myCount;String name;

myCount = 2;name = new String(“Bob”); null

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“Bob”

store 2

(Memory Address)

When the constructor is invoked (called) Java

allocates memory for the object.

The object handle now refers to the newly created object.

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Object AliasesTwo References to the Same Object

(Multiple References to

the Same Object)

String name = “Bob”;String aName;aName = name;name = null;String anotherName = “Bob”;

Your Computer’s Memory(RAM)

(Memory Address)

“Bob”

null(Memory Address)

null

“Bob”

(Memory Address)

null

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PackagesThe Final Frontier What are they and why should we create them?

Collection of classes, organized by functionality Define encapsulation at a Class Level

Aided by access modifiers

Package names begin with lowercase letters by convention

Must be inside a directory (folder) of the same name// Comments here.package banking;

public class BankAccount {

…

}

Must be the first line of code

(Only whitespace and comments can precede package declarations)

banking

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Sub-Packages

Further organization by functionality

package banking.eCommerce;

public class OnlineAccount {

…

}banking

eCommerce

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Libraries A Library is really nothing more than a package that was

written by somebody else Usually compressed in a .jar file

Like a .zip file or a .tar file Jar: Java Archive

The .jar file that contains the library is full of classfiles (a .class file is the result when you compile a class / a .java file)

[ Extracting rt.jar / Sun Java Library Documentation ]

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Using Libraries: The import Statement

The import statement tells Java you want to use an external Class or Package

Must precede all class declarations… but after package declaration

Terminated with a semicolon// Correct importimport banking.BankAccount;

public class MyBankApp {…

}

public class MyBankApp {…

}

// Compile error!import banking.BankAccount

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Using Libraries …with fully qualified classnames

Instead of importing, we can refer to external classes explicitly with their fully qualified name

This is perfectly legal, but makes code slightly more difficult to read Not as good from a style point of view as using import

public class MyBankApp {

public void doSomething () {BankAccount b; // Compile Error! Not imported…

}}

fully qualified name = banking.BankAccountPackage NamePeriod Class Name

banking.BankAccount b;

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Access Modifiers Define who can use class members (fields &

methods) Allow us to encapsulate data

public Everyone, and their dog

protected Subclasses of this class, Classes in my

package Note: Subclasses can be in a different

package! (Not shown in visual) (not specified: default, a.k.a. package

protected) Classes in my package

private Only other instances of this class

MyClass

myPackage

anotherPackage

myPackage.subPackage

OtherClass

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Boolean Operators & ExpressionsReview

Boolean Operators! // Not – Negate the value of the operator&& // And – true if both operators are true|| // Or – true if one or both operators are true^ /* Exclusive Or (xor) – true if exactly one

operator is true */

boolean a = true;boolean b = false;boolean negated = !a;boolean anded = a && b; boolean ored = a || b;boolean exored = a ^ b;

// false// false// true// true

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Boolean Operators & Expressions

Boolean operators are used to produce a logical result from boolean-valued operands

Like arithmetic expressions (*+/%), boolean expressions can be complex E.g. isDone || !found && atEnd

Like arithmetic operands, boolean operands have precedence Highest ! && ^ || Lowest

… precedence can be overridden with parenthesis: E.g. (isDone || !found) && atEnd

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Relational Operators Used for comparing variables Evaluate to a boolean result

== // Equal to!= // Not equal to< // Less than> // Greater than<= // Less than or equal to>= // Greater than or equal to

int a = 1;int b = 2;boolean equal = (a == b); boolean lessThan = (a < b);

// false// true

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Relational Operators

… have a lower precedence than arithmetic operators

Can be combined with boolean operators we already know to form complex expressions:

int a = 1;int b = 2;boolean equal = a == b-1; // trueboolean lessThan = a < b-1; // false

int a = 1;int b = 2;boolean lessThanOrEqual = a < b || a == b;boolean lessThanEqual = a <= b;

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Relational OperatorsFloating Point Numbers

Computations often result in slight differences that may be irrelevant

The == operator only returns true if floating point numbers are exactly equal

In many situations, you might consider two floating point numbers to be "close enough" even if they aren't exactly equal

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Comparing Floating Point Values To determine the equality of two doubles or floats, you

may want to use the following technique:

boolean equal = (Math.abs(f1 - f2) < TOLERANCE);

If the difference between the two floating point values is less than the tolerance, they are considered to be equal

The tolerance could be set to any appropriate level, such as 0.000001

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Comparing Objects .equals (Pronounced “Dot equals”)

When comparing variables which are objects, the == operator compares the object references stored by the variables true only when variables are aliases of each other

Two objects may be semantically equivalent, but not the same object .equals allows a programmer to define equality

String name = “Bob”;String otherName = “Bob”;boolean sameObject = (name == otherName);boolean equivalent = name.equals(otherName);

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Flow of Control Flow of Control (a.k.a. control flow) is a fancy way of

referring to the order of execution of statements

The order of statement execution through a method is linear - one statement after another in sequence

Some programming statements allow us to:

decide whether or not to execute a particular statement

execute a statement over and over, repetitively

These decisions are based on boolean expressions (or conditions) that evaluate to true or false

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The if Statement

The if statement has the following syntax:

if ( condition ) statement;

if is a Javareserved word

The condition (a.ka. predicate) must be a boolean expression. It must evaluate to either true or false.

If the condition is true, the statement is executed.If it is false, the statement is skipped.

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Control Flow of an if statement

conditionevaluated

statement

truefalse

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The if Statement

An example of an if statement:

if (balance > amount) balance = balance - amount;System.out.println (amount + “ was withdrawn.”);

First the condition is evaluated -- the value of balance is either greater than amount, or it is not

If the condition is true, the assignment statement is executed -- if it isn’t, it is skipped.

Either way, the call to println is executed next

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Conditional Statements: Indentation The statement controlled by the if statement is indented to indicate that relationship

The use of a consistent indentation style makes a program easier to read and understand

Although it makes no difference to the compiler, proper indentation is crucial

if (balance > amount)balance-= amount;

if (balance > amount)balance-= amount;

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The if-else Statement

An else clause can be added to an if statement to make an if-else statement

if ( condition ) statement1;else statement2;

If the condition is true, statement1 is executed; if the condition is false, statement2 is executed

One or the other will be executed, but not both

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Control Flow of an if-else statement

conditionevaluated

statement1

true false

statement2

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Indentation Revisited

Remember that indentation is for the human reader, and is ignored by the computer

if (balance < amount) System.out.println ("Error!!"); errorCount++;

Despite what is implied by the indentation, the increment will occur whether the condition is true or not

So what if we want to conditionally execute multiple statements?

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Block Statements

Several statements can be grouped together into a block statement delimited by curly braces (‘{‘ and ‘}’)

A block statement can be used wherever a statement is called for in the Java syntax rules

if (balance < amount){ System.out.println ("Error!!"); errorCount++;}

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Block Statements

In an if-else statement, the if portion, or the else portion, or both, could be block statements

public double withdraw (double amount) {if (balance < amount){

System.out.println(“Error!!”);return 0;

}else {

System.out.println("Total: " + total);balance -= amount;

}return amount;

}

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Blocks Statements Scope

The term ‘scope’ describes the relevance of variables at a particular place in program execution

Variables declared in blocks leave scope (“die”) when the block closes

if (balance < amount){double shortage = amount – balance;System.out.println(“Error!”);

}System.out.println(“Lacking “ + shortage);/* Compile error: ‘shortage’ has already left scope. */

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Nested if Statements The statement executed as a result of an if

statement or else clause could be another if statement

These are called nested if statements

if (balance < amount)if (overdraftProtection())

borrow(amount-balance);else balance -= amount;

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Nested if Statements An else clause is matched to the last

unmatched if (no matter what the indentation implies)

Braces can be used to specify the if statement to which an else clause belongs

if (balance < amount) {if (overdraftProtection())

borrow(amount-balance);}else balance -= amount;

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Questions?

Quiz #1 Lab 1.5 Assigned

Deals with: String Manipulation Interacting with Objects

Due Next Tuesday Next time:

The switch statement A ternary conditional statement

I will be in Lab now