1

Boolean expressionsBoolean expressions truth tablestruth tables conditional operatorconditional operator switch statementswitch statement repetition statements:repetition statements:

• whilewhile• do/whiledo/while• forfor

Lecture 3Lecture 3

2

Boolean ExpressionsBoolean Expressions A condition often uses one of Java's A condition often uses one of Java's equality operators equality operators or or

relational operatorsrelational operators, which all return boolean results:, which all return boolean results:

==== equal toequal to!=!= not equal tonot equal to<< less thanless than>> greater thangreater than<=<= less than or equal toless than or equal to>=>= greater than or equal togreater than or equal to

Note the difference between the equality operator (Note the difference between the equality operator (====) and ) and the assignment operator (the assignment operator (==))

3

Logical OperatorsLogical Operators Boolean expressions can also use Boolean expressions can also use logical operatorslogical operators::

!! Logical NOTLogical NOT&&&& Logical ANDLogical AND|||| Logical ORLogical OR

operands and result are booleanoperands and result are boolean

4

Logical NOTLogical NOT logical negationlogical negation or or logical complementlogical complement

If boolean condition If boolean condition aa is true, then is true, then !a!a is false; if is false; if aa is false, is false, then then !a!a is true is true

Logical expressions can be shown using Logical expressions can be shown using truth tablestruth tables

a

truefalse

!a

falsetrue

5

Logical AND and Logical ORLogical AND and Logical OR Logical and:Logical and:

a && b

true if both true if both aa and and bb are true, and false otherwise are true, and false otherwise

Logical or:Logical or:

a || b

true if true if aa or or bb or both are true, and false otherwise or both are true, and false otherwise

6

Truth TablesTruth Tables A truth table shows the possible true/false combinations of A truth table shows the possible true/false combinations of

the termsthe terms Since Since &&&& and and |||| each have two operands, there are four each have two operands, there are four

possible combinations of true and falsepossible combinations of true and false

a

truetruefalsefalse

b

truefalsetruefalse

a && b

truefalsefalsefalse

a || b

truetruetruefalse

7

Logical OperatorsLogical Operators Conditions in selection statements and loops can use logical Conditions in selection statements and loops can use logical

operators to form complex expressionsoperators to form complex expressions

if (total < MAX && !found) System.out.println ("Processing…");

total < MAX

falsefalsetruetrue

found

falsetruefalsetrue

!found

truefalsetruefalse

total < MAX && !found

falsefalsetruefalse

8

More OperatorsMore Operators

increment and decrement operators: ++, --increment and decrement operators: ++, -- assignment operators: +=, *=, ...assignment operators: +=, *=, ... conditional operator conditional operator

9

Increment and Decrement Increment and Decrement OperatorsOperators

The The increment operatorincrement operator ( (++++) adds one to its operand) adds one to its operand The The decrement operatordecrement operator ( (----) subtracts one from its operand) subtracts one from its operand The statementThe statement

count++;

is essentially equivalent tois essentially equivalent to

count = count + 1;

10

Increment and Decrement Increment and Decrement OperatorsOperators Increment and decrement operators can be applied in:Increment and decrement operators can be applied in:• prefix formprefix form (before the variable) (before the variable) • postfix formpostfix form (after the variable) (after the variable)

When used alone in a statement, the prefix and postfix When used alone in a statement, the prefix and postfix forms are basically equivalent. That is,forms are basically equivalent. That is,

count++;

is equivalent tois equivalent to

++count;

11

Increment and Decrement Increment and Decrement OperatorsOperators ““prefix” and “postfix” refer to prefix” and “postfix” refer to whenwhen the value of the the value of the

variable gets incremented/decremented relative to when it variable gets incremented/decremented relative to when it is used in the expression.is used in the expression.• Prefix incrementPrefix increment: increment first, then use: increment first, then use• Postfix incrementPostfix increment: use, then increment: use, then increment• similarly with decrementsimilarly with decrement

12

Increment and Decrement Increment and Decrement OperatorsOperators Example: Suppose Example: Suppose count = 1. . Complete the following table:Complete the following table:

Statementx = count++x = ++countx = count--x = --count

count

x

13

Assignment OperatorsAssignment Operators Often we perform an operation on a variable, then store the Often we perform an operation on a variable, then store the

result back into that variableresult back into that variable Java provides Java provides assignment operatorsassignment operators to simplify that process to simplify that process For example, the statementFor example, the statement

num += count;

is equivalent tois equivalent to

num = num + count;

14

Assignment OperatorsAssignment Operators There are many assignment operators, including the There are many assignment operators, including the

following:following:

Operator

+=-=*=/=%=

Example

x += yx -= yx *= yx /= yx %= y

Equivalent To

x = x + yx = x - yx = x * yx = x / yx = x % y

15

Assignment OperatorsAssignment Operators The right hand side of an assignment operator can be a The right hand side of an assignment operator can be a

complete expressioncomplete expression The entire right-hand expression is evaluated first, then the The entire right-hand expression is evaluated first, then the

result is combined with the original variableresult is combined with the original variable ThereforeTherefore

result /= (total-MIN) % num;

is equivalent tois equivalent to

result = result / ((total-MIN) % num);

16

The Conditional OperatorThe Conditional Operator Java has a Java has a conditional operatorconditional operator that evaluates a boolean that evaluates a boolean

condition that determines which of two other expressions is condition that determines which of two other expressions is evaluatedevaluated

The result of the chosen expression is the result of the entire The result of the chosen expression is the result of the entire conditional operatorconditional operator

Its syntax is:Its syntax is:

condition ? expression1 : expression2

If the If the conditioncondition is true, is true, expression1expression1 is evaluated; if it is false, is evaluated; if it is false, expression2expression2 is evaluated is evaluated

17

The Conditional OperatorThe Conditional Operator The conditional operator is similar to an if-else statement, The conditional operator is similar to an if-else statement,

except that it is an expression that returns a valueexcept that it is an expression that returns a value

For example:For example:

larger = (num1 > num2) ? num1 : num2;

If If num1num1 is greater that is greater that num2num2, then , then num1num1 is assigned to is assigned to largerlarger; otherwise, ; otherwise, num2num2 is assigned to is assigned to largerlarger

The conditional operator is The conditional operator is ternaryternary, meaning that it , meaning that it requires three operandsrequires three operands

18

The switch StatementThe switch Statement The The switch statementswitch statement provides another means to decide provides another means to decide

which statement to execute nextwhich statement to execute next

The switch statement evaluates an expression, then The switch statement evaluates an expression, then attempts to match the result to one of several possible attempts to match the result to one of several possible casescases

Each case contains a (constant) value and a (possibly Each case contains a (constant) value and a (possibly empty) list of statementsempty) list of statements

The flow of control transfers to first statement in list The flow of control transfers to first statement in list associated with the first value that matches and continues associated with the first value that matches and continues thereonthereon

19

The switch StatementThe switch Statement The general syntax of a switch statement is:The general syntax of a switch statement is:

switch ( expression ){ case value1 : statement-list1 case value2 : statement-list2 case value3 : statement-list3 case ...

}

switchswitchandandcasecaseareare

reservedreservedwordswords

If If expressionexpressionmatches matches value2value2,,control jumpscontrol jumpsto hereto here

20

The switch StatementThe switch Statement Often a Often a break statementbreak statement is used as the last statement in each is used as the last statement in each

case's statement listcase's statement list

A break statement causes control to transfer to the end of A break statement causes control to transfer to the end of the switch statementthe switch statement

If a break statement is not used, the flow of control will If a break statement is not used, the flow of control will continue into the next casecontinue into the next case

Sometimes this can be helpful, but usually we only want to Sometimes this can be helpful, but usually we only want to execute the statements associated with one caseexecute the statements associated with one case

21

The switch StatementThe switch Statement A switch statement can have an optional A switch statement can have an optional default casedefault case

The default case has no associated value and simply uses the The default case has no associated value and simply uses the reserved word reserved word defaultdefault

If the default case is present, control will transfer to it if no If the default case is present, control will transfer to it if no other case value matchesother case value matches

Though the default case can be positioned anywhere in the Though the default case can be positioned anywhere in the switch, it is usually placed at the endswitch, it is usually placed at the end

If there is no default case, and no other value matches, If there is no default case, and no other value matches, control falls through to the statement after the switchcontrol falls through to the statement after the switch

22

The switch StatementThe switch Statement The expression of a switch statement must result in an The expression of a switch statement must result in an

integral data typeintegral data type, like an integer or character; it cannot be , like an integer or character; it cannot be a floating point valuea floating point value

Note that the implicit boolean condition in a switch Note that the implicit boolean condition in a switch statement is equality - it tries to match the expression with statement is equality - it tries to match the expression with a valuea value

You cannot perform relational checks with a switch You cannot perform relational checks with a switch statementstatement

See See http://www.csc.villanova.edu/~map/7000/progs/SwitchExample.javahttp://www.csc.villanova.edu/~map/7000/progs/SwitchExample.java

23

Repetition StatementsRepetition Statements Repetition statementsRepetition statements allow us to execute a statement allow us to execute a statement

multiple times repeatedlymultiple times repeatedly

They are often simply referred to as They are often simply referred to as loopsloops

Like conditional statements, they are controlled by boolean Like conditional statements, they are controlled by boolean expressionsexpressions

Java has three kinds of repetition statements: the Java has three kinds of repetition statements: the while while looploop, the , the do loopdo loop, and the , and the for loopfor loop

24

The while StatementThe while Statement The The while statementwhile statement has the following syntax: has the following syntax:

while ( condition ) statement;whilewhile is a is a

reserved wordreserved word

If the condition is true, the statement is executed.If the condition is true, the statement is executed.Then the condition is evaluated again.Then the condition is evaluated again.

The statement is executed repeatedly untilThe statement is executed repeatedly untilthe condition becomes false.the condition becomes false.

25

Logic of a while loopLogic of a while loop

statement

truetrue

conditionevaluated

falsefalse

26

The while StatementThe while Statement if condition of whileif condition of while statement is false initially, the statement is false initially, the

statement is never executedstatement is never executed

Therefore, the body of a while loop will execute zero or Therefore, the body of a while loop will execute zero or more timesmore times

See Counter.java (page 133)See Counter.java (page 133)

See Average.java (page 134)See Average.java (page 134)

See WinPercentage.java (page 136)See WinPercentage.java (page 136)

27

Infinite LoopsInfinite Loops The body of a whileThe body of a while loop must eventually make the loop must eventually make the

condition falsecondition false

If not, it is an If not, it is an infinite loopinfinite loop, which will execute until the user , which will execute until the user interrupts the programinterrupts the program

See Forever.javaSee Forever.java (page 138) (page 138)

28

Nested LoopsNested Loops Similar to nested if statements, loops can be nested as wellSimilar to nested if statements, loops can be nested as well

See See PPalindromeTesteralindromeTester..java java (page 137)(page 137)

29

The do StatementThe do Statement The The do statementdo statement has the following syntax: has the following syntax:

do{ statement;}while ( condition )

Uses bothUses boththe the dodo and andwhilewhile

reservedreservedwordswords

The statement is executed once initially, then the condition is evaluatedThe statement is executed once initially, then the condition is evaluated

The statement is repetitively executed until the condition becomes falseThe statement is repetitively executed until the condition becomes false

30

Logic of a do loopLogic of a do loop

truetrue

conditionevaluated

statement

falsefalse

31

The do StatementThe do Statement A do loop is similar to a while loop, except that the A do loop is similar to a while loop, except that the

condition is evaluated after the body of the loop is executedcondition is evaluated after the body of the loop is executed

Therefore the body of a do loop will execute at least one Therefore the body of a do loop will execute at least one timetime

See Counter2.javaSee Counter2.java (page 143)(page 143)

32

Comparing the while and do Comparing the while and do loopsloops

statement

truetrue

conditionevaluated

falsefalse

while loopwhile loop

truetrue

conditionevaluated

statement

falsefalse

do loopdo loop

33

The for StatementThe for Statement The The for statementfor statement has the following syntax: has the following syntax:

for ( initialization ; condition ; increment ) statement;

ReservedReservedwordword

The The initializationinitialization portion portionis executed onceis executed once

before the loop beginsbefore the loop begins

The statement isThe statement isexecuted until theexecuted until the

conditioncondition becomes false becomes false

The The incrementincrement portion is executed at the end of each iteration portion is executed at the end of each iteration

34

The for StatementThe for Statement A for loop is equivalent to the following while loop A for loop is equivalent to the following while loop

structure:structure:

initialization;while ( condition ){ statement; increment;}

35

Logic of a for loopLogic of a for loop

statement

truetrue

conditionevaluated

falsefalse

increment

initialization

36

The for StatementThe for Statement Like a while loop, the condition of a for statement is tested Like a while loop, the condition of a for statement is tested

prior to executing the loop bodyprior to executing the loop body Therefore, the body of a for loop will execute zero or more Therefore, the body of a for loop will execute zero or more

timestimes It is well suited for executing a specific number of times It is well suited for executing a specific number of times

that can be determined in advancethat can be determined in advance

See Counter3.javaSee Counter3.java (page 146)(page 146)

See Multiples.javaSee Multiples.java (page 147)(page 147)

See Stars.javaSee Stars.java (page 150)(page 150)

37

The for StatementThe for Statement Each expression in the header of a for loop is optionalEach expression in the header of a for loop is optional

• If the initialization is left out, no initialization is performedIf the initialization is left out, no initialization is performed• If the condition is left out, it is always considered to be true, and If the condition is left out, it is always considered to be true, and

therefore creates an infinite looptherefore creates an infinite loop• If the increment is left out, no increment operation is performedIf the increment is left out, no increment operation is performed

Both semi-colons are always required in the for loop headerBoth semi-colons are always required in the for loop header

38

Program DevelopmentProgram Development The creation of software involves four basic activities:The creation of software involves four basic activities:

• establishing the requirementsestablishing the requirements• creating a designcreating a design• implementing the codeimplementing the code• testing the implementationtesting the implementation

The development process is much more involved than this, The development process is much more involved than this, but these basic steps are a good starting pointbut these basic steps are a good starting point

39

RequirementsRequirements RequirementsRequirements specify the tasks a program must accomplish specify the tasks a program must accomplish

(what to do, not how to do it)(what to do, not how to do it) They often include a description of the user interfaceThey often include a description of the user interface An initial set of requirements are often provided, but An initial set of requirements are often provided, but

usually must be critiqued, modified, and expandedusually must be critiqued, modified, and expanded It is often difficult to establish detailed, unambiguous, It is often difficult to establish detailed, unambiguous,

complete requirementscomplete requirements Careful attention to the requirements can save significant Careful attention to the requirements can save significant

time and money in the overall projecttime and money in the overall project

40

DesignDesign An An algorithmalgorithm is a step-by-step process for solving a problem is a step-by-step process for solving a problem A program follows one or more algorithms to accomplish A program follows one or more algorithms to accomplish

its goalits goal The The designdesign of a program specifies the algorithms and data of a program specifies the algorithms and data

neededneeded In object-oriented development, the design establishes the In object-oriented development, the design establishes the

classes, objects, and methods that are requiredclasses, objects, and methods that are required The details of a method may be expressed in The details of a method may be expressed in pseudocodepseudocode, ,

which is code-like, but does not necessarily follow any which is code-like, but does not necessarily follow any specific syntaxspecific syntax

41

ImplementationImplementation ImplementationImplementation is the process of translating a design into is the process of translating a design into

source codesource code Most novice programmers think that writing code is the Most novice programmers think that writing code is the

heart of software development, but it actually should be the heart of software development, but it actually should be the least creative stepleast creative step

Almost all important decisions are made during Almost all important decisions are made during requirements analysis and designrequirements analysis and design

Implementation should focus on coding details, including Implementation should focus on coding details, including style guidelines and documentationstyle guidelines and documentation

See ExamGrades.javaSee ExamGrades.java (page 155)(page 155)

42

TestingTesting A program should be executed multiple times with various A program should be executed multiple times with various

input in an attempt to find errorsinput in an attempt to find errors DebuggingDebugging is the process of discovering the cause of a is the process of discovering the cause of a

problem and fixing itproblem and fixing it Programmers often erroneously think that there is "only Programmers often erroneously think that there is "only

one more bug" to fixone more bug" to fix Tests should focus on design details as well as overall Tests should focus on design details as well as overall

requirementsrequirements