Conditional Statements and Control Structures in Java

Making Choices

The “if” Control

The “else” Control

Boolean Variables

Overview

•

Having the program choose an action

–

the if control

–

comparing Strings

–

the else control

–

nesting controls

–

combining boolean conditions

•

boolean variables

–

remembering true/false values

Making a Choice

•

Calculating an employee’s pay

–

get their hourly pay

–

get how many hours they worked

–

calculate their standard pay

–

if they worked more than 40 hours:

•

add overtime pay

–

issue a cheque for the total pay

Sometimes overtime pay is added;

sometimes it’s not.

Depends on how many hours they worked.

Conditional Commands

•

“Add overtime pay” is 

conditional

–

computer doesn’t always do that

•

“worked more than 40 hours” is the 

condition

–

it’s either 

true

 or 

false

•

if it’s 

true

, then computer adds overtime pay

•

if it’s 

false

, the computer 

doesn’t

 add overtime pay

•

In English/pseudocode:

–

“if 

they worked more than 40 hours

,

  then 

add overtime pay

”

The “if” Control

•

Java syntax:

if (

condition

) {

conditionalAction

;

}

–

note braces

•

go around the command you 

maybe

 want to do

–

note 

indentation

:

•

conditional action indented another four spaces!

–

also note:  no semi-colon on the “if” line

•

“if they worked over 40 hours” is 

not

 a command

if (hours > 40) {

    pay = pay + …

}

Remember the 

Format

 command in NetBeans!

It will do the indentation for you.

“if” Control Example

System.out.print("Hours this week: ");

hours = kbd.nextDouble();

kbd.nextLine();

pay = rate * hours;

if (hours > 40) {

System.out.println("You get overtime pay.");

    pay = pay + rate * 0.5 * (hours – 40);

}

Hours this week: 

30

Hours this week: 

45

You get overtime pay.

no overtime pay

overtime pay

Condition

Conditional Actions

Simple Comparisons

•

if (hours > 40) 



 “if hours is greater than 40ˮ

•

Can also put “ifˮ in front of these:

–

a == b

“a is equal to bˮ

–

a != b

“a is not equal to bˮ

–

a < b

“a is less than bˮ

–

a <= b

“a is less than or equal to bˮ

–

a > b

“a is greater than bˮ

–

a >= b

“a is greater than or equal to bˮ

You can’t use the ≤ or ≥ signs in Java.

It doesn’t understand them!

Important Difference

•

Check

whether

 a is equal to b: 

a 

==

 b

–

two equals signs

•

Make

 a equal to b: 

a 

=

 b

–

one equals sign

•

Do not get them confused

–

hours == kbd.nextDouble();

if (hours = 40.0) {

Exercises

•

Write if statements:

–

if grade is greater than or equal to 50, print out

“You passed!”

–

if age is less than 18, print out “I’m sorry, but

you’re not allowed to see this movie.”

–

if guess is equal to secretNumber, print out “You

guessed it!”

–

if y plus 9 is less than or equal to x times 2, set z

equal to zero.

Sequential “if” Controls

•

Each “if” is separate

if (midtermGrade < 50) 

{

     System.out.println(

"

You failed the midterm!

 "

);

}

if (finalGrade < 50) 

{

     System.out.println(

"

You failed the final!

 "

);

}

You failed the midterm!

You failed the final!

one

other

neither

You failed the midterm!

You failed the final!

both

Sequential “if” Controls

•

Each “if” is separate

if (grade < 50) 

{

 System.out.println("I’m sorry.  You failed.");

}

if (grade > 90) 

{

 System.out.println("That is an excellent grade!");

}

What grade did you get? 

41

I’m sorry.  You failed.

What grade did you get? 

95

That is an excellent grade!

What grade did you get? 

75

one

other

neither

Sequential “if” Controls

•

Each “if” is separate

if (grade >= 50) 

{

 System.out.println("You passed.");

}

if (grade < 80) 

{

 System.out.println("You didn’t get an A.");

}

What grade did you get? 

41

You didn’t get an A.

What grade did you get? 

95

You passed.

What grade did you get? 

75

You passed.

You didn’t get an A.

one

other

both

Exercise

•

What is the output of the following code?

int age = 24;

int height = 180;

int weight = 70;

int income = 120000;

if (age < 18)  {

System.out.println(

"

Young!");  }

if (height > 180) { 

System.out.println(

"

Tall!");  }

if (weight <= 50)  {

System.out.println(

"

Thin!");  }

if (income >= 100000) { 

System.out.println(

"

Rich!");  }

Making a Choice

•

Taking a lunch order at Greesieberger’s

–

ask what sandwich they want

–

add that sandwich to the order

–

ask if they want fries with that

–

if they say yes:

•

add fries to the order

–

ask what drink they want

–

add that drink to the order

Sometimes fries are added to the order;

sometimes they’re not.

Depends on what the user wants.

Choosing Fries

•

Ask the user: Do you want fries with that?

•

Get user’s answer (should be yes or no)

–

if the answer was yes, add fries to the order

•

In Java (but not right)

System.out.print("Do you want fries? ");

String answer = kbd.next();

kbd.nextLine();

if (

"yes" == answer

)

…

Object Variables

•

Strings are objects

–

== means “Are they the very same object?” as

in “My car and my wife’s car are the same car”

–

want “Are they the same value?” as in “I have

the same car as my neighbour”

My Car

My Wife’s Car

My Neighbour’s Car

The String in the  code

What the user typed in

"yes"

"yes"

Objects and Primitives

•

Strings are objects

String name = "Mark";

•

Scanners are objects, too

Scanner kbd = new Scanner(System.in);

•

Data type name starts with a Capital Letter

–

String, Scanner, Car, Color, …

•

NOTE: the 

variable name 

starts with a small letter

•

name, kbd, …

•

Data types without capitals are 

primitives

–

int, double, …

Talking to Objects

•

You can talk to an object:

–

“Hey, kbd, give me the next int.”

kbd.nextInt();

–

“Hey, kbd, give me the next line.”

kbd.nextLine();

–

object name, dot, request name, parentheses

•

Also known as 

calling a method

–

nextInt and nextLine are methods

–

each 

returns

 a value that your program can use

Comparing Strings

•

Ask one String about the other String

–

don’t use ==, !=, <, <=, >, >=

–

they don’t work (or they do the wrong thing)

==



oneString

.equals(

anotherString

)

!=



! 

oneString

.equals(

anotherString

)

<



oneString

.compareTo(

anotherString

)

 < 0

<=



oneString

.compareTo(

anotherString

)

 <= 0

>



oneString

.compareTo(

anotherString

)

 > 0

>=



oneString

.compareTo(

anotherString

)

 >= 0

We won’t use compareTo very often in CSCI 1226.

String Equality

•

Strings are equal if 

exactly

 the same

"First String".equals("First String")

•

Any other strings are different!

"First String".equals("Second String")

"First String".equals("first string")

"First String".equals("FirstString")

"First String".equals("First String ")

•

Exercise:

–

In each of the false conditions above, what’s

different between the Strings?

true

false

false

false

false

Choosing Fries

•

In Java

System.out.println("Do you want fries? ");

String answer = kbd.next();

kbd.nextLine();

if (

"yes".equals(answer)

)

…

–

only accepts “yes”

–

does not accept “Yes”, “YES”, …

NOTE:  We could have asked answer if it was equals to “yes”:

answer.equals("yes")

We’d get the same answer.

String.

equalsIgnoreCase

•

Sometimes don’t care if capital or small

"First String".equalsIgnoreCase("First String")

"First String".equalsIgnoreCase("first string")

•

Any other strings are different!

"First String".equalsIgnoreCase("Second String")

"First String".equalsIgnoreCase("FirstString")

"First String".equalsIgnoreCase("First String ")

true

false

false

false

true

Choosing Fries

•

In Java

System.out.println("Do you want fries?");

String answer = kbd.next();

kbd.nextLine();

if (

"yes".equalsIgnoreCase(answer)

)

…

–

accepts “yes”, “Yes”, “YES”, …

–

(does not accept “yeah”)

Again, we could have asked answer about “yes”:

answer.equalsIgnoreCase("yes")

We’d get the same answer.

String.

startsWith

•

Accept any answer starting with “y” as yes

System.out.println("Do you want fries?");

String answer = kbd.next();

kbd.nextLine();

if (

answer.startsWith("y")

)

…

–

Accepts “yes”, “yeah”, “yup”, but not “Yes”

•

sadly, there is no “startsWithIgnoreCase” method

There’s also an 

endsWith

 method.

And 

lots

 of others!  Google 

java string

.

String

.toUpperCase/toLowerCase

•

Ask a string for upper/lower case version

System.out.println("Do you want fries?");

String answer = kbd.next();

kbd.nextLine();

String upperAnswer = answer.toUpperCase();

if (

upperAnswer.

startsWith("Y"))

…

–

now accepts “Yes”, “yes”, “Yeah, “yup”, …

•

upperAnswer

 is “YES”, “YES”, “YEAH”, “YUP”

•

answer

 is still “Yes”, “yes”, “Yeah”, “yup”

String.oneThing

().

another

()

•

Can combine two steps into one

–

and more, if you want

System.out.println("Do you want fries?");

String answer = kbd.next();

kbd.nextLine();

if (

answer.toUpperCase().startsWith("Y")

)

…

–

if answer is “yes”, then answer.toUpperCase() is

“YES”, and 

that

 does start with a “Y”

This works because answer.toUpperCase() is another String.

We can ask 

any

 String a question, even if we haven’t given it a name.

Scanner.oneThing

().

another

()

•

Can save the String in upper case to start with

–

and more, if you want

System.out.println("Do you want fries?");

String answer = 

kbd.next().toUpperCase()

;

kbd.nextLine();

if (

answer.startsWith("Y")

)

…

–

change user’s answer to upper case 

before

 saving it

–

user enters “yes”; answer is “YES”

–

user enters “Yup”; answer is “YUP”

This works because kbd.nextLine() is another String.

We asked kbd to give us the next line, and it gives us back a String.

Exercise

•

Right now we accept anything that starts with

the letter Y (capital or small)

if (answer.toUpperCase().startsWith("Y"))

–

accepts “You need to speak louder” as “Yes”

–

it starts with the letter Y

•

Change the code so that it only accepts

abbreviations of “YES” as “Yes”.

–

but still allow capital letters

•

Y, y, YE, ye, YES, yes, Ye, yE, YeS, Yes, …

HINT: we still need to use startsWith and toUpperCase.

But who do we ask, and what do we ask about? 

The “if-else” Control

•

Do 

exactly one of two 

things

if (

grade < 50

) {

 System.out.println(

"

You cannot continue to 1227.

 "

);

}

else {

 System.out.println(

"

You can continue to 1227!

 "

);

}

–

note the indentation

What grade did you get? 

41

You cannot continue to 1227.

What grade did you get? 

95

You can continue to 1227!

one

other

The “if-else” Control

•

Java syntax:

if (

condition

) {

ifSoAction

;

} else {

otherwiseAction

;

}

•

Does 

exactly

 one of

those two things

–

two-way choice

•

do this or do that

–

“if” is one-way

choice

•

do this or not

Better Than Two if Controls

•

Could rewrite if-else into two if controls:

if (

grade < 50

) {

 System.out.println(

"

You cannot continue to 1227.

 "

);

}

if

(

grade >= 50

) {

 System.out.println(

"

You can continue to 1227!

 "

);

}

–

but that’s 

error-prone

•

makes it more likely you’ll make a mistake

Error-Prone Version

•

What’s wrong with this:

if (

grade < 50

) {

 System.out.println(

"

You cannot continue to 1227.

 "

);

}

if

(

grade > 50

) {

 System.out.println(

"

You can continue to 1227!

 "

);

}

Error-Prone Version

•

For 1228 you need a 60, so…

–

copy the code, change 1227 to 1228, and 50 to 60

if (

grade < 

60) {

 System.out.println(

"

You cannot continue to 

1228

.

 "

);

}

if

(

grade >= 50

) {

 System.out.println(

"

You can continue to 

1228

!

 "

);

}

–

what went wrong?

Binary Choices

•

Else is for 

either-or

 situations

–

either you pass or you fail

•

no program option for incomplete

–

either you can proceed or you cannot

–

either go left or go right

•

no program option for straight

•

State the condition once

•

Split options between if-body and else-body

Exercise

•

Write if-else controls for

–

if temp is over 30, print “Go swimming”;

otherwise print “Play video games”

–

if count is zero, print sum divided by count;

otherwise print “No numbers entered”.

–

if answer starts with a y (or Y), then set price to

$10; otherwise set it to $15.

–

if num is even, set it equal to n divided by 2;

otherwise set it to n times 3, plus 1

•

num is even 



num % 2 == 0

“if” or “if-else”?

•

Look at the possible outcomes:

–

outcome #1

•

computer prints A B C D

–

outcome #2

•

computer prints A D

–

code:

Sometimes prints B and C;

Sometimes doesn’t:

(if control)

“if” or “if-else”?

•

Look at the possible outcomes:

–

outcome #1

•

computer prints A B D

–

outcome #2

•

computer prints A C D

–

code:

Sometimes prints B;

Sometimes prints C:

(if-else control)

Exercise

•

Possible outcomes:

–

outcome #1:  A B C E F

–

outcome #2:  A D E F

–

What is the code?

•

Possible outcomes:

–

outcome #1:  A B C D

–

outcome #2:  A B

–

What is the code?

Exercise

•

Possible outcomes:

–

outcome #1:  A B C F G

–

outcome #2:  A D E F G

–

outcome #3:  A B C F

–

outcome #4:  A D E F

–

What is the code?

A Common Mistake

•

Putting braces around everything after if

if (thisIsTrue) {

    doThis();

    andThat();

else

    thisOther();

}

–

the 

else

 must be 

after

 the closing brace of the 

if

•

otherwise can’t find the 

if

 it’s the 

else

 for

if-else inside if-else

What time is it? 

9 pm

Good evening!

What time is it? 

9 am

Good morning!

What time is it? 

3 pm

Good afternoon!

What time is it? 

3 am

Good grief!

Evening: 6pm to 11pm.

Afternoon: 12pm to 5pm.

Morning: 7am to 11am.

12am to 6 am.

pm

am

A

E

M

G

Get the Time

// create variables

Scanner kbd = new Scanner(System.in);

int hour;

String amPM;

// get the time

System.out.println("What time is it?");

hour = kbd.nextInt();

// fix weird clock

if (hour == 12) { hour = 0; }

// NOTE: read next word in lower case

amPM = kbd.next().toLowerCase();

kbd.nextLine();

// read the Enter key!

Print the Message

// print the appropriate message

if ( amPM.equals("pm") ) {

    if ( hour < 6 ) {

        System.out.println("Good afternoon!");

    } else {

        System.out.println("Good evening!");

    }

} else {

    if ( hour > 6 ) {

        System.out.println("Good morning!");

    } else {

        System.out.println("Good grief!");

    }

}

A

E

M

G

9 AM Message

// print the appropriate message

if ( amPM.equals("pm") ) {

    if ( hour < 6 ) {

        System.out.println("Good afternoon!");

    } else {

        System.out.println("Good evening!");

    }

} else {

    if ( hour > 6 ) {

        System.out.println("Good morning!");

    } else {

        System.out.println("Good grief!");

    }

}

Note Indentation

// print the appropriate message

if ( amPM.equals("pm") ) {

    if ( hour < 6 ) {

        System.out.println("Good afternoon!");

    } else {

        System.out.println("Good evening!");

    }

} else {

    if ( hour > 6 ) {

        System.out.println("Good morning!");

    } else {

        System.out.println("Good grief!");

    }

}

8

12

16

Remember the 

Format

 command!

if inside if

•

You can put if controls inside if controls

–

if first condition is true, test second condition

System.out.println("A");

if (condition1) {

    System.out.println("B");

    if (condition2) {

        System.out.println("C");

    }

}

possible output:

A

AB

ABC

pattern:

always A

sometimes B

when B, sometimes C

Eligible for Pension

•

Who is eligible for a pension

–

seniors (age is 65+)

–

disabled people 55+ who live on their own

•

Only ask question if need to know

–

ask age

–

if 65+ 



 eligible

–

if 55+ and not already eligible

•

ask if disabled

•

if disabled

–

ask if live on own

–

if live on own 



 eligible

Eligible for Pension

// ask age

System.out.print("How old are you? ");

age = kbd.nextInt();

kbd.nextLine();

eligible = (age >= 65);

// if 55..64, ask if disabled

if (age >= 55 && !eligible) {

    System.out.print("Are you disabled? ");

    answer = kbd.nextLine().toLowerCase();

// if disabled, ask if live on own

if (answer.startsWith("y")) {

        System.out.print("Do you live on your own? ");

        answer = kbd.nextLine().toLowerCase();

        eligible = answer.startsWith("y");

    }

}

Exercise

•

Write nested if/if-else to generate:

–

either "A", "AB", "ABC" or "ABD"

–

either "A", "ABC" or "AC"

–

either "A", "ABC", or "D"

Logical Operators

•

p && q

p 

and

 q both true

(0 < n) && (n < 100)

•

p || q

either p 

or

 q (or 

both

) are true

(m > 0) || (n > 0)

•

!p

p is 

not

 true

!answer.equals("yes")

On Translating from English

•

In English we can leave things unsaid

–

if x is greater than 0 and less than 100, ...

•

In Java, we can’t

if (x > 0 && 

< 100

) 

computer:  Huh??

•

Start by saying the whole thing in English

–

if x is greater than 0 and 

x is 

less than 100, ...

if (x > 0 && x < 100)

computer:  OK

Warnings

•

Use the right operators

==

not

=

&&

not

&

||

not

|

•

Nothing goes without saying!

a > 0  && a < 10

not

a > 0 && 

< 10

b == 0 || c == 0

not

b || 

c == 0

0 < d  && d < 10

not

0 < d 

< 10

Exercises

•

Write Boolean expressions for whether…

–

x is greater than 0 and y is less than 5

–

x is greater than zero or y is equal to z

–

answer does not start with N

–

it’s not the case that x is greater than the product of

y and z

•

NOTE:  translate 

directly

 to Java

–

y is greater than x and less than z

–

x is equal to y but not to z

–

m or n is greater than 0

Three-Way Choice

•

Exactly one of three options

–

either "A", "B", or "C"

if (age < 18) {

System.out.print("Child");

}

if (age >= 18 && age < 65) {

System.out.print("Adult");

}

if (age >= 65) {

System.out.print("Senior");

}

Child

age is 15

Adult

age is 25

Senior

age is 65

Sequential If and If-Else

•

Be careful mixing if with if-else

–

else parts go with each if separately

if (age < 18) {

System.out.print("Child");

}

if (age >= 18 && age < 65) {

System.out.print("Adult");

} else {

System.out.print("Senior");

}

What happens when age is 15?

Adult

age is 25

Senior

age is 65

If-Else Inside Else

•

If-Else inside an Else

–

only do Adult/Senior choice if Child 

not 

chosen

if (age < 18) {

System.out.print("Child");

} else {

    if (age >= 18 && age < 65) {

    System.out.print("Adult");

    } else {

    System.out.print("Senior");

    }

}

Adult

age is 25

Senior

age is 65

What happens when age is 15?

If-Else Inside Else

•

So common it has a special style

–

second 

if

 comes 

right after

 the 

else

 (no braces)

if (age < 18)  {

System.out.print("Child");

} else if (age >= 18 && age < 65) {

System.out.print("Adult");

} else {

System.out.print("Senior");

}

–

behaves exactly the same as previous slide

Cascading If Statements

•

Simplified conditions

–

already 

know

 age >= 18 in part 2

if (age < 18) {

System.out.print("Child");

} else if (age < 65) {

System.out.print("Adult");

} else {

System.out.print("Senior");

}

Adult

age is 25

Senior

age is 65

What happens when age is 15?

Multi-Way Choice

A or B or C or ... or Z

if (...) {

doA();

} else if (...) {

doB();

...

} else {

doZ();

}

A or B or ... or Z

or 

none of the above

if (...) {

doA();

} else if (...) {

doB();

...

} else if (...) {

doZ();

}

If we didn’t do any of the others,

then we’ll do Z for sure

If we didn’t do any of the others,

we 

still

 might not do Z

Exercises

•

Write “Good morning

”

 if time < 12, “Good

afternoon

”

 if 12 



time < 17, “Good evening

”

otherwise

•

Write “Small

”

 if size is 1, “Medium

”

 if size is

2, “Large

”

 if size is 3, “X-Large

”

 if size is 4,

and “XX-Large

”

 if size is 5

–

don’t write anything if size is not 1..5

Boolean Values

•

Comparison operators evaluate to 

true

 or 

false

–

20 < 40

 is 

true

; 

40 < 20

 is 

false

–

NOTE: 

true

 and 

"true" 

are entirely different

•

just like 

65.0

 and 

"65.0"

–

true

 and 

false

 are called “Boolean” values

•

after George Boole, who studied logic

–

20 < 40

 is called a “Boolean expression”

•

its value is a Boolean value

–

&&

, 

||

 and 

!

 are called “Boolean operators”

•

allow us to make bigger Boolean expressions

Boolean Variables

•

Can save the answer to a comparison

–

use a boolean variable

boolean workedOvertime = (hours > 40);

•

you don’t 

need

 parentheses, but it’s easier to read

•

Can use the saved result as a condition

if (

workedOvertime

) {

overtimeHours = hours – 40;

regularPay = 40 * rate;

overtimePay = overtimeHours * rate * 1.5;

}

Boolean Variable

•

Answer to a “True or False” question

True or False?

a.

The hours worked is over 40.

workedOvertime = (hoursWorked > 40);

b.

The midterm grade is less than 50.

failedMidterm = (midtermGrade < 50);

Complex Expressions

•

Break complex expressions down

boolean failedMidterm, failedFinal, …;

failedMidterm = (midtermGrade < 50);

failedFinal = (finalGrade < 50);

failedBothTests = failedMidterm && failedFinal;

blewOffLabs = (labGrade < 30);

blewOffAsgns = (asgnGrade < 30);

blewOffWork = blewOffLabs && blewOffAsgns;

if (

failedBothTests || blewOffWork

)

 {

System.out.println("Special fail rule!");

courseGrade = "F";

}

Remember!

•

Changing the value of one variable doesn’t

change the value of any other variables!

length = 10;

width = 2;

area = length * width;

// area is 10 * 2 = 20

width = 5;

// area is 

still

 20 -- NOT 50

•

Applies to boolean variables, too!

age = 15;

isMinor = (age < 18);

// isMinor is true

age = 20;

// isMinor is 

still

 true -- NOT false

Exercise

•

Create a variable named saidYes

•

Ask user if they are an adult

•

Set saidYes to true or false

–

true if they said “Yes” (ignoring case)

–

false otherwise

•

Can you do it without using any variables for

what the user typed in?

Questions