Chapter 2

Elementary Programming

The Basics of Java

Created by: Jonathan Cazalas

Edited by: Hanaa Abdulaziz

1
 Writing a Simple Program
✦ Write a program that will calculate the area of a
circle.
✦ Remember:
– Step 1: Problem-solving Phase
– Step 2: Implementation Phase

4
 Writing a Simple Program
✦ Write a program that will calculate the area of a
circle. input

✦ Step 1: Design your algorithm

Process
output

1. Get the radius of the circle.

2. Compute the area using the following formula:
1. area = radius x radius x π
3. Display the result

5
 Writing a Simple Program
✦ Write a program that will calculate the area of a
circle.
✦ Step 2: Implementation (code the algorithm)

i
Process

6
 Writing a Simple Program
✦ Write a program that will calculate the area of a
circle.
✦ Step 2: Implementation (code the algorithm)
– In order to store the radius, the program must declare
a symbol called a variable.
– A variable represents a value stored in the
computer s memory
– You should choose good names for variables with meaning
◆ Do not choose x or y …these have no meaning
◆ Choose names with meaning… area or radius

7
 Writing a Simple Program
✦ Write a program that will calculate the area of a
circle.
✦ Step 2: Implementation (code the algorithm)
– What value do you want to store in radius?
p 65 6
– What about area?
44s Lintor
as

2
◆ Integer? Real number? Something else maybe? do 2

– The variable s data type is the kind of data that you

can store in that particular variable.
5ns
– So when you declare (create) a variable, you must
state its data type and the variable name.
8
 Writing a Simple Program
✦ Write a program that will calculate the area of a
circle.
✦ Step 2: Implementation (code the algorithm)
– Java provides simple data types for integers, real
numbers, characters, and Boolean types.
– These basic data types are known as primitives.
– Here are two example primitive data types:
◆ int: used to store an integer
◆ double: used to store a real number

9
 Writing a Simple Program
✦ Write a program that will calculate the area of a
circle.
✦ Step 2: Implementation (code the algorithm)

10
 Writing a Simple Program
✦ Write a program that will calculate the area of a
circle.
✦ Step 2: Implementation (code the algorithm)
– Now, we set a value for radius.
◆ Later we will learn how to ask the user to input the value
for radius!
– We then perform the calculation to get the area.
– And we print/display the result.

11
 Writing a Simple Program
✦ Write a program that will calculate the area of a
circle.
✦ Step 2: Implementation (code the algorithm)

12
animation

Trace a Program Execution

public class ComputeArea {
/** Main method */ memory
public static void main(String[] args) {
double radius; radius 20
double area; area 1256.636
// Assign a radius
radius = 20;

// Compute area
area = radius * radius * 3.14159; print a message to the
console
// Display results
System.out.println("The area for the circle of radius " +
radius + " is " + area);
}
}

17
a Writing a Simple Program
✦ Discussion:
– Variables such as radius and area refer to memory
locations
– Each variable has a name, a type, a size, and value
– Line 3 says that radius can store a double value.
◆ But the value is not defined until you assign a value.
– Line 7 assigns the value 20 into radius.
– Similarly, line 4 declares the variable area.
– Line 10 then assigns a value into area.

18
 Writing a Simple Program
✦ Discussion:
– The following table shows the value in memory for
the variables area and radius as the program is
executed.

– This method of reviewing a program is called

tracing a program .
– Helps you to understand how programs work.
19
 Writing a Simple Program
✦ Discussion:
– The plus sign (+) has two meanings in Java:
ad att At
◆ Addition A 7 2 azz
a cause
a 72 9
◆ Concatenation (combining two strings together)

– The plus sign (+) in lines 13-14 is called a string

concatenation operator.
◆ It combines two strings into one string.
◆ If a string is concatenated with a number, the number is
converted into a string and then concatenated with the other
string.
– See sample program: StringPractice.java
20
 Writing a Simple Program
✦ Discussion:
– In Java, a string cannot be written across multiple
lines in the source code.
– The following statement would result in a compile
error:
System.out.println( Introduction to Java Programming,
by Y. Daniel Liang );
– To fix the error, break the string into separate
substrings and use concatenation (+) to combine
System.out.println( Introduction to Java Programming, +
by Y. Daniel Liang );
21
 Reading Input from the Console
✦ In the last example, the radius was fixed.
✦ Your program can be better, and more
interactive, by letting the user enter the radius.
✦ Java uses the Scanner class for console input
– System.out refers to the standout output device
◆ By default, standard output is the monitor
– System.in refers to the standard input device
◆ By default, standard input is the keyboard
tale s Jia
import
java.util.gr
if Easy 61 22
 Reading Input from the Console
✦ Java uses the Scanner class for console input
– So how do you read from the keyboard?
– You make a Scanner object!
– And you tell this Scanner object to read from
System.in (they keyboard)
– Here is the code:
tf
np Scanner input = new Scanner (System.in);
b
I
object import se
from java utilscanner
import
1 scannerinputs
scanner class
systemin
scanner
Scannerinputnew
methods
gpjergsfinput.nextD.my
23
Reading Input from the Console
Scanner input = new Scanner (System.in);
– new Scanner (System.in) creates an object of the
Scanner type.
– Scanner input says that input is a variable whose type
is Scanner.
– The whole line creates a Scanner object and then
saves the reference of this object into the variable
called input.
– Now, we can use this new Scanner object.
◆ Specifically, this Scanner object has helpful methods that
allow us to read/scan data from the user.
24
 Reading Input from the Console
✦ Methods of Scanner object:
– We can invoke a method on the Scanner object.
– What does this mean?
– It means we are asking the Scanner object to perform
a specific task.
– Example:
◆ We can invoke the nextDouble() method
◆ This allows us to read a double value from the input

double radius = input.nextDouble();

25
 Reading Input from the Console
✦ Summary: import

– Create a Scanner object:

Scanner input = new Scanner (System.in);

– Use the method nextDouble() to read a double value

from the user/keyboard:
double radius = input.nextDouble();

– Now, let us revisit the Compute Area of a Circle

– We will get the radius from the user…
26
 Program 2: Compute Area with
Console/User Input

✦ See sample program: AreaCircle.java

27
 Program 2: Compute Area with
Console/User Input
✦ Discussion:
– Before you can use the Scanner class, you must
import it!
– The Scanner class is in the java.util package.
– We import this on Line 1 of the program.
– Notice that we do this import before we start coding
our actual class.

28
 Program 2: Compute Area with
Console/User Input
✦ Discussion:
– There are two types of import statements:
◆ Specific import: specifies a single class that should be
imported
– Example: import java.util.Scanner;
◆ Wildcard import: imports all the classes in a package by
using the asterisk as the wildcard.
– Example: import java.util.*;
– You can use either methods to import classes.
– The choice is up to you! You are the programmer!

29
 Program 3: Compute Average
✦ Write a program to get three values from the user
and compute their average.
✦ Remember:
– Step 1: Problem-solving Phase
– Step 2: Implementation Phase

30
 Program 3: Compute Average
✦ Write a program to get three values from the user
and compute their average.
✦ Step 1: Design your algorithm
1. Get three numbers from the user.
◆ Use Scanner object
2. Compute the average of the three numbers:
◆ average = (num1 + num2 + num3) / 3
3. Display the result

31
 Program 3: Compute Average
✦ Write a program to get three values from the user
and compute their average.
✦ Step 2: Implementation (code the algorithm)

32
 Program 3: Compute Average

✦ See sample program: AverageFourNumbers.java

33
 Program 3: Compute Average
✦ Example runs of the program:

34
 Identifiers
✦ What is an identifier?
✦ Identifiers are the names that identify elements of your
program, such as classes, methods, and variables.
– An identifier is a sequence of characters that consist of letters,
digits, underscores (_), and dollar signs ($).
– An identifier must start with a letter, an underscore (_), or a
dollar sign ($). It cannot start with a digit.
– An identifier cannot be a reserved word. (See Appendix A,
“Java Keywords,” for a list of reserved words).
– An identifier cannot be true, false, or
null.
– An identifier can be of any length.
35
 Identifiers
✦ Examples of legal identifiers:
– area, radius, ComputeArea, $2, average
✦ Examples of illegal identifiers:
– 2A and d+4
◆ These two do not follow the rules
◆ Java will report that you have a syntax error!

✦ Note: Java is case sensitive

– area, Area, and AREA all are different identifiers

36
 Variables
✦ Variables are used to represent values that may be
changed in the program.
– In the previous programs, we used variables to store
values
◆ area, radius, average, etc.
✦ They are called variables because their values can
be changed!

37
 Variables

✦ Discussion:
– radius is initially 1.0 (line 2)
– then changed to 2.0 (line 7)
– area is computer as 3.14159 (line 3)
– then changed to 12.56636 (line 8)

38
 Declaring Variables
✦ Syntax for declaring a variable:
datatype variableName;

✦ Examples:
int x; // Declare x to be
2 59 //an integer variable;
double radius; // Declare radius to be
//a double variable;
char a; //Declare a to be
//a character variable;

39
 Declaring Variables
✦ If variables are of the dame data type, they can be
declared together:
datatype var1, var2, var3,…, varn;
✦ Example:
int i, j, k;
✦ Variables often have initial values
✦ You can declare and initialize in one step:
int count = 1;
double pi = 3.14159;

40
 Declaring Variables
✦ You can also use shorthand form to declare and
initialize variables of the same type together:
✦ Example:
int i = 62, j = 78, k = 35;
✦ Tip:
– A variable must be declared before it can be assigned
a value, and a value must be assigned to the variable
before it can be used.
– Try to declare and initialize in one step.
◆ This makes program easier to read and helps to avoid errors
41
 Assignment Statements
✦ After a variable has been declared, we can give that
variable a value.
✦ This is called assigning a value to the variable.
✦ In Java, we do this with the assignment statement.
– The equal sign (=) is used as the assignment operator.
– The syntax for assignment statement is as follows:
variable = value;
or
variable = expression;

42
 Assignment Statements
✦ Sometimes we assign an exact values into variables:
– Examples:
int y = 1; // assign 1 to y
double w = 3.0; // assign 3.0 to w
✦ Other times we assign the value of an expression into the
variable:
– Examples:
int x = 5 * (3 / 2);
double area =radius * radius *3.14159;

43
 Assignment Statements
✦ If a value is assigned to multiple variables, you can use
this syntax:
i = j = k = 5;
✦ This is equivalent to:
k = 5;
j = k;
i = j;

44
 Assignment Statements
✦ You can also use the same variable on both sides
of the assignment statement
✦ Example:
x = x + 1;
– First, the right side of the assignment is calculated.
– Then, the new value is assigned into the variable on
the left (x).
◆ So if the value of x was 7 before the statement is executed,
then x will become 8 after the statement is executed.

✦ See sample program: VariablePractice.java

45
 Assignment Statements
✦ Note: in an assignment statement, the data type of
the variable on the left must be compatible with
the data type of the value on the right.
✦ Example:
int x = 1.0;
– This would be illegal!
– The data type of x is an int.
– You cannot assign a double value (1.0) into an int
variable unless you use type casting.
◆ Type casting is coming later…

46
 Named Constants
✦ A named constant is an identifier that represents a
permanent value.
– The value of a variable can change during execution
of a program.
– However, a named constant, or simply constant,
represents a permanent data that never changes.
– Here is the syntax:
final datatype CONSTANTNAME = value;
– Example:
final double PI = 3.14159;
final int SIZE = 15;
47
Program 4: Compute Area with a
Constant

48
 Named Constants
✦ So what are the benefits of using constants?
1. You don t have to repeatedly type the same value if
it is used multiple times
2. If you have to change the value, you only need to
change it in a single location in the code
◆ Instead of changing it at all locations
3. A descriptive name for a constant makes the
program easier to read

49
 É k
Naming Conventions
✦ Choose meaningful and descriptive names.
– Do not use abbreviations
✦ Variables and method names:
– Use lowercase. If the name consists of several words,
concatenate all in one, use lowercase for the first
word, and capitalize the first letter of each subsequent
word in the name.
– For example, the variables radius and area, and the
method computeArea.

50
 Naming Conventions
✦ Class names:
– Capitalize the first letter of each word in the name.
– For example, the class name ComputeArea.
✦ Constants:
– Capitalize all letters in constants, and use underscores
to connect words.
– For example, the constants PI and MAX_VALUE
✦ Do you have to follow these rules?
– No. But it makes your program MUCH easier to
read!!!
51
 Numerical Data Types
✦ Every data type has a range of possible values
that it can have/hold
✦ Whenever you make a variable or constant, the
compiler will allocate (create) memory based on
the data type requested
✦ Java provides eight primitive data types
✦ The following table lists the six numeric data
types and their ranges and storage sizes

52
Numerical Data Types

I'm j
53
 Numerical Data Types
✦ Example:
– The largest value you can save into an integer data
type is 2,147,483,647.
◆ This is just over 2billion
– So what if you try to store a value larger than this in
to an integer data type?
int x=2147483648;
– Answer: you will get an error.
– This will not work.
– Solution: use a different data type
◆ double, float, long
54
 Numerical Data Types
✦ Java uses four data types for integers:
– byte, short, int, long
✦ Which should you use?
– Choose the type that is most appropriate for your
variable.
– Long is usually unnecessary for most int types
◆ It is larger than needed.
✦ Normal usage:
– int is normally used for integers
– double is normally used for real numbers
55
 Number Literals
✦ A literal is a constant value that appears directly
in the program.
✦ For example, 34, 1,000,000, and 5.0 are literals in
the following statements:
int i = 34;
long x = 1000000;
double d = 5.0;

56
 Number Literals
✦ An integer literal can be assigned to an integer
variable as long as it can fit into the variable.
✦ A compilation error would occur if the literal
were too large for the variable to hold.
✦ For example, the statement
byte b=1000;
– Would cause a compilation error
– 1000 can not be stored in a variable of the byte type.
– The range for byte is -128 to 127
◆ Anything smaller or larger will result in an error!
57
 Number Literals
✦ An integer literal is assumed to be of the int type
– The range for int is between -231(-2147483648) to
231–1 (2147483647).
– If you want an int, but you need to store a larger
number, then you should use the type long
– To denote an integer literal of the long type, append it
with the letter L or l.
◆ L is preferred because l (lowercase L) can easily be
confused with 1 (the digit one).

58
 Floating-Point Literals
✦ Floating-point literals are written with a decimal
point.
✦ By default, a floating-point literal is treated as a
double type value.
– For example, 5.0 is considered a double value, not a
float value.
– You can make a number a float by appending the
letter f or F, and make a number a double by
appending the letter d or D.
◆ For example, you can use 100.2f or 100.2F for a float
number, and 100.2d or 100.2D for a double number.
59
 double vs. float
The double type values are more accurate than the
float type values. For example,
System.out.println("1.0 / 3.0 is " + 1.0/3.0);
displays 1.0 / 3.0 is 0.3333333333333333

16 digits

System.out.println("1.0F / 3.0F is " + 1.0F /

3.0F);
displays 1.0F / 3.0F is 0.33333334

7digits

60
Reading Numbers from the Keyboard
Scanner input = new Scanner(System.in);
int value = input.nextInt();

double grade = input.nextDouble();

61
Numeric Operators

62
 Assignment Statements
✦ Normal division: 7 / 2 = 3.5
✦ In Computer Science, when we say division,
majority of the time, we mean integer division.
– When both operands of a division are integers, we
will use integer division.
✦ What is integer division?
– Easiest to explain with examples:
◆ 5/2=2 12 / 5 = 2
◆7/2=3 15 / 4 = 3
◆ 15 / 2 = 7 33 / 8 = 4

63
 Remainder Operator
✦ The % operator is known as the remainder
operator, or also as the modulo operator
– This operator will give the remainder after division
– Examples:
◆ 7%3=1 3%7=3
◆ 12 % 4 = 0 26 % 8 = 2
◆ 20 % 13 = 7

64
 Remainder Operator
✦ Remainder is very useful in programming.
✦ For example, an even number % 2 is always 0
✦ An odd number % 2 is always 1
✦ So you can use this property to determine
whether a number is even or odd.
✦ You can also mod by other values to achieve
valuable results.

65
 Remainder Operator
✦ Example:
– If today is Saturday, it will be Saturday again in 7
days. Suppose you and your friends will meet in 10
days. What day is it in 10 days?
– Let us assume Sunday is the 1st day of the week.
– We can find that in 10 days, the day will be Tuesday
by using the following equation:
Saturday is the 7th day in a week
A week has 7 days
(7 + 10) % 7 is 3
The 3nd day in a week is Tuesday
After 10 days
66
 Program 5: Convert Time
✦ Write a program to get an amount of time from
the user in seconds. Then your program should
convert this time into minutes and the remaining
seconds.
✦ Remember:
– Step 1: Problem-solving Phase
– Step 2: Implementation Phase

67
 Program 5: Convert Time
✦ Step 1: Problem-solving Phase
– If you are given seconds, how do you then calculate
the minutes and remaining seconds?
◆ Example:
– Given 624 seconds, how do we calculate the minutes?
– We divide by 60!
• We see how many complete 60s are in 624.
• Answer: 10 of them. 10x60 = 600.
– So in 624 seconds, there are a full 10 minutes.
– After we remove those 10 minutes, how many seconds are
remaining?
• 624 –(10x60) = 24 seconds remaining
• We can use mod! 624%60 = 24 seconds remaining.
68
 Program 5: Convert Time
✦ Step 1: Design your algorithm
1. Get amount of seconds from the user.
◆ Use Scanner object
◆ Save as an int

2. Compute the minutes and seconds remaining:

◆ From these seconds, determine the number of minutes
◆ Example:
– 150 seconds => 2 minutes and 30 seconds
– 315 seconds => 5 minutes and 15 seconds
• 315 / 60 = 5 and 315 % 60 = 15
3. Display the result
69
 Program 5: Convert Time
Step 2: Implementation

70
 Program 5: Convert Time

✦ See sample program: TimeCalculation.java

71
 NOTE
✦Calculations involving floating-point numbers are
approximated because these numbers are not stored
with complete accuracy.
–For example,
System.out.println(1.0 - 0.1 - 0.1 - 0.1 - 0.1 - 0.1);
◆displays 0.5000000000000001, not 0.5, and
System.out.println(1.0 - 0.9);

◆displays 0.09999999999999998, not 0.1.

–Integers are stored precisely.
–Therefore, calculations with integers yield a precise integer
result.
72
 Exponent Operations
6
System.out.println(Math.pow(2, 3));
// Displays 8.0 W dosa a
System.out.println(Math.pow(2, 5));
// Displays 32.0
System.out.println(Math.pow(4, 0.5));
// Displays 2.0
System.out.println(Math.pow(2.5, 2));
// Displays 6.25
System.out.println(Math.pow(2.5, -2));
// Displays 0.16
73
 Scientific Notation
✦ Floating-point literals can also be specified in
scientific notation
✦ Example:
– 1.23456e+2, same as 1.23456e2, is equivalent to
123.456
– and 1.23456e-2 is equivalent to 0.0123456
– E (or e) represents an exponent and it can be either in
lowercase or uppercase.

74
 Arithmetic Expressions
✦ Java expressions are written the same way as
normal arithmetic expressions.
✦ Example:

3 + 4 x 10( y − 5)(a + b + c) 4 9+ x
− + 9( + )
5 x x y

✦ is translated into
(3+4*x)/5 –10*(y-5)*(a+b+c)/x + 9*(4/x + (9+x)/y)

75
 How to Evaluate an Expression
✦ Summary: you can safely apply the arithmetic
rule for evaluating a Java expression
140 – Operators inside parenthesis are evaluated first
◆ Parenthesis can be nested
◆ Expression in inner parenthesis is evaluated first

– Use normal operator precedence

◆ Multiplication, division, and remainder are done first
– if an expression has several multiplication, division, and remainder
operators, you evaluate them from left to right
◆ Addition and subtraction are done last
– and again, if an expression has several addition and subtraction
operators, you evaluate them from left to right
76
 How to Evaluate an Expression
✦ Example of how an expression is evaluated:

3 + 4 * 4 + 5 * (4 + 3) - 1
(1) inside parentheses first
3 + 4 * 4 + 5 * 7 – 1
(2) multiplication
3 + 16 + 5 * 7 – 1
(3) multiplication
3 + 16 + 35 – 1
(4) addition
19 + 35 – 1
(5) addition
54 - 1
(6) subtraction
53

77
 Program 6: Convert Temperature
✦ Write a program that converts a temperature in
Fahrenheit into Celsius.
– You will need the following formula:

celsius = ( 95 )( fahrenheit − 32)

✦ Remember:
– Step 1: Problem-solving Phase
– Step 2: Implementation Phase

78
 Program 6: Convert Temperature
✦ Step 1: Design your algorithm
1. Get temperature in Fahrenheitfrom the user.
◆ Use Scanner object
◆ Save temperature as an int

2. Compute the temperature into Celsuis:

◆ Use formula:
◆ But write the formula in Java:
celsius= (5.0 / 9) * (fahrenheit–32);
3. Display the result

79
 Program 6: Convert Temperature
✦ Step 2: Implementation

80
 Program 6: Convert Temperature
✦ Discussion: be careful when dividing integers
– Notice the formula has 5 divided by 9
celsius = ( 95 )( fahrenheit − 32)
– What happens if we write this formula as:
celsius= (5 / 9) * (fahrenheit–32);
– (5 / 9) evaluates to zero!
◆ Integer division!
– So we use (5.0 / 9) instead, which gives a number

✦ See sample program: TempConvert.java

81
 Program 7: Show Current Time
✦ Write a program that displays current time in
GMT in the format hour:minute:second such as
1:45:19.

✦ Remember:
– Step 1: Problem-solving Phase
– Step 2: Implementation Phase

82
 Program 7: Show Current Time
✦ Step 1: Problem-solving Phase
– Remember how you print to the screen?
◆ You use the method println
◆ This method is inside the System class

System.out.println
– Well, there are many beneficial methods inside this
System class.
– Java provides a method to return the current time
System.currentTimeMillis()
◆ This method returns the current time, in milliseconds, in
milliseconds since midnight, January 1, 1970 GMT.
83
 Program 7: Show Current Time
✦ Step 1: Problem-solving Phase
System.currentTimeMillis()
◆ This method returns the current time, in milliseconds, in
milliseconds since midnight, January 1, 1970 GMT.
◆ Why this specific date?

◆ This was known as the UNIX epoch

– The point in time when UNIX started
• Important? Not really. Just a neat fact!
Elapsed
time
Time
Unix Epoch Current Time
01-01-1970 System.currentTimeMills()
00:00:00 GMT

84
 Program 7: Show Current Time
✦ Step 1: Problem-solving Phase
System.currentTimeMillis()
◆ So this method returns the number of milliseconds since
1970.
◆ That s a LOT of milliseconds

◆ It s 2015…so 45 years since 1970

◆ Now take a calculator…

– That comes to 1,419,210,000,000 milliseconds
– The point: this methods returns a HUGE number
◆ So how can we calculate the time from this number???
85
 Program 7: Show Current Time
✦ Step 1: Problem-solving Phase
1. Get the total milliseconds since midnight, January
1, 1970 by invoking:
◆ System.currentTimeMillis();
– Example: 1203183068328 milliseconds
2. Obtain the total number of seconds by dividing
totalMilliseconds by 1000
◆ totalSeconds= totalMilliseconds/ 1000;
– Example: 1203183068328 ms/ 1000 = 1203183068 seconds
3. Compute the current seconds from totalSeconds
◆ currentSeconds= totalSeconds% 60;
– Example: 1203183068 % 60 = 8, which is the current second86
 Program 7: Show Current Time
✦ Step 1: Problem-solving Phase
4. Obtain the total minutes, totalMinutes, by dividing
totalSeconds by 60
◆ totalMinutes= totalSeconds/ 60;
– Example: 1203183068 seconds / 60 = 20053051 minutes
5. Compute the current minute from totalMinutes
mod 60
◆ currentMinute= totalMinutes% 60;
– Example: 20053051 minutes % 60 = 31, the current minute

87
 Program 7: Show Current Time
✦ Step 1: Problem-solving Phase
6. Obtain the total hours, totalHours, by dividing
totalMinutesby 60
◆ totalHours= totalMinutes/ 60;
– Example: 20053051 minutes / 60 = 334217 hours
7. Compute the current hour from totalHours% 24
◆ currentHour= totalHours% 24;
– Example: 334217 hours % 24 = 17, which is the current hour

✦ The final time:

– 17:31:8 GMT, or 5:31 PM and 8 seconds

88
 Program 7: Show Current Time
✦ Step 1: Problem-solving Phase
– All these numbers are HUGE
– The int data type is not large enough
– All variables should be declared as the long data
type for this program

89
 Program 7: Show Current Time
✦ Step 2: Implementation

90
 Program 7: Show Current Time
✦ Step 2: Implementation

91
 Augmented Assignment Operators
✦ Very often, we use the current value of a
variable, we modify it, and then save it back to
the same variable.
– Example:
count = count + 1;
– Java allows you to combine this addition and
assignment into one operator, which is called the
augmented assignment operator.

j
– Example:
count += 1;

92
Augmented Assignment Operators

93
 Augmented Assignment Operators
✦ The augmented assignment operator is
performed last after all the other operators in the
expression are evaluate
– Example:
x /= 4 + 5.5 * 1.5;
is same as
x = x / (4 + 5.5 * 1.5);
then
– Caution: there are no spaces in the augmented
operators
◆ For example, + = should be += (with no space)

94
Increment and Decrement Operators
✦ Another common expression is to simply
increment (increase) a variable by one
– Such as x = x + 1;
– Because this is so common, Java gives you special
increment and decrement operators
◆ Increment operator: ++ I
◆ Decrement operator: --
That
– Examples:
int i = 3, j = 3;
i++; // i becomes 4
j--; // j becomes 2
95
Increment and Decrement Operators
✦ More details:
◆ i++ is pronounced as i plus plus
◆ i-- is pronounced as i minus minus

– These two operators are known as post-increment

and post-decrement
– Why?
◆ Because the operators (++ and --) are placed after the
variable
– The operators can also be placed before the variable

96
Increment and Decrement Operators
✦ More details:
– The operators can also be placed before the variable
int i = 3, j = 3;
++i; // i becomes 4
--j; // j becomes 2
– Again, ++i increments i, and --j decrements j
– In this small example, result is the same
◆ Meaning i++ and ++i both increase i from 3 to 4.
◆ And both --j and j–decrease j from 3 to 2.

97
Increment and Decrement Operators
✦ More details:
– If the statement is ONLY doing increment or
decrement, the effect of j++and ++j is the same.
– However, the effect changes when these operators
are used in other types of statements.
◆ Specifically:
– ++i: the increment is done before evaluating an expression
– i++: the increment is done aftere valuated an expression

3 – Study the following table and examine the results…

x it

a I 98
Increment and Decrement Operators

Pk

99
Increment and Decrement Operators
✦ Consider the following code:

– Details:
◆ Here, we first get the value of i (which is 10)and calculate

To
newNum.
◆ Then, we increment i.
– i is not incremented until AFTER the expression is evaluated.
100
Increment and Decrement Operators
✦ Consider the following code:

– Details:
◆ Here, we first increment i

t
◆ Then, after i is incremented, we calculate newNum.

101
Increment and Decrement Operators
✦ Another example:
– Consider the following code:
double x = 1.0; 2 7
1 6
double y = 5.0; X O
6
It
double z = x--+ (++y);
– What is the value of x, y, and z after are three lines
are executed?
◆ x becomes 0.0
◆ y becomes 6.0

◆ z becomes 7.0

102
Increment and Decrement Operators
✦ More details:
– Using increment and decrement operators makes
expressions short
◆ but it also makes them complex and difficult to read
– Avoid using these operators in expressions that
modify multiple variables, or the same variable for
multiple times such as this:
int k = ++i + i; if i 3
K 8
andIT
◆ Is this legal?
– Yes. But it is confusing!
– Message: don t use increment/decrement like this.
103
 Numeric Type Conversion
✦ Can you perform binary operations with
operands of different types?
– Meaning, can we add an integer literal with a double
literal?
– Answer: YES.
◆ If you add an integer with a floating-point number, Java
automatically coverts the int to a floating point value.
– Example:
◆ 3 * 4.5 is the same as
◆ 3.0 * 4.5

104
 Numeric Type Conversion
✦ Details:
– You can always assign a value to a numeric variable
just so long as the value is within the limits/range of
the data type.
– Example:
◆ You can save an int into a double, because the double is
much wider (larger) than the int
int x = 4;
double y;
y = x;
◆ This is allowed, because x can easily fit into y.

105
 Numeric Type Conversion
✦ Details:
– However, you cannot assign a value to a variable of
a data type with a smaller range of values.
◆ Unless you use type casting
– Casting is an operation that coverts a value of one
data type into a value of another data type
◆ Casting a type with a small range to a type with a larger
range is known as widening a type
Large small É
casting

◆ Casting a type with a large range to a type from a smaller

range is known as narrowing a type YA
Small Large

106
 Numeric Type Conversion
✦ Casting:
– Java will automatically widen a type, but you must
request a narrowing explicitly
– Syntax:
◆ specify the target type in parentheses, followed by the
variable s name or the value to be cast
– Example: int x int 8.5
System.out.println((int)1.7); x 8
◆ 1 gets printed.

◆ Why? Because 1.7 was converted into an int.

107
 Numeric Type Conversion
✦ Casting:
– Example:
System.out.println((double)1/2);
◆ 0.5 gets printed.

◆ Why? Because 1 is cast into 1.0. Then 1.0 is divided by 2.

– Example:
System.out.println(1 / 2);
◆ Be careful!

◆ Here, 0 (zero) gets printed. Why? 1 and 2 are both inters

and the result should be an integer.

108
 Type Casting
Implicit casting
double d = 3; // (type widening)
Explicit casting
int i = (int)3.0; // (type narrowing)
int i = (int)3.9; // (Fraction part is truncated)
What is wrong? int x = 5 / 2.0;

range increases
I
byte, short, int, long, float, double

HE
✦ See sample program: JavaBasics.java
109
 Conversion Rules
When performing a binary operation involving two
operands of different types, Java automatically
converts the operand based on the following rules:

1. If one of the operands is double, the other is

converted into double.
2. Otherwise, if one of the operands is float, the other is
converted into float.
3. Otherwise, if one of the operands is long, the other is
converted into long.
4. Otherwise, both operands are converted into int.

110
 Program 8: Sales Tax
✦ Write a program that reads a purchase amount
from the user, calculates the sales tax, and then
displays the result.
– But we want to print the tax with only two decimal
places

✦ Remember:
– Step 1: Problem-solving Phase
– Step 2: Implementation Phase

111
 Program 8: Sales Tax
✦ Step 1: Design your algorithm
1. Get purchase amount from user.
◆ Use Scanner object
◆ Save purchaseAmount as a double

2. Compute the tax:

◆ Use a simple tax rate (6%)
◆ Use the formula:
– tax = purchaseAmount* 0.06
3. Display the result
◆ Print the result, but show only two decimal places

112
 Program 8: Sales Tax
✦ Example:
– If purchaseAmount is entered as 197.55
– Then the sales tax is evaluated as 197.55 * 0.06
◆ This equals 11.853
– So how do we display this with only two decimals?
1. Multiply 11.853 by 100
• 11.853 * 100 = 1185.3
2. Cast the result as an integer
• Now 1185.3 becomes 1185 (the .3 is removed)
3. Now, divide by 100.0, which is a double
• And we get 11.85
113
 Program 8: Sales Tax
✦ Step 2: Implementation

✦ See sample program: SalesTax.java

114
Casting in an Augmented Expression
✦InJava, an augmented expression of the form x1
op= x2 is implemented as
– x1 = (T)(x1 op x2) Here OP means operation.

◆where T is the type for x1.

✦Example:
int sum = 0;
sum += 4.5; // sum becomes 4 after this statement

6
–This is equivalent to
◆sum = (int)(sum + 4.5);

115
 Common Errors and Pitfalls ion
✦ Common Error # 1: Undeclared/Uninitialized
Variables and Unused Variables
– a variable must be declared and a value assigned to it
before you use it.
– Common errors include not declaring or not
initializing a variable
– Example:
double interestRate= 0.05;
double interest = interestrate * 45;
– Java is case sensitive. In the 2nd line, interestrate is not
defined, because the R is not capitalized
128
 Common Errors and Pitfalls ion
✦ Common Error #2: Integer Overflow wedigits II

– Remember: numbers are stored with a limited

number of digits
– If you try to save a value that is too large for a
variable, an overflow error will occur
– Example:
◆ int value = 2147483647; // allowed
– but this number is the biggest possible int, therefore…
◆ value++; // will result in an error

129
 Common Errors and Pitfalls ion
EE 6
✦ Common Error #3: Round-off Errors IT
– a round-off error, also called a rounding error, is the
difference between the exact mathematical value of
a number and the approximated value that was
calculated.
– Example:
◆ 1/3 is approximately 0.333 if you use three decimals
◆ and it is 0.3333333 if you use seven decimal places

◆ The number of digits you can store is limited

◆ So having round-off errors is inevitable (a guarantee!)

130
 Common Errors and Pitfalls ion
✦ Common Error #3: Round-off Errors
– Calculations with floating-point numbers are also
approximated
◆ Because they are not stored with complete accuracy

– Example:
◆ System.out.println(1.0 -0.1 -0.1 -0.1 -0.1
-0.1);
– The output should be 0.5, but the output is really
0.5000000000000001
◆ System.out.println(1.0 -0.9);
– The output should be 0.1, but the output is really
0.09999999999999998
– Message: use integers for exact/precise results!
131
 Common Errors and Pitfalls ion
✦ Common Error #4: Unintended Integer Division
– Perhaps you want normal division Ew double ints tilt
µ I 405 I
◆ Example: 3 / 2 = 1.5 100 05 4.4
– However, if both operands are integers, Java will
automatically use integer division
– If you do not want integer division, you should make
one of the operands a floating-point number
K

(a) I (b) 1,5

132
 Common Errors and Pitfalls ion
✦ Common Pitfall #1: Repeated Input Objects
– New programmers often make many Scanner objects
each time they want input
◆ This is a mistake!
– See the code below:

HE
g

133
 Common Errors and Pitfalls ion
✦ Common Pitfall #1: Repeated Input Objects
– The code is not wrong, but it is inefficient (slow)
– It creates two input objects, which is a waste
– The correct code is below:

134