Download Top Down Design

Objectives:
 Learn about the 8 primitive data types
 Learn how to declare variables
 Learn about arithmetic operators
 Learn how to read numbers from the
keyboard using the Scanner class.
Lab 02-2
Primitive Data Types
All information in digital computers is
represented by numbers!
Lab 02-4
Primitive Data Types
All information in digital computers is
represented by numbers!
There is no such thing as letters of the
alphabet. Words, sounds and images are all
digitally stored as binary numbers.
Lab 02-5
Primitive Data Types
Digital computers are capable of storing
numbers in two different formats.
Lab 02-6
Primitive Data Types
Digital computers are capable of storing
numbers in two different formats.
1. Integers – (whole numbers)
Lab 02-7
Primitive Data Types
Digital computers are capable of storing
numbers in two different formats.
1. Integers – (whole numbers)
2. Real numbers – numbers that have a
fractional part.
Lab 02-8
Primitive Data Types
•
•
•
•
int
double
char
boolean
•
•
•
•
byte
short
long
float
Most
frequently
used
Lab 02-9
Primitive Data Types (whole numbers)
•
•
•
•
byte
short
int
long
(8 bits)
(16 bits)
(32 bits)
(64 bits)
It’s all about size. The more bits used by a data type the larger
the values it can contain.
Lab 02-10
Primitive Data Types (whole numbers)
•
•
•
•
byte
short
int
long
(8 bits)
(16 bits)
(32 bits)
(64 bits)
An int uses four bytes to represent a whole number from -231 to
231-1.
−2,147,483,648 to 2,147,483,647
Lab 02-11
Primitive Data Types (real numbers)
• float
(32 bits)
• double (64 bits)
A double can hold any number from -1.7 * 10308 to 1.7 * 10308
(+ or -) 17,976,931,348,623,157.0
Lab 02-12
Primitive Data Types (char & boolean)
• char
(16 bits)
• boolean (8 bits)
A char holds a 16 bit unicode character.
A boolean is a logical data type that holds two values - true
and false.
Lab 02-13
Variables
In Java, variable names
typically begin with lower
case letters.
• A variable is a “named container” that
holds a value.
• int q = 100;
means:
100
q
1. q is a whole number (data type int)
2. q is storing the value 100
Lab 02-15
Variables (cont’d)
• A variable must be declared before it can be
used:
int
Type
count;
double
x, y;
long
size;
float
area;
short age;
Name(s)
Declarations
Lab 02-16
Variables (cont’d)
• The assignment operator = sets the variable’s
value:
count = 5;
x = 0;
age = 16;
area = 45;
Assignments
• A variable can be initialized in its declaration:
int count = 5;
double pi = 3.14;
Short age = 16;
Declarations
with
initialization
Lab 02-17
Constants
Constants are exactly like fields except their
values cannot be changed. In the field declaration
the final modifiers is used.
final double PI = 3.142;
Lab 02-19
Constants
Constants are exactly like fields except their
values cannot be changed. In the field declaration
the final modifiers is used.
final double PI = 3.142;
PI = 3.14;
Lab 02-20
Constants
Constants are exactly like fields except their
values cannot be changed. In the field declaration
the final modifiers is used.
final double PI = 3.142;
PI = 3.1415;
Lab 02-21
Fields
Fields are variables declared in the class. All
methods of the class can access the fields.
public class MyClass
{
public int age = 16;
Field
public void output() {
System.out.println(age);
}
}
Lab 02-23
Local Variables
Local variables are declared in a method. They
can only be used in the method in which they are
declared.
public class MyClass
{
public void output() {
Local Variable
int age = 16;
System.out.println(age);
}
}
Lab 02-24
Arithmetic (cont…)
• Operators: +, -, /, * , %
• The precedence of operators and
parentheses work the same way as in
algebra.
• m % n means the remainder when m is
divided by n (e.g. 17 % 5 is 2).
• % has the same rank as / and *
• Same-rank binary operators are
performed in order from left to right.
Lab 02-26
Arithmetic
• Caution: if a and b are ints, then a / b is
truncated to an int…
17 / 5 yeilds 3
3 / 4 yeilds 0
• …even if you assign the result to a double:
double ratio = 2 / 3;
The double type of
the result doesn’t
help: ratio still gets
the value 0.0.
Lab 02-27
Arithmetic (cont’d)
• The type of the result is determined by the
types of the operands, not their values; this
rule applies to all intermediate results in
expressions.
• If one operand is an int and another is a
double, the result is a double; if both
operands are ints, the result is an int.
Lab 02-28
Type Casting
Java is a strong-typed language. Many
conversions, specially those that imply
a different interpretation of the value,
require an explicit conversion.
Lab 02-30
Type Casting (cont…)
double d = 6.3;
int n = d;
Can you put a double into an int?
Lab 02-31
Type Casting (cont…)
double d = 6.3;
int n = (int) d;
Note: The data type not the value is
placed inside a set of parentheses.
Lab 02-32
Type Casting (cont…)
int n = 35;
int divisor = 3;
double quotient = n / divisor;
We can type cast either n or divisor into
a double thereby forcing the solution to
be performed using floating point math.
Lab 02-33
Type Casting (cont…)
int n = 35;
int divisor = 3;
double quotient = (double) n / divisor;
Lab 02-34
Possible Loss Of Precision
What happens when you pour a gallon of
milk into a one quart container?
You get an overflow!
Lab 02-36
Possible Loss Of Precision
The same thing may happen when you
try to assign a double to an int or an int
to a byte or an int to a char.
Lab 02-37
Possible Loss Of Precision
As a programmer, you needs to be able
to recognize a loss of precision and
you need to be able to fix the problem.
Lab 02-38
Possible Loss Of Precision
int n = 4.5;
int n = 4;
double n = 4.5;
Lab 02-39
Possible Loss Of Precision
double d = 6.3;
int n = d;
int n = (int) d;
double n = d;
Lab 02-40
Possible Loss Of Precision
int n = 4.2 / 2.1;
int n = (int) (4.2 / 2.1);
double n = 4.2 / 2.1;
Lab 02-41
Possible Loss Of Precision
int n = 65;
char c = n;
char c = (char) n;
Lab 02-42
Scanner
• The Scanner class provides methods to
read data from the keyboard.
• To use the Scanner class you must
import java.util.Scanner or java.util.*.
• Requires an InputStream object as an
argument.
• Contains methods to read words, lines,
integers and doubles.
Lab 05-44
Scanner (cont’d)
Instantiating an object of the Scanner class:
Scanner reader = new Scanner(System.in);
Default InputStream
(Keyboard)
Lab 05-45
Scanner (cont’d)
Read the next integer:
int number = reader.nextInt();
• nextInt() reads a single integer and places the
number that is read into the variable number.
• The end of an number is indicated by any white
space. This could be the Space Bar, the Tab Key,
or the Enter key.
• When nextInt() reads a number, it leaves the
white space in the input buffer.
• If the number begins with white space, the white
space is ignored and does not become part of the
number.
Lab 05-46
Scanner (cont’d)
Read the next double:
double number = reader.nextDouble();
• nextDouble() reads a single double and
places the number that is read into the variable
number.
• The end of an number is indicated by any white
space. This could be the Space Bar, the Tab Key,
or the Enter key.
• When nextDouble() reads a number, it leaves
the white space in the input buffer.
• If the number begins with white space, the white
space is ignored and does not become part of the
number.
Lab 05-47
Scanner (cont’d)
InputMismatchException
nextInt() and nextDouble() will
throw an InputMismatchException if
the next value read is not an int or a
double, respectively.
Lab 05-48
Top Down Design
Planning a trip to Europe? Save your money. The dollar
doesn’t hold up well in Europe. You will only get .7522
Euros for every dollar you convert.
Lets write a program that converts Dollars to Euros.
$1 = € 0.7522
Lab 02-50
Top Down Design
Lab 02-51
Top Down Design
 Start JCreator.
 Create a new file called “Lab02.java”.
 Save the new file in your Lab02 folder.
Lab 02-52
Top Down Design – import statements
An import statement is a way of making
more of the functionality of Java available to
your program. Java can do a lot of things,
and not every program needs to do
everything. So, to cut things down to size,
so to speak, Java has its classes divided
into "packages." Your own classes are part
of packages, too.
Lab 02-53
Top Down Design
import java.util.Scanner;
Lab 02-54
Top Down Design – define the class
Every program in Java is a class.
Lab 02-55
Top Down Design
import java.util.Scanner;
public class Lab02
{
}
Lab 02-56
Top Down Design (cont’d)
We need two instance fields and one
constant to solve the problem.
Instance fields are values that are needed
by more than one method. Usually they will
be values read from the keyboard and the
values we are solving for.
Lab 02-57
Top Down Design - Fields
public class Lab02
{
private int dollars;
private double euros;
private static final double rate = .7522;
}
Lab 02-58
Top Down Design
The “big problem” is defined in the main
method.
Lab 02-59
Top Down Design – main method
public class Lab02
{
private int dollars;
private double euros;
private static final double rate = .0705;
public static void main(String[ ] args)
{
}
}
Lab 02-60
Top Down Design – main method
public static void main(String[ ] args)
{
Lab 02 lab = new Lab02();
}
Lab 02-61
Top Down Design – main method
public static void main(String[ ] args)
{
Lab 02 lab = new Lab02();
lab.input();
}
Lab 02-62
Top Down Design – main method
public static void main(String[ ] args)
{
Lab 02 lab = new Lab02();
lab.input();
lab.process();
}
Lab 02-63
Top Down Design – main method
public static void main(String[] args)
{
Lab 02 lab = new Lab02();
lab.input();
lab.process();
lab.output();
}
Lab 02-64
Top Down Design
input(), process() and output( ) are the
smaller parts of the problem.
Lab 02-65
Top Down Design
input()
Lab 02-66
Top Down Design – input method
public void input() {
}
Lab 02-67
Top Down Design – input method
Declare an instance of a Scanner
public void input() {
Scanner reader = new Scanner(System.in);
}
Lab 02-68
Top Down Design – input method
Prompt the user.
public void input() {
Scanner reader = new Scanner(System.in);
System.out.print("Enter an amount in dollars: ");
}
Lab 02-69
Top Down Design – input method
Read from the keyboard.
public void input() {
Scanner reader = new Scanner(System.in);
System.out.print("Enter an amount in dollars: ");
dollars = reader.nextInt();
}
Lab 02-70
Top Down Design
process()
Lab 02-71
Top Down Design – process method
public void process() {
}
Lab 02-72
Top Down Design – process method
public void process() {
euros = dollars * rate;
}
Lab 02-73
Top Down Design – process method
public void process() {
euros = dollars * rate;
}
dollars
rate
euros (dpllars * rate)
35
.7522
26.326999999999998
Lab 02-74
Top Down Design
output()
Lab 02-75
Top Down Design – output method
public void output() {
}
Lab 02-76
Top Down Design – output method
dollars
rate
euros (dpllars * rate)
35
.7522
26.326999999999998
public void output() {
}
Lab 02-77
Top Down Design – output method
dollars
rate
euros (dpllars * rate)
35
.7522
26.326999999999998
public void output() {
}
35 Dollars = 26.326999999999998 Euros
Lab 02-78
Top Down Design – output method
dollars
rate
euros (dpllars * rate)
35
.7522
26.326999999999998
public void output() {
System.out.print(dollars
}
35 Dollars = 26.326999999999998 Euros
Lab 02-79
Top Down Design – output method
dollars
rate
euros (dpllars * rate)
35
.7522
26.326999999999998
public void output() {
System.out.print(dollars + “ dollars = ”
}
35 Dollars = 26.326999999999998 Euros
Lab 02-80
Top Down Design – output method
dollars
rate
euros (dpllars * rate)
35
.7522
26.326999999999998
public void output() {
System.out.print(dollars + “ dollars = ” + euros
}
35 Dollars = 26.326999999999998 Euros
Lab 02-81
Top Down Design – output method
dollars
rate
euros (dpllars * rate)
35
.7522
26.326999999999998
public void output() {
System.out.print(dollars + “ dollars = ” + euros +
“ Euros”);
}
35 Dollars = 26.326999999999998 Euros
Lab 02-82
Top Down Design – output method
Lab 02-83
Top Down Design
import java.util.Scanner;
import java.text.DecimalFormat;
public class Lab02
{
/* code not shown */
}
Lab 02-85
Top Down Design – output method
public void output() {
DecimalFormat df = new DecimalFormat(“#.00”);
System.out.println(dollars + " dollars = " +
euros + " Euros");
}
Lab 02-86
Top Down Design – output method
public void output() {
DecimalFormat df = new DecimalFormat(“#.00”);
System.out.println(dollars + " dollars = " +
df.format(euros) + " Euros");
}
Lab 02-87
Top Down Design
Lab 02-88
Questions?
Lab 02-89