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Literals, Variables, Data types
• Constants in Java are called literals.
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123
123.45
'a'
'&‘
"Gosling“
"this is a line“
true
false
Int
double
char
String
boolean
Later on we’ll see another way to define constants…..
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Literals, Variables, Data types
Variables
• Named location in memory
• Program controls its contents:
• Initialize, change, display, access
• Java is strongly typed
• You must declare the type of each variable … cannot
change this later
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Literals, Variables, Data types
Data types: 8 primitive types
Need to know int, double, char, boolean
Numeric
int
long
float
double
char
boolean
100
234
0
100L
234L 0L
100.12f 234.0f 0.0f
100.12 234.0 0.0
'a', 'b', … 'A', 'B',… '0', '1', '2', … '~', '@', '#', …
true, false
int and double are default types for numeric literals.
For numeric types: we will focus on int and double.
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Literals, Variables, Data types
Integer numeric types vary according to
the amount of memory
smallest/largest values
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Literals, Variables, Data types
Integer numeric types
Multiplication
Division
Modulo
Sample program : IntegerArithmetic.java
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Literals, Variables, Data types
Decimal numeric types
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Literals, Variables, Data types
Decimal numeric types
10.0 / 4.0
Correction to page 26
10.0/4.0 is 2.5
10/4 is 2
Sample program : FuelConsumption.java
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Literals, Variables, Data types
Decimal – values are often approximations
Sample program : Approximations.java
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boolean
boolean type has 2 values: true, false
Operators && || !
boolean xyz = true ;
boolean found ;
If (xyz) System.out.println("the variable is true");
If (xyz && found ) …………….
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char
char is used for single characters
Usual operators <
<= == !=
>
>=
char a, b, c;
a = '*';
b = 'q';
c = '1';
If (a == b) ……………
To compare two things
for equality we normally
use ==
= is discussed much later
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String
Literals enclosed in double quotes
"this is a line of text"
"any character such as $ % * . < etc can be included"
Variables
Declared as a String type
e.g.
String firstName, lastName, address;
firstName = "Jones";
firstName.length() ;
// see next page
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String
Memory for Strings is handled differently from primitive data types
Primitive
type
Object
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String
Many useful methods for handling String data
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System.out.println(…)
Used to start a new line of output on the Terminal Window
What is output for:
public static void main (){
System.out.println("123");
System.out.println("456");
System.out.println("789");
System.out.println("0");
}
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System.out.println(…)
Used to complete a line of output on the Terminal Window
What is output for:
public static void main (){
System.out.println("123");
System.out.println("456");
System.out.println("789");
System.out.println("0");
}
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System.out.print(…)
Used to continue a line displayed on the Terminal Window
What is the output for:
public static void main (){
System.out.print("123");
System.out.print("456");
System.out.print("789");
System.out.print("0");
}
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System.out.print(…)
Used to continue a line displayed on the Terminal Window
What is the output for:
public static void main (){
System.out.print("123");
System.out.print("456");
System.out.print("789");
System.out.print("0");
}
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System.out….(…)
Used to continue a line displayed on the Terminal Window
What is the output for:
public static void main (){
System.out.println("start of display");
System.out.print("123");
System.out.println("456");
System.out.print("789");
System.out.println("0");
System.out.println("end of display");
}
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System.out….(…)
Used to continue a line displayed on the Terminal Window
What is the output for:
public static void main (){
System.out.println("start of display");
System.out.print("123");
System.out.println("456");
System.out.print("789");
System.out.println("0");
System.out.println("end of display");
}
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Expressions
Complex – more
than one operator
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Expressions
If there is more than one operator at the same priority the evaluation goes from
left to right.
Suppose c=2.0;
9.0 / 5.0 * c + 32.0
1.8
3.6
35.6
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Expressions
Suppose c=2.0;
Like the previous example, but what if the addition is written first:
32.0 + 9.0 / 5.0 * c
1.8
3.6
35.6
/ and * have higher priority
than +
Where priority is same,
evaluate from left to right
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Expressions
Priorities (and so order of evaluation) can be overridden with a subexpression …
operations enclosed in parentheses
A sub-expression is always evaluated before the expression in which it is
contained is evaluated. Of course the sub-expression is evaluated
according to the rules of expressions.
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Expressions
A sub-expression is always evaluated before the expression in
which it is contained is evaluated
Suppose c=2.0;
Consider the following:
(9.0 / (5.0 * ( c + 32.0 ) ) )
34.0
170.0
0.0529
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Mixed Mode Expressions
An expression that involves ints and doubles is a mixed-mode expression
Example: 9 / 5.0 * 2 + 32
If an operation involves two operands where one is an int and the other is
a double, then the int is converted automatically to its double
equivalent before the operation occurs.
9 / 5.0 * 2 + 32
1. The first operation is “/” … the 9 is converted to 9.0 and we have
9.0/5.0 * 2 + 32
And so we have
1.8 * 2 + 32
2. “*” is performed next … the 2 is converted to 2.0
1.8 * 2.0 + 32
And so we have
3.6 + 32
3. 32 becomes 32.0 and we have the final result of 35.6
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Mixed Mode Expressions
Example: 9 / 5 * 2 + 32.0
5 and not 5.0
9 / 5 * 2 + 32.0
1. The first operation is “/” … 9/5 is 1
And so we have
1 * 2 + 32.0
2. “*” is performed next
And so we have
2 + 32.0
3. 2 becomes 2.0 and we have the final result of 34.0
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Expressions
Suppose we need to determine an employee’s net pay where this is calculated
as:
Step 1: subtract deductions from gross pay, and multiply that result by tax
rate giving taxes paid
Step 2: subtract taxes paid and deductions from gross pay giving net pay
Example:
Gross pay is $10,000
Deductions are $1,500
Tax rate is 20%
Taxes paid are ($10,000 – $1,500) * 20% yields $1,700
Net pay is $10,000 - $1,500 - $1,700 yields $6,800
Note that at the bottom of page 30 we have expressions
grossPay - deductions * taxRate
(grossPay - deductions) * taxRate
both of which are wrong …
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Expressions
How do we code this in Java ? How do we determine netPay?
netPay = grossPay - grossPay - deductions * taxRate;
or
netPay = grossPay - (grossPay-deductions) * taxRate;
or
netPay = grossPay - deductions - (grossPay - deductions) * taxRate;
or
… perhaps do this in a few steps:
netPayLessDeductions = grossPay – deductions;
taxesPaid = netPayLessDeductions * taxRate;
netPay = netPayLessDeductions – taxesPaid;
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Expressions
 Program to calculate net pay given the user supplies gross pay,
deductions, and tax rate.
Examine the Scanner class first
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Expressions
Area of a Trapezoid
How do we express the calculation of area in Java?
area = a + b / 2 h
or
area = a + b / 2 * h
or
area = (a + b) / 2 * h
or
area = ((a + b) / 2) * h
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Expressions
We will write a program to calculate
1. Net pay (slide 34)
2. Area of a trapezoid (to do in class)
but where the user supplies the necessary values.
But first, to handle input, we examine the Scanner class first
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Input Using the Scanner Class
•System - a pre-defined Java class that has an object named in.
•The standard input stream is System.in.
•A typical statement is:
Scanner keyboard = new Scanner(System.in);
•The keyboard object above is a Scanner object to be used to
get user input from the keyboard
•The Scanner class is defined in the Java library named java.util
 need an import statement for this
import java.util.Scanner;
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Input Using the Scanner Class
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Sample program – calculate gross pay
Taxes paid are (gross pay minus deductions) * tax rate
Net pay is gross pay minus deductions minus taxes
import java.util.Scanner;
public class NetPayCalculation
{
public static void main(String[] args)
{
double grossPay, deductions, taxRate, taxes, netPay;
String name;
// Declare a scanner object for the keyboard
Scanner keyboard = new Scanner(System.in);
// Prompt the user for a name
System.out.println("\n\nEnter your name (no spaces) and press enter");
name = keyboard.next();
Continued 
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Sample program – calculate gross pay
// Prompt the user for gross pay
System.out.println( "Enter the gross pay");
grossPay = keyboard.nextDouble();
// Prompt the user for deductions
System.out.println("Enter the deductions");
deductions = keyboard.nextDouble();
// Prompt the user for tax rate
System.out.println( "Enter the tax rate");
taxRate = keyboard.nextDouble();
// Calculate net pay and display all the information
taxes = (grossPay - deductions) * taxRate;
netPay = grossPay - deductions - taxes;
System.out.println("\n Your name: "+name
+"\n gross pay: "+ grossPay
+"\n deductions: "+ deductions
+"\n taxRate: "+taxRate
+"\n taxes: "+taxes
+"\n net pay: "+netPay);
}
}
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Sample program – calculate area of trapezoid
import java.util.Scanner;
public class TrapezoidAreaCalculation
{
public static void main(String[] args)
{
}
}
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JOptionPane – for input and output
Just another way to manage input and output
User interacts with program through pop-up windows
User must click OK for the program to continue
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JOptionPane – for input and output
import javax.swing.JOptionPane;
public class HelloWorld
{
public static void main(String[] args)
{
String message = "Hello World";
JOptionPane.showMessageDialog (null, message);
}
}
User sees
showMessageDialog
to display information
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JOptionPane – for input and output
import javax.swing.JOptionPane;
public class HelloYou
showInputDialog
{
to get information
public static void main(String[] args)
{
String message = "What is your name?";
String firstName = JOptionPane.showInputDialog (" Enter your name: ") ;
JOptionPane.showMessageDialog (null, " Hello "+firstName) ;
}
}
User sees
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