Download Chapter 2 Basics of Java

Programming Fundamental I
ACS-1903
Chapter 2
Basics of Java
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1
Basics of Java

What we will learn
literals
variables
primitive data types
the String class
input
output
The first three concepts are common in
most of the programming languages
The last three will help you to understand
what is an “object-oriented” language like
Java
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Literals

Literals
They are the formal names for
constants in Java
Examples include: 123, 123.45, 'a',
"Gosling", true
Most of the time numeric literals and
Boolean literals are written in the
same way we would normally write
them down
e.g. 123, 123.45, true, false
For text literals
single quotes to specify a single character
(e.g. 'a')
or double quotes to specify a text string
(e.g. "Gosling")
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Variables

Variable
A fundamental concept in
programming
a piece of memory
that holds a value
that a program can use and change as
it executes

Java is strongly typed
You MUST declare the type of each
variable before you use it
You cannot change this type later
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Variables

Using a variable
1 /**
2 * This Java class declares
3 * an int variable named i,
4 * assigns the value 14 to i,
5 * and displays i.
6 */
7 public class Variable
8{
9
public static void main ( String [] args ){
10
int i;
11
i = 14;
12
System.out.println (i);
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i = 30;
14
System.out .println (i);
15 }
16 }
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Variables

Naming Variables
Choose names that are concise yet
meaningful
Names that can indicate the intent of its
use
Camel case
The way a Java programmer will often
name the variable if this name needs more
than one word
It is a style where words are catenated
together
the first word is all lower case, and the
second and subsequent words have only the
first letter capitalized
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Variables
Java variable names are casesensitive
NetPay and netPay are different
variables
You cannot use keywords for
variable names
Keywords are reserved for special
purposes
public, int, void, static, class, …, and more
are coming
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Primitive Data Types

The Java language contains eight
primitive data types
byte
short
int
long
float
double
char
boolean
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Primitive Data Types

Numeric
 Integers
byte
short
int – 100, 234, 0
long – 100L, 234L, 0L
 Real numbers
double – 100.12, 234.0, 0.0
float – 100.12f, 234.0f, 0.0f
 int and double are default types for numeric
literals

Individual characters
Char – 'a‘, 'b', 'q', '$'.

Logical values
boolean – true and false
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Primitive Data Types

Primitive Data Types: Byte/short/int/long
 These types differ with respect to the amount of
memory used
therefore minimum and maximum values
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Primitive Data Types

Calculations
Addition +
Subtraction Multiplication *
Division /
Modulo %
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Primitive Data Types

Integer Arithmetic
If the operands of an arithmetic operation
are both integers, the result is an integer
Consider division - there is no
remainder
Modulo gives the remainder when the
first operand is divided by the second
operand
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Primitive Data Types
1 public class IntegerArithmetic
2{
3 public static void main ( String [] args )
4
{
5
// Use integer arithmetic
6
// Division : no remainder
7
// Modulo : yields the remainder
8
int number , digit ;
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number = 1297;
10
// Get right - most digit
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digit = number % 10;
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System.out.println ( digit );
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// Decrease number by a factor of 10
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// and get next digit
15
number = number / 10;
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digit = number % 10;
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System.out.println ( digit );
18
}
19 }
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Primitive Data Types

Numeric Data Types: float, double
 They are used to represent values that have decimal places
 They differ with respect to
the number of significant digits they store
 approximately 7 for float and 16 for double
the overall magnitude of a value that can be
represented
Just for comparison, it is estimated that the there are
between 1078 to 1082 atoms in the known, observable
universe
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Primitive Data Types

Arithmetic calculations on doubles and floats
 Operators that we will discuss at this time include
+, -, *, and /
 There is a mistake in above table!
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Primitive Data Types

Doubles as approximations
Programmers must be aware that not
every number can be represented exactly
as a double or float
Think of ¼ and 1/3

Default Decimal Data Type
double is the default data type for values
with a decimal point
If you want to use a float value then the suffix
“f” needs to be used
 100.25f

Default numeric data types
? and ?
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Primitive Data Types

Numeric Expressions
Formed by operators and operands
Operators
For now: +, -, *, /, and %
 They are all binary operators,
These operands all take two operands
Expressions with these operands are written in
an infix manner where one operand is on the
left of the operator and the other operand in on
the right
Operands
Literals
Variables
Sub-expressions
 The expressions enclosed in parentheses, “()”
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Primitive Data Types

Operator Priorities
Java gives each operator a priority
It then uses those priorities to control the
order of evaluation for an expression
 Higher priority operators are executed before
lower priority operators
 You can use a sub-expression to override these
priorities
A sub-expression is always evaluated before
the expression in which it is contained
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Primitive Data Types

Operator Associativity
When an expression involves more than
one operator of the same priority, they
are evaluated from left to right
These operators are called left associative in
programming term

Mixed Mode Expressions
Expressions that contain a mixture
of types
Java permits conversions between
integer and floating-point types
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Primitive Data Types
There are two types of conversions
 Widening
 The type being converted can contain all values of the other type
 Example: a value of the short type (a 2-byte integer) is converted
as an int type (a 4-byte integer)
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/**
* This Java class declares a short variable,
* assigns it a value, and then assigns the
* value to a variable of type int
*/
public class ShortToInt
{
public static void main(String[] args){
short s;
int t;
s = 100;
t = s;
System.out.println ("s is: "+s);
System.out.println ("t is: "+t);
}
}
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Primitive Data Types
Java allows these widening conversions
automatically:
*from byte to short, int, or long, float, or double
*from short to int, long, float, or double
*from int to long, float, or double
*from long to float or double
*from float to double
*from char to int, long, float, or double
More examples …
 Narrowing
Cases where there could be a loss of precision
after conversion
Examples: converting from a double to an int
Cannot be performed automatically – you have to
directly indicates that casting is to be performed
(this will be discussed later)
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Primitive Data Types

Unary operators
the operators that takes one
operand
Unary minus is one that commonly
used
It is placed immediately in front of an
expression to negate the value of the
expression
Its priority is higher than multiplication,
division, and modulo
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Primitive Data Types

boolean Data Types
 This type has only two values
True
False
 It can most often be used for control structures
 The operators for booleans
and
 &&
or
 ||
not
!
First two are binary, and the last one is unary
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Primitive Data Types
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Primitive Data Types
Example:
boolean xyz = true ;
boolean found ;
If (xyz) System.out.println(“the variable is true”);
If (xyz && found ) ................
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Primitive Data Types

Operator priorities
 Java assigns priorities to boolean operators
From high to low
 not
 and
 or
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Primitive Data Types

Relational Operators
 The operators for comparing one value to another
 The operations with these operators evaluate to a
boolean (true or false)
 They are summarized in the table below (assume
x and y are of type int)
Example of CompareNumber
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Primitive Data Types

char Data Type
char is used when you need to handle
individual characters
a char value is enclosed in single quotes
How char values are stored
Each char value is stored using two bytes of memory
 So a character can be represented by a bit sequences and
corresponding integer value
The corresponding integer value is called ASCII code
http://www.ascii-code.com
The table in the textbook is NOT correct
 Because of this we can apply the relational operators to char
values too
Example: CompareChar
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Primitive Data Types

char Data Type
char is used when you need to handle
individual characters
a char value is enclosed in single quotes
How char values are stored
Each char value is stored using two bytes of memory
 So a character can be represented by a bit sequences and
corresponding integer value
The corresponding integer value is called ASCII code
http://www.ascii-code.com
The table in the textbook is NOT correct
 Because of this we can apply the relational operators to char
values too
Example: CompareChar
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Operators

Operators
 The operators that we have learned and those that are
popularly used
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Operators

Complex expression
Expressions can be very complex
Example: boolean answer = a+b > c+d &&
x<z
You need to evaluate this expression based on
the order of priorities shown in the previous slide
You can use extra spaces and parentheses in
expressions to make it easier to read the
expressions – as long as you do not change the
order of operations
boolean answer = ((a+b)> (c+d)) && (x<z)
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Operators
The assignment operator
The assignment statement is really a Java expression
followed by a semicolon
The assignment operator is usually the last operator
to be evaluated
 With a priority of 2
The assignment operator is right associative
 when several assignment operators appear in an
expression they are evaluated/performed from right to
left
int q = (j=1)+1;
int i = j = k = 1;
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The String Class

The String Class
The String class
It is provided to facilitate the many things
that programmers need to do with text
strings
String literals are written as a sequence of
characters that are delimited by double
quotes
"this is a line of text“
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The String Class
Variable of type String (Capital!)
How Java compiler allocates memory for a
variable of type String
The memory location for this variable will contain
a reference (an address) to the storage location
where the text string is actually stored
Memory for primitive types is handled
differently
The memory location associated with a primitive
type contains the value (not an address) of the
variable
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The String Class
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The String Class
 Allocating strings
The formal way to declare fullName and assign it a value is
to use the "new" operator
 Since text strings are objects of type String
 String fullName = new String("Joe Smith")
In general the new operator is used to instantiate (to
create) an object
Because text strings are so common Java provides the
short cut for allocating a string
 String fullName = "Joe Smith“
 The only way to work with objects is through the
methods that are defined in the class from which
the object is instantiated
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The String Class
String class provides many methods
Some are listed below:
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The String Class
Checking the documentation of classes
It is very easy in BlueJ
Taste of “object-oriented”
To use a String method you must reference
the object and the method
Example: to obtain the length of a variable s (of
String class) you must use the expression
s.length()
The variable name is followed by a period which is
followed by the method name and any arguments
enclosed in parentheses
In object-oriented terminology we are asking the
object s to execute its length() method
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The String Class
 Examples of using String methods
UsingStringLength
UsingStringCharAt
UsingStringEquals
 Catenation operator +
The + operator can also be used to add (catenate) strings
 It is used very often in statements that generate output
If at least one operand is a string then a result is formed by
joining two strings
When one operand is not a string then the equivalent string
representing that non-string's value is generated, and then the
catenation of two strings is carried out forming a new string
 Example:
int x = 10;
int y = 11;
System.out.println("the total is "+x+y);
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Output

System.out
 Generate output for the user
So far we can use
 println(. . . )
 print(. . . ) -- it does not automatically advance to a new line
They are methods belonging to the pre-defined Java class
named "System" and an object within System named "out"
The output generated is said to go to the standard output
device
 When you use this type of output with BlueJ you will see a
window pop up named "Terminal Window"
It contains the output produced by the program
Example: UsingPrintln
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Output
Redirecting System.out
By default the println() and print() methods create output
displayed on the standard output device
 also called the Console
The default value of System.out is a PrintStream object directed
to standard output
 In BlueJ it is the Terminal Window
We can also redirect the output to a file
 What you need to do
Create a new file: File f = new File (“<file name>");
Set a new FileOutputStream:
FileOutputStream fs =new FileOutputStream (f);
Set a new PrintStream
PrintStream ps = new PrintStream (fs);
Set the System’s setOut to ps: System.setOut (ps);
Start the output using System.out.println
Do not forget to close the file using ps.close ()
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Output
 Example: RedirectOutputToFile
Note the first 3 lines with the import statements
JOptionPane
In some situations you may want to use JOptionPane
message dialogs
 in order to provide the user a more interactive
experience
 It can display the output to the user and then the
program waits for the user to respond with the click of
a button
When the pop-up window appears, the program is
suspended until the user clicks the OK button
 Example: UsingDialogBox
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Input

Get input from the user
by using pre-defined Java classes
Scanner class
JOptionPane class
Scanner class
A Scanner object can be used with the
standard input stream System.in
A typical statement is:
Scanner keyboard = new Scanner(System.in)
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Input
System is a pre-defined Java class that has an
object named “in”
After a variable (like keyboard) is defined the
programmer can use methods defined for a
scanner object to get values the user has typed
on the keyboard
These values input by the user are referred
to as tokens in Java
Some of the most useful methods for scanner:
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Input
 System is a pre-defined Java class that has an object named
“in”
 Example: UsingScannerForInput
use next(), nextDouble(), and nextInt() to obtain a user's
inputs (of different types!)
 JOptionPane class
It provides a user with a more interactive experience
 One popular method defined in JOptionPane is
showInputDialog()
It prompts the user to enter text
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Input
The text the user enters becomes the value of the
method
Example:
String name = OptionPane.showInputDialog("Enter name");
Example: UsingJOptionPane – again, do not forget the
import statement
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