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Chapter 2
Basic Elements of Java
continued……..
Increment & Decrement Operator Examples
Revision
common code
int n = 3;
int m = 4;
int result;
What will be the value of m and result after each of
these executes?
(a) result = n * ++m; //pre-increment m
(b) result = n * m++; //post-increment m
(c) result = n * --m; //pre-decrement m
(d) result = n * m--; //post-decrement m
Answers to Increment/Decrement
Operator Questions
(a) 1) m = m + 1;//m = 4 + 1 = 5
2) result = n * m;//result = 3 * 5 = 15
(b) 1) result = n * m;//result = 3 * 4 = 12
2) m = m + 1;//m = 4 + 1 = 5
(c) 1) m = m - 1;//m = 4 - 1 = 3
2) result = n * m;//result = 3 * 3 = 9
(b) 1) result = n * m;//result = 3 * 4 = 12
2) m = m - 1;//m = 4 - 1 = 3
Two Main Kinds of Types in Java
primitive data types
class types
• the simplest types
• cannot decompose into other
types
• values only, no methods
• Examples:
int - integer
double - floating point
(real)
char - character
• more complex
• composed of other types
(primitive or class types)
• both data and methods
• Examples:
String
The class String
• Used to manipulate strings (methods)
• String
–
–
–
–
–
–
Sequence of zero or more characters
Enclosed in double quotation marks
Null or empty strings have no characters
Numeric strings consist of integers or decimal numbers
Length is the number of characters in a string
String class has methods to operate on strings
char charVariable = `a`;
//single quotes
String stringVariable = "a"; //double quotes
String sentence = "Hello, world";
Strings and the Operator +
(Concatenation)
• Operator + can be used to concatenate two strings
or a string and a numeric value or character
• Example:
Indexing Characters within a String
• The index of a character within a string is an integer starting at 0
for the first character and gives the position of the character
• The charAt(Position)method returns the char at the
specified position
• substring(Start, End)method returns the string from
position Start to position End
• For example:
String greeting = "Hi, there!";
greeting.charAt(0)returns H
greeting.charAt(2)returns ,
greeting.substring(4,7)returns the
H
i
,
0
1
2
3
t
h
e
r
e
!
4
5
6
7
8
9
Parsing Numeric Strings
• String to int
Integer.parseInt(strExpression)
• String to float
Float.parseFloat(strExpression)
• String to double
Double.parseDouble(strExpression)
Input
•
Named constant
–
–
–
content cannot be changed during program execution
declared by using the reserved word final
initialized when it is declared
final double CONVERSION = 2.45;
final char BLANK = ‘ ‘;
•
Variable (name, value, data type, size)
–
–
–
–
–
content may change during program execution
must be declared before it can be used
may not be automatically initialized
if new value is assigned, old one is destroyed
value can only be changed by an assignment statement or an input (read)
statement
Input
•
Standard input stream object: System.in
•
Input numeric data to program
–
•
Separate by blanks, lines, or tabs
To read a line of characters:
1. Create an input stream object of the class
BufferedReader
2. Use the method readLine
To read characters from the input stream:
1. Declare & initialise a variable to the standard input device
(System.in) to read only one character at a time.
InputStreamReader charReader
= new InputStreamReader(System.in);
2. Declare & initialise another input stream variable to read an entire
line of characters.
BufferedReader keyboard
= new BufferedReader(charReader);
3. Declare String variable to contain the input.
String inputStr;
4. Read & assign input line of characters to variable.
inputStr = keyboard.readLine();
Inputting Numeric Data
• Numeric data read as a sequence of characters.
• Must parse the numeric string into an appropriate
number.
String numString;
double miles;
BufferedReader keyboard =
new BufferedReader(new InputStreamReader(System.in));
numString = keyboard.readLine();
miles = Double.parseDouble(numString);
Reading a single character
• Use the method read (from BufferedReader).
• Returns the integer value of the character.
– A returns a value of 65
• Will :. have to use a cast operator to convert this
integer value in a char value.
char ch;
BufferedReader keyboard =
new BufferedReader(new InputStreamReader(System.in));
ch = (char) keyboard.read();
Output
• Standard output object: System.out
• Methods
– print
– println
– flush
• Syntax
System.out.print(stringExp);
System.out.println(stringExp);
System.out.flush();
Commonly Used Escape
Sequences
Packages, Classes, Methods, &
the import Statement
• Package: collection of related classes
• Class: consists of methods
• Method: designed to accomplish a specific
task
import Statement
• Used to import the components of a package into a
program
• Reserved word
• For example import java.io.*;
imports the (components of the) package java.io into the
program
• Primitive data types and the class String
– Part of the Java language
– Don’t need to be imported
Java Application Program
• Syntax of the main method
More on Assignment Statements
• variable = variable * (expression);
is equivalent to
• variable *= expression;
Similarly,
• variable = variable + (expression);
is equivalent to:
• variable += expression;
Documentation & Style
• Use meaningful names for variables, classes, etc.
• Use indentation and line spacing as shown in the
examples in the text
• Always include a “prologue” (an brief explanation of
the program at the beginning of the file)
• Use all lower case for variables, except capitalize
internal words (eggsPerBasket)
• Use all upper case for variables that have a constant
value, PI for the value of pi (3.14159…) (see text for
more examples)
Documentation & Style
• Use meaningful names for variables, classes, etc.
• Use indentation and line spacing as shown in the
examples in the text
• Always include a “prologue” (an brief explanation of
the program at the beginning of the file)
• Use all lower case for variables, except capitalize
internal words (eggsPerBasket)
• Use all upper case for variables that have a constant
value, PI for the value of pi (3.14159…) (see text for
more examples)
Programming Examples
• Make Change Program
– Input: Change in cents
– Output: Equivalent change in half-dollars,
quarters, dimes, nickels, and pennies
import java.io.*;
public class MakeChange
{
static BufferedReader keyboard = new
BufferedReader(new InputStreamReader(System.in));
static final int HALFDOLLAR = 50;
static final int QUARTER = 25;
static final int DIME = 10;
static final int NICKEL = 5;
public static void main (String[] args) throws IOException
{
//declare variables
int change;
System.out.print("Enter the change in cents: ");
System.out.flush();
change = Integer.parseInt(keyboard.readLine());
System.out.println();
System.out.println("The change you entered is "
+ change);
System.out.println("The number of half dollars "
+ "to be returned are "
+ change / HALFDOLLAR);
change = change % HALFDOLLAR;
System.out.println("The number of quarters to be returned are "
+ change / QUARTER);
change = change % QUARTER;
System.out.println("The number of dimes to be returned are "
+ change / DIME);
change = change % DIME;
System.out.println("The number of nickels to be returned are "
+ change / NICKEL);
change = change % NICKEL;
System.out.println("The number of pennies to be "
+ "returned are " + change); }
}
}