Introduction to Java Programming

1
Introduction to Java Programming

• Today we start off with basics of Java
  programming
• Java programs are written as classes
• This will be important later on when we want to
  build large programs or ones that use built-in
  routines for graphics
• For now, we will write single, stand-alone
  classes
• The syntax will be a bit weird, just get used to
  it
• All classes start with a class definition of the
  form
• public class name
• name must match the file name, for instance if we
  name a class HelloWorld, then the file must be
  named HelloWorld.java (or HelloWorld.jav, etc)
• Our class will comprise one piece of code, a main
  method
• The main method must always appear as
• public static void main(String args)
• where the are the instructions that we want to
  execute

2
Example Hello World
class name defined
public class HelloWorld public static void
main(String args) System.out.println("Hell
o World!")
the main method
one executable statement, it outputs a message to
System.out (a window)
used to delimit (start and stop) blocks of
code
Running the program produces this output
3
Statements

• Inside the of our main method, we list our
  statements
• These will include variable and constant
  declarations (we had none in this program)
• And executable statements
• Input statements, output statements
• Assignment statements
• Loops and selection statements
• For the hello world program, the only executable
  statement is println
• This is an output statement
• It outupts whatever is placed inside of the ( )
• In this case, it outputs the literal message
  Hello World!

4
Variables, Values, Literals

• We want to store information for our program to
  use
• values are either
• placed directly in the program by the programmer,
  called literals
• or input or computed by the program and stored in
  variables
• Hello World is a literal, it will always appear
  this way
• Variables are memory locations where values can
  be stored and retrieved
• The advantage of a variable is that the value can
  change, that is, the variable can hold multiple
  values
• Values have types, in Java the types are
• int, short, long
• char
• boolean
• float, double
• objects

We declare variables using the form type
variable name As in int x or char a, b, c
5
Example 2 Revised Hello World
public class HelloWorld2 public static void
main(String args) String name name
"Frank Zappa" System.out.println("Hello "
name)
declaring a variable, name is a String Set name
to store Frank Zappa
output now contains a literal (Hello ) and a
variable notice the blank space after o
The output
6
Another Way to Output

• System.out represents a window on your monitor
• often known as the console window or the system
  window
• Java has numerous GUI classes, one of which is
  JOptionPane, which creates pop-up windows of
  different types
• To use this class (or others), you must import it
• import javax.swing.JOptionPane or
• import javax.swing.
• Replace the System.out.println statement with an
  appropriate message to JOptionPane
• JOptionPane.showMessageDialog(null, Hello
  World, title", JOptionPane.INFORMATION_MESSAGE)

A JOptionPane window created by the program on
the next slide
7
Example 3 New Hello World
import all classes from this library (these are
all GUI classes)
import javax.swing. public class
HelloWorld3 public static void main(String
args) String name name "Frank
Zappa" String title title "output
GUI" JOptionPane.showMessageDialog(null,
"Hello " name, title, JOptionPane.INFORMATION_
MESSAGE) System.exit(0)
add a title variable and store our title there
output message sent to pop-up window of type
INFORMATION_MESSAGE
System.exit(0) makes sure that the program
ends once our pop-up window is closed
8
String Concatenation

• Notice in our previous programs, we used a in
  our output statements
• Hello name
• The is used to concatenate (join together)
  Strings
• So this results in the word Hello followed by a
  blank followed by whatever is stored in name
• Imagine that the int variable age stores a
  persons age, then we could output a message
  like
• JOptionPane.showMessageDialog(null, "Hello "
  name ", you are " age " years old "
  ,title, JOptionPane.INFORMATION_MESSAGE)
• Notice that
• we separate the literal parts of the message and
  the variables using signs
• we enclose all literals in quote marks
• we include blank spaces inside of quote marks to
  enforce blanks

9
More on Variables

• We divide variables between
• primitive types (int, double, char, boolean, etc)
  
• and objects (Strings and others)
• Primitive types are handled through assignment
  statements using arithmetic operators (, -, ,
  /, )
• Objects are handled through message passing
• variable_name.message(parameters)
• Examples name is a String, to get its length
• name.length( )
• To get name as all upper or lower case
  characters
• name.toUpperCase( ) name.toLowerCase( )
• NOTE to name a variable, use characters, digits
  and _ as in name1 or last_name or lastName
  (variables cannot start with a number and cannot
  be any of Javas reserved words like public,
  void, int)

10
Assignment Statements

• To assign a variable to a value
• The form of an assignment statement is
• variable expression
• expression is some arithmetic expression using
  variables, literals, and arithmetic operators ,
  -, , /, (mod)
• For an assignment statement to properly work, the
  expression must compute the same (or a
  compatible) type as the variable
• x 5 y z // x must be a compatible type
  with y and z
• for instance, if y and z are ints, x can be an
  int, long, float or double, if y is a double,
  then x must be a double
• We can also have expressions that work on Strings
  using the String concatenation operator () or
  logical expressions that work on true and false
  values

11
More on Assignments

• Expressions may also contain messages to various
  objects as long as the messages invoke operations
  that return the proper type of value
• for instance, the Math class has a number of such
  operations such as sqrt, whereas
  JOptionPane.showMessageDialog does not return any
  value
• x z Math.sqrt(y) // what if y is negative?
• x z Math.sqrt(Math.abs(y))
• To compute the average of 5 test scores
• average (t1 t2 t3 t4 t5) / 5
• Note that if t1, t2, t3, t4 and t5 are int
  values, the expression computes an int value even
  if average is a float or double

12
Numeric Conversions

• Two types of conversion processes in Java
• Implicit conversions (called coercions) occur
  automatically if the left-hand side variable is a
  different type as to the right-hand side
  expression
• x y // y is an int, x is a double
• Casts occur because you force a value to change
  types
• casts are performed by placing the new type in (
  ) as in (int)
• average ((double) t1 t2 t3 t4 t5) / 5
• changes t1 to a double, then the rest of the
  expression computes a double value not an int
  value
• we could force a coercion instead by doing
  average (t1 t2 t3 t4 t5) / 5.0 // 5.0
  is a double, so the entire thing becomes a double

13
Getting Input

• The easiest way to get input from the user is to
  use JOptionPane again
• In this case, the message to JOptionPane is
  showInputDialog
• Example name JOptionPane.showInputDialog(Ente
  r your name)
• This opens a pop-up window (see below) with the
  question Enter your name and a box for the user
  to enter something, plus two buttons, OK and
  Cancel
• When the user clicks on the OK button or presses
  the enter key, whatever was typed in is returned
  and stored in the String name

14
Example Input and Output
import javax.swing.JOptionPane public class
HelloWorld4 public static void
main(String args) String firstName
JOptionPane.showInputDialog("Enter your first
name") String lastName JOptionPane.showInputDi
alog("Enter your last name") JOptionPane.show
MessageDialog(null, "Hello " firstName " "
lastName, "Greeting", JOptionPane.INFORMATION_M
ESSAGE) int many firstName.length( )
lastName.length( ) JOptionPane.showMessageDialog
(null, "Your name has " many " characters
in it", "Length", JOptionPane.INFORMATION_MESSAGE)
System.exit(0)
15
More on Input

• showInputDialog is only one way to obtain input
• we will use it for now and study other approaches
  later
• one restriction on using showInputDialog is that
  it can only input and return a String
• If you want to input some other type (int,
  double, char) you have to convert it
• To convert input into integer
• Integer.parseInt(JOptionPane.showInputDialog())
• int age Integer.parseInt(JOptionPane.showInputDi
  alog(Enter your age))
• To convert an input into double
• Double.parseDouble(JOptionPane.showInputDialog())
  
• double gpa double.parseDouble(JOptionPane.showIn
  putDialog(Enter your GPA))
• To convert an input into char
• JOptionPane.showInputDialog().charAt(0)
• char sex JOptionPane.showInputDialog(Enter
  your sex).charAt(0)

16
Using charAt

• charAt returns a single character from a String
  at the position of the value indicated in the
  parens (this will be an int value or a variable
  storing an int)
• charAt is a message passed to a String object
• Example firstName.charAt(0) returns the first
  letter
• The integer value passed to charAt must be an int
  between 0 and the length of the String 1
• For instance, to get someones initials, you
  might use
• initials firstName.charAt(0)
  middleName.charAt(0) lastName.charAt(0)
• or more appropriately
• initials firstName.charAt(0) .
  middleName.charAt(0) . lastName.charAt(0)
  .
• To get the last character in a String, use length
  but remember that the last character is at length
  1
• lastLetters firstName.charAt(firstName.leng
  th( ) 1) middleName.charAt(middleName.length(
  ) 1) lastName.charAt(lastName.length( ) 1)

Notice the use of which casts the expression
into a String
17
More String Messages

• replace(oldchar, newchar)
• return a String with all of the chars that match
  oldchar replaced by newchar
• substring(startingpoint, endingpoint)
• return a String that consists of all of the
  characters of this String that start at the index
  startingpoint and end at endingpoint - 1
• toUpperCase( ), toLowerCase( )
• return new Strings that replace all letters in
  the old String with their uppercase (or
  lowercase) equivalents but leave all other
  characters (digits, punctuation marks) unchanged
• length( )
• already seen, returns an int value
• concat(str)
• takes the String str and concatenates it to this
  String, so that str1.concat(str2) does the same
  as str1 str2

18
String Using Example
import javax.swing. public class
StringUsingExample public static void
main(String args) // get
the user's full name in 3 inputs String
firstName JOptionPane.showInputDialog("Enter
your first name") String middleName
JOptionPane.showInputDialog("Enter your middle
name") String lastName JOptionPane.showInputD
ialog("Enter your last name") String name
firstName " " middleName " "
lastName name name.replace('e', '3') //
store in name the new String once we name
name.replace('E', '3') // replace each of 'e',
'i', 'g', also name name.replace('i',
'1') // replacing the upper case
versions name name.replace('I', '1') name
name.replace('g', '6') name
name.replace('G', '6') // output a
response JOptionPane.showMessageDialog(null,
"Hello " name ".\nI do not like the
letters 'e', 'i' or 'g'\n" "so I have
replaced them with '3', '1' and '6'",
"Response", JOptionPane.INFORMATION_MESSAGE)
System.exit(0) // end the program once the
dialog window is closed
19
The if Statement

• A common form of selection statement is the if
  statement
• In the if statement, a condition is tested, and
  if it evaluates to true
• then the statement which follows is executed,
  otherwise the
• statement which follows is skipped

20
Why Use an if?

• We use the if statement so that our program can
  make decisions and carry out actions
• consider computing the users age as the current
  year their birth year
• age currentYear birthYear
• this is accurate as long as they have already had
  a birthday, but if not, it is 1 year too many
• consider that you were born in December of 1989,
  this would make you 16 but our code above would
  compute your age as 17
• input whether the user has had their birthday yet
  (use the charAt(0) to take whatever they entered
  and only store the first letter as a char)
• if(birthday n) age age 1 // no
  birthday yet then deduct 1

21
Examples
if (total gt amount) total total (amount
1) if (sex m) pay pay 500 if
(age gt 21) System.out.println("Ok, you can
drink") if (sunny) System.out.println("Wear
your sunglasses!") if (rainy)
System.out.println("Take an umbrella") if (x gt
y) x x 5 y
sex is a character sunny and rainy
are boolean variables
Warning Do not confuse and
22
if-else statement

• Used if there are two possibilities
• For instance, if you want to calculate someones
  pay, there are two formulas, one for normal pay
  and another for overtime pay
• Example below to the right
• Note that the if-else statement has two separate
  statements, one following the condition, one
  following the word else
• Neither the condition nor else have a after
  it, the follows the statement
• We refer to the statements as they if clause
  and the else clause

if (hours lt 40) pay hours wages else
pay 40 wages (hours 40)
wages 1.5
23
More Examples
if (score gt 60) grade P else grade
F if (number gt 0) sqt
Math.sqrt(number) else System.out.println("Ca
nt take the square root of a negative
number!") if (number 0)
System.out.println("Cant take reciprocal of
0") else rec 1.0 / (float) number if
(age gt 21) canGamble true else
canGamble false
Use if-else to decide what value should be
assigned a variable Use if-else to decide if an
operation could/should be performed
24
Block Statements

• All of our examples so far have had a single
  statement as the if clause and the else clause
• What if we needed to accomplish more than one
  instruction in an if or else clause?
• We use block statements by wrapping all of the
  instructions inside of
• Blocks will be used often in our Java programs

If (temperature gt 80)
System.out.print("Wear shorts and t-shirt ")
effectiveTemp temperature HEATINDEX
humidity System.out.println("because it
is " effectiveTemp " degrees today")
Else System.out.println("It is not hot
today, the temperature is " temperature) The
if-clause has 3 instructions, so we place them in
a block