Java Basics II

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Java Basics - II Yangjun Chen Dept. Business
Computing University of Winnipeg
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Outline Java Basics - II

• Modifiers (Specifiers)
• Statements
• Array
• Control flow
• - condition statements
• if
• switch
• - loop structure
• for
• while
• do
• others String, print, new, constructor

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Modifiers

• Modifiers are special keywords that modify the
  definition of a class, method, or variables.

modifiers
Modifiers for methods
Modifiers for variables
Modifiers for classes
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Modifiers for Methods and Variables

• static
• final
• private
• protected
• public

access modifiers
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Modifiers for Methods and Variables

• Static and final Modifiers
• - The static keyword is used in the declaration
  of class variables and class methods.
• - At times we want to associate a variable or a
  method with the class rather than with every
  instance (every object) of the class.
• - Within a class, a static variable can be
  accessed by referring to its name.
• - From outside of the class, a static variable
  can be visited using the dot notation
• className.variableName.
• - A static method can be visited from outside of
  the class using the dot notation
  className.methodName.

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Modifiers for Methods and Variables
Class Circle static double
PI3.141592635 double radius double
circumference double diameter
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Modifiers for Methods and Variables

• Here, PI shouldnt be a variable. (It is just a
  constant.)
• To make it not changeable, we add the keyword
  final to the declaration.
• Class Circle
• final static double PI3.141592635
• - Any attempt to modify PI now will generate an
  error.
• - Final modifier can not be used for methods.
• Access modifiers allow you to control the
  visibility and access to variables and methods
  inside your class.

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• Difference between instance and class variables
• Class Circle
• static double PI3.141592635
• double radius 2.0
• double circumference 12.56
• double diameter 4.0
• Class ComputationCircles
• double x, y, z, w
• double computation()
• x Circle.PI
• Circle o new Circle()
• y o.radius z o.circumference w
  o.diameter

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• Difference between instance and class methods
• Class Circle
• ...
• static double m1( )
• double m2 ( )
• Class ComputationCircles
• double x, y, z, w
• double computation()
• ...
• x Circle.m1()
• Circle o new Circle()
• y o.m2()

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Modifiers for Methods and Variables

• There are three access modifiers in Java
• - private
• protected
• public
• private Access Modifier
• - The private modifier creates a non-accessible
  member.
• - A private method or variable restricts access
  only to those methods in the same class.
• - The private identifiers are unknown to other
  classes even if the extend (subclass) from the
  class.
• - To declare a member private, simply put the
  keyword private in front of its declaration.
• private int number

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Modifiers for Methods and Variables

• protected Access Modifier
• - The protected modifier allows access to a
  member within the same package and within any
  subclasses.
• - We wont study packages at this point. (They
  are basically a collection of classes that you
  can define yourself and used as a library.)
• - You can declare an identifier protected by
  placing the protected keyword in front of its
  declaration.
• protected int number
• public Access Modifier
• - The public modifier allows access to a member
  inside or outside of the class.
• public int number

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Modifiers for Classes

• There are three modifiers for classes in Java.
• - public
• This makes a class accessible to outside of the
  package.
• - final
• This prevents a class from being extended.
• - abstract
• In an abstract class, some abstract methods
  (with only method signature no implementation)
  are defined. Therefore, an abstract class can
  not be instantiated.

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Statements

• There are several types of statements
• - Declaration statements, expression statements,
  return statements and compound statements.
• Declaration Statements
• - declare new variables by specifying the type,
  variable name, properties and optionally
  initialize the variable.
• - some example are
• Button clear
• private Point spnew Point(45, 15),
  ep final int Max100000
• int almostMaxMax - 500

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Statements

• Expression Statements
• - Expression statements cause the expression to
  be evaluated and then any other actions in the
  statement are executed.
• - some example are
• clear new Button(Clear)
• i
• p q

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Statements

• Return Statements
• - statements in a method with non-void return
  types that return some information back to the
  location where the method was called.
• - A return statement is of the following form
• return (expression)
• - The expression can be any expression that is
  of the same type as the header declaration of
  the method.
• - When a return is encountered, it forces the
  immediate exit from the method.

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Statements

• - Example
• double magnitude(double x, double y)
• double sqrSum xx yy
• return Math.sqrt(sqrSum)
• Compound Statements
• - a collection of statements enclosed in curly
  braces.
• - Example
• Integer obj new Integer(8)
• System.out.println(obj.value)

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Array

• An array is a collection of items.
• Each array has some number of slots, each of
  which can hold an individual item.
• - An item can be any object or primitive
  variable but an array can be only of one type
  of items
• - You cant have an array with different types
  in it.
• Steps to create an array
• - 1. Declare a variable to hold the array
• int temp
• - 2. Create new array object and assign it
• temp new int10
• - 3. Store things in the array
• temp0 6

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Array

• Note that the array index starts from zero, so if
  the array is of size 10, the index values will
  run from 0 to 9.
• It is also possible to initialize the array
  during declaration
• - int temp 1, 2, 3, 4, 5

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Multidimentional Array

• This is just an array of arrays and is declared
  in the same manner as regular arrays.
• - Example int temp new int105
• - This will create an array that has 10 rows and
  5 columns.
• - This will allow us to store 50 values in temp.
• - You would access each item of the array by
  specifying both indices,
• for example,
• temp63 12
• This will store the value 12 into location
  with row index 6 and column index 3.
• - int temp 33, 71, -16, 45, 99, 27

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if Statement

• The if statement
• - written as
• if (boolean expression)
• statement //controlled statement
• - If the boolean expression evaluates true, then
  the controlled statement is executed. If it is
  false, then the controlled statement is skipped.
• - The controlled statement can also be a
  compound statement.

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if Statement

• The if else statement
• - This is another form of the if statement.
• - It allows some code to be executed if the
  boolean expression is true and some other code
  if false.
• - written as
• if (boolean expression)
• statement1 //expression is true
• else
• statement2 //expression is false

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switch Statement

• The switch statement
• - Instead of chaining if else statements, a
  switch statement can be used for testing
  equality.
• - This is like a case statement in some other
  languages.
• - The expression must evaluate to a primitive
  type and the constants must be literals of final
  variables.
• - The defaul case is evaluated if the expression
  fails to match any of the constants.
• (The default may be omitted, but it is a good
  idea to have it here.)

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switch Statement
Switch (expression) case constant1 some
statements case constant2 some
statements default some statements
//end of switch
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switch Statement

• Whenever a label (case constant) is matched,
  execution of the program continues from there
  through the switch statement executing each line.
  
• - This is the fall through behavior.
• To avoid this, a statement is used.
• - A break statement will force the exit from a
  switch statement
• - In almost all cases, you would write a switch
  statement with a break statement separating each
  case statement.

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switch Statement
Switch (expression) case constant1 some
statements break case constant2 some
statements break default some
statements //end of switch
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for Statement

• The for loop in Java looks as follows
• for ( init expression increment)
• some statements
• //end of the for loop
• The init is an expression that will introduce the
  start of the for loop.
• - Example int i 0
• - declares and initializes the index i for this
  loop.

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for Statement

• The expression is a boolean expression that is
  evaluated before each pass of the loop.
• - If it is true, then the loop is executed.
• - Example i lt 10
• The increment expression is any expression of
  function call.
• The increment is commonly used to bring the state
  of the loop closer to returning false and
  completing.
• - Example int i

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for Statement

• So the for loop looks like this
• for (int i 0 i lt 10 i )
• System.out.println(i)
• //end of the for loop
• What is the output?

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while Statement

• while loops like for loops will repeat the
  execution of a block of code until a specific
  condition is met.
• A while loop looks like the following
• while (boolean expression)
• statements
• //end of the while loop
• As long as the boolean expression evaluates true,
  the code in the loop is executed.

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do Statement

• Another loop that is quite similar to the while
  loop is called the do loop.
• A do loop takes the form
• do
• statements
• while (boolean expression)
• The difference between this loop and the while
  loop is that the condition is tested at the end
  of the loop.
• - do loop always execute the loop at lease once.

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Loop Constructs

• For all these loops, you can use the break
  statement to exit the loop at any time.
• There is also a continue statemment that forces
  the loop to start over at the next iteration.
• Label1
• outer-iteration
• inner-iteration
• break //break out of the inner iteration
• continue //move back to the beginning of the
  inner iteration
• continue label1 //move all the way back to
  Lable1
• break lable1 //break out of both iterations

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int i 0 label for (int j 0 j lt 10
j) while (i lt 10) System.out.println(
i) break i
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label for (int j 0 j lt 10 j) int
i 0 while (i lt 10) System.out.println(
i) continue i
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int k0 label for (int j k j lt 10
j) int i 0 while (i lt 10)
System.out.println(i) k continue
label
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label for (int j 0 j lt 10 j) int
i 0 while (i lt 10) System.out.println(
i) break label i
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big_loop while (!done) if (test(a, b) 0)
continue //Control goes to point 2 try
for (int j 0 j lt 10 j) if (aj
null) continue //Control goes to
point 1 else if bj null) continue
big_loop //Control goes to point 2 doit(aj,
bj) //point 1. Increment and start loop
again with the test finally
cleanup(a, b) //point2. Start loop again
with the (!done) test
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break_test if (check(i)) try for (int j
0 jlt10 j) if (j gt i )
break //Terminate just this loop. if
(aij null) break break_test //Do the
finally clause and //terminate the if
statement. finally cleanup(a, i, j)
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Strings

• Strings are a combination of characters and
  instances of the class String in Java.
• - They are not simply an array of characters.
• String literals are a series of characters
  enclosed in quotation marks. A string declaration
  would look like
• - String myStringBrand new String
• Unlike many other language strings can not be
  indexed character by character.
• - There are methods in the String class that
  will retrieve characters for you.
• String s This is a string!
• char char_a s.charAt0

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Print Statements

• The System.out.println method prints massage out
  to the standard output of your system - to the
  screen, to a special window, or maybe to a log
  file.
• It also prints a new line character at the end of
  a string.
• System.err.println() prints on standard err
  instead.
• You can concatenate arguments to println() using
  the operator.
• Example
• - System.out.println(Hello World!)

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Print Statements

• Using print() instead of println() does not break
  the line.
• Example
• System.out.print(Hello)
• System.out.print(World!)
• System.out.println()
• This will yield the same output as the previous
  example.
• System.out.println() breaks the line and flushes
  the output.

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new Operator

• The new operator is used for creating new
  instance of classes or objects.
• To create a new object, use the new operator with
  the name of the class you want to create an
  instance of, then parentheses after that.
• String str new String()
• Random r new Random()
• Triple origin new Triple()
• - The parentheses need not be empty they can
  contain arguments that determine the initial
  values of instance variables or other properties.
• - Example
• Point pt new Point(0, 0)

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new Operator

• The number and type of arguments that can be used
  in the parentheses with new are defined by the
  class constructor method.
• - Example
• Date d1 new Date()
• Date d2 new Date(71, 7, 1, 7, 30)
• Date d3 new Date(April 3 1993 324 PM)

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Constructors
Class Date Date ( )
Date (int x, int y, int z, int u, int v)
Date (String s) ...
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Constructors

• In the above example, we have shown three
  different constructors for the Date class. This
  is called method overloading or in this case
  constructor overloading.
• Overloading is when you have methods with the
  same name but with different arguments. The
  compiler will distinguish between the methods by
  different arguments
• - different arguments mean
• different number of arguments or
• different type of arguments

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Constructors

• Constructors are special methods that are used to
  initialize new object, set its variables, and
  perform any other functions the object needs to
  initialize itself.
• These methods are called by Java automatically
  when you create a new object.
• Constructors always have the same name as class
  with no return type.
• For example, say we had a class Circle and always
  wanted the center to start at (100, 100).

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Constructors

• Class Circle
• int x, y
• Circle( )
• x 100
• y 100
• So every time we create an object of class
  Circle, the constructor will be called
  automatically setting x and y to 100,
  respectively.

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Backslash codes Code Meaning code Meaning \b Ba
ckspace \N Octal constant \n Newline (where N
is 0 to 7) \r Carriage return \NN Octal
constant \f from feed (where N is 0 to
7) \t Horizental tab \MNN Octal
constant \ Double quote (where M is 0 to 3
and \ Single-quote character N is 0 to
7) \0 Null \uxxxx Unicode character \\ Backslash
(where xxxx are four hexadecimal constants)