Java I Refresher

1
Java I Refresher

• Jonathan F. Gemmell
• Saturday, January 7th / Friday, January 13th
• Room 819 CTI

2
Before we begin

• Sign up sheet
• Survey
• Does everyones computer work?
• These slide are new (let me know if you spot a
  typo)
• This is meant to be a hands-on review. We will
  spend about 50 of the time working through
  problems. Make sure I have your email address,
  so I can send you the solutions to the problems.
• Please feel free to raise your hand, shout out
  questions, or throw rotten fruit during the
  review.

3
Outline

• Basic structure of an application
• Demo HelloWorld
• Primitive data types and operations
• Demo MathProblems
• Using predefined objects
• Demo StringDemo, ScannerDemo
• Branching structures (if, if-else, switch)
• Demo ZipCode
• Loops (for, while, do-while)
• Demo Factor
• Demo MultiplicationTable

4
Outline continued

• Arrays
• Demo CalculateAverage
• More on the structure of an application
• Demo CircleCalculations
• Method Overloading
• Demo PrintNames
• Objects
• Demo Rectangle
• Demo Country and CalculatePopulations

5
Basic structure of an application

• A Java application consists of a definition of a
  class.
• The name of the class (ClassName) and the name of
  the file (ClassName.java) must be identical.
• The class definition has a header (public class
  ClassName) followed by a body inside braces.
• The body of the class may contain a method called
  main. The main method always has the modifiers
  public, static and void in front of it.

6
Demo HelloWorld

• Write the following program on your computer
• public class HelloWorld
• public static void main (String args)
• System.out.println("Hello World!")

7
Primitive data types and operations

• There are eight java primitives
• boolean true or false
• byte signed 8-bit integer
• char 16-bit Unicode 2.0 character
• short signed 16-bit integer
• int signed 32-bit integer
• long signed 64-bit integer
• float signed 32-bit floating-point
• double signed 64-bit floating-point

8
Primitive data types and operations

• Most often you will use.
• booleans for true/false variables
• true, false
• chars for characters
• a, b, c
• ints for integers
• 1, 2, 3 (not 1.00)
• doubles for decimals
• 1.0, 3.14, 9.9999999

9
Primitive data types and operations

• Java is a strongly typed language. An explicit
  type must be assigned to every data value.
• Ex
• int x 5
• int y
• y 7
• int z x y
• System.out.println(z)
• Output 12
• Notice that
• x, y and z are all declared as ints
• You can only declare a variable once!!!
• int x 5
• int x 6 ? Cant re-declare x!!

10
Primitive data types and operations

• Common operations on primitives
• Addition, Multiplication, etc
• double z x y,
• double z x y
• Modulus
• int z x y
• Numerical comparison and equality
• boolean z ( x
• boolean a (x y)
• Recall that one equals sign ( ) is an
  assignment
• Two equals signs ( ) is a comparison

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Primitive data types and operations

• Casting
• Will this compile?
• double x 3
• double y 2
• int z x / y
• Notice that a double has 64 bits, while an int
  has only 32. Trying to assign the value of x/y
  to z is like trying to pour a gallon of water
  into a 2-liter bottle. Try this instead.
• double x 3
• double y 2
• int z (double)(x / y)
• This is called casting. But be careful! You
  could lose precision. In the above example z
  will equal 1, not 1.5

12
Primitive data types and operations

• What is the value of z? Why?
• int x 3
• int y 2
• int z x / y
• In integer math the decimal gets thrown out. z
  equals 1, not 1.5

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Primitive data types and operations

• Modulus is a useful operation. It results in the
  remainder of the division operation.
• 10 3 1
• 6 2 0
• 5 3 2
• 14 5 4
• What is the value of z?
• int z 8 3

14
Demo MathProblems (5 minutes)

• Write an application called MathProblems.
  Declare variables where appropriate. Try some of
  the examples in the previous slides. Be sure to
  try at least one example of primitive operations,
  casting, integer math, and modulus.

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Using predefined objects

• One of the strengths of Java is the ability to
  create objects in your program from a large
  variety of pre-defined classes.
• REMEMBER THIS WEBSITE!!!!!
• http//java.sun.com/j2se/1.5.0/docs/api/index.html
  
• Notice that primitive data types create a
  variable, whereas a class produces an object.
• Each pre-defined class has a collection of public
  methods defined for it.

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Using predefined objects

• The String class
• One of the first classes you used in Java 211 was
  the String class. A String represents a sequence
  of characters.
• To create a String object
• String s new String(A string of characters)
• There is also a short-hand way to create a
  String
• String s Another string of characters

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Using predefined objects

• Methods of the String class
• int length() returns the number of characters in
  a String
• String s "this is a String"
• int i s.length()
• char charAt(int index) returns the character at
  the index specified
• String s "this is a String"
• char c s.charAt(3)
• int indexOf(char target) returns the index of
  the first occurrence of the target. -1 otherwise.
• String s "this is a String"
• int i s.indexOf('S')

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Using predefined objects

• String substring(int start, int end) returns a
  String that represents the characters from
  positions start to end
• String s "this is a String"
• String t s.substring(5,10)
• Many more!
• Concatenation
• You can concatenate two Strings together with the
  operator. You can also concatenate a String
  and a primitive.
• String s "this is a String."
• String t "this is also a String"
• String u s t

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StringDemo

• Write an application that declares some Strings.
  Use the methods described in the previous slide.
  Concatenate two Strings together. Try
  concatenating a String and a primitive.

20
Using predefined objects

• The Scanner class
• The Scanner class allows a program to read input
  taken from standard input. It can be found in
  the java.util package so you must put import
  java.util. in the program in order to use it.
• Declare and initialize a Scanner object
• Scanner s new Scanner(System.in)
• Use the Scanner object to obtain an integer
  value
• int x s.nextInt()
• Use the Scanner object to obtain a decimal value
• double y s.nextDouble()

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Using predefined objects

• The Scanner class cont.
• In order to use the Scanner class to input a
  String
• 1. Declare and initialize the Scanner object
• 2. Change the delimiter from whitespace to the
  line separator
• 3. Use the Scanner object to input a String
• Scanner s new Scanner(System.in)
• s.useDelimiter( System.getProperty("line.separator
  ") )
• String str s.next()

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ScannerDemo

• Write an application that declares and
  initializes a Scanner object. Use it to input
  integer values, decimal values, and Strings.
  Print the input values to the screen.

23
Branching structures

• This is the most basic if-statement
• if (boolean expression)
• //do this block of
• If the boolean expression is true, the program
  will execute the next block of code.

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Branching structures

• Compound ifelse statements
• if (boolean expression)
• //do this
• else if (boolean expression)
• //do something else
• else
• //do yet something else

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Branching structures

• A switch statement evaluates the given integer
  and execute the appropriate block of code.
• switch (x)
• case 1 //this block of code will be executed
  if x equals 1
• System.out.println("first case")
• break
• case 2 //this block of code will be executed
  if x equals 2
• System.out.println("second case")
• break
• default //this block of code will be executed
  as a default
• System.out.println("third case")
• break

26
Demo ZipCode

• Write an application that takes a three digit
  area code from a user and then tells a user where
  s/he lives.
• 312 Chicago
• 212 New York
• 510 San Francisco
• 972 Dallas
• Write one version that uses a switch statement
  and a second version that uses a compound if-else
  statement.

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Loops

• The purpose of a loops is to repeat a block of
  code many times.
• for loop
• while loop
• do- while loop

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Loops

• for loop
• Use a for loop when you know exactly how many
  times you want to repeat the loop. You may know
  the particular value, or know a variable that
  contains the value.
• The for loop has three crucial parts
• The initialization of the counter
• The boolean statement that tests the continuation
  condition
• Incrementing the counter
• For loops are useful when working with arrays (as
  we will see)
• EX print hello 5 times.
• for(int i0 i
• System.out.println("hello")

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Loops

• while loop
• Use a while loop when you want to repeat a block
  of code an unknown number of times (maybe not at
  all)
• A while loop will repeat the block of code until
  the boolean statement is no longer true
• while (boolean expression)
• //do this block of code

30
Loops

• do-while
• Use a do-while loop when you want to repeat a
  block of code an unknown number of times, but at
  least once.
• Perfect for user inputs.
• EX ask a user for a number between 1 and 10,
  repeat until the user enters a valid number.
• Scanner s new Scanner(System.in)
• int input
• do
• System.out.print("Enter a number between 1 and
  10 ")
• input s.nextInt()
• while(input 10)

31
Demo Factor

• Write an application that prompts the user for a
  positive integer and prints to the screen the
  factors of the input. The program should not
  accept negative numbers as an input, and should
  continue to ask for a value until the user enters
  a proper input.
• For example, if the user enters 7, the output
  should be 1 and 7. If the user enters 12, the
  output should be 1, 2, 3, 4, 6, and 12.

32
Demo MultiplicationTable

• It is crucial for you to understand and be able
  to implement nested loops. A nested loop is a
  loop inside another. Using nested loops, write
  an application that prints to the screen a five
  by five multiplication table.

33
Arrays

• In Java, arrays are objects
• You can create an array of any primitive or
  object class by using
• The declaration of an integer array
• int x
• The declaration of a String array
• String s
• Declaring an array does not create the object or
  allocate memory. It merely creates the variable.

34
Arrays

• Initializing arrays
• Use the key word new to initialize an array and
  provide the type of the array, and the size of
  the array within the brackets.
• int x new int100
• Creates an int array x, 100 elements long
• String s new String10
• Creates a String array s, 10 elements long

35
Arrays

• Modifying elements
• int x new int10
• x0 5
• x2 8
• In the above example, a new array is declared and
  initialized. The 0th element is set to 5, while
  the 2nd element is set to 8. Recall that a 10
  element array is numbered 0 through 9.
• Accessing elements
• int i x2
• In the above example, the program will access the
  2nd element of the array and assign its value to
  the integer variable i.
• Array Length
• In Java, an array knows how long it is.
• int x new int20
• int l x.length

36
Demo CalculateAverage

• Write an application that creates an int array of
  size 20, fills it with random numbers in the
  range 0 through 99, prints the array to the
  screen, and then calculates and prints the
  average of the numbers. (hint use for loops)
• To generate a random number in the range 0
  through 99
• int x (int)(Math.random() 100)

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More on the structure of an application

• So far the example programs have had only one
  method (the main method) and none of the examples
  have had any class variables. In practice, most
  programs will have many class variables and many
  methods.

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More on the structure of an application

• Class variables
• A class variable is declared at the class level,
  rather than inside the body of a method.
  Consequently, the scope of variable extends
  throughout the entire class.
• public class Test
• static int i
• public static void main (String args)
• i 5
• Notice that even while the variable i is declared
  outside the main method, it can still be accessed
  inside the main method.
• Also notice the variable i has the modifier
  static. More on this later.

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More on the structure of an application

• Scope
• public class Scope
• int x 5
• public static void main (String args)
• int y 6
• if (y 6)
• int z 7
• What is the scope of the variables x, y and z?
  Notice how proper indentation makes determining
  the scope easier.

40
More on the structure of an application

• Methods A method has two parts
• Header Describes the method in general terms,
  including its name, visibility, inputs and
  outputs.
• Visibility modifier
• Return type
• Method Name
• List of parameters
• Body Describes the specific actions the method
  will perform
• The body consists of a sequence of program
  statements to be executed. If the method returns
  a value, there must be a return statement.

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More on the structure of an application

• Visibility modifiers
• public
• Visible outside the class
• Called a service method
• private
• Not visible outside the class
• It can only be used by methods inside the class
• Called a support method

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More on the structure of an application

• Return type
• Can be any primitive, a class, or void.
• It tells us what kind of result comes back from
  the method when it is evaluated.
• If a method is supposed to return a value, there
  must be a return statement.

43
More on the structure of an application

• Method Name
• Gives the method a name, so that it can be called
  from elsewhere in the program.
• The convention is to use lower case letters
  except for the first letter of subsequent words.
  No white spaces.
• EX calculateSum
• EX getLastName
• EX SetPhoneNumber ? not convention

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More on the structure of an application

• List of parameters
• A list of the type and name of the expected
  inputs
• Type gives what kind of data is expected for that
  input
• Name gives the name by which it will be referred
  to during the methods execution
• Parameters are given in parentheses, separated by
  commas.
• There does not need to be any parameters at all
  if the method does not require them.

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More on the structure of an application

• An example of a method
• public static int squareNumber(int n)
• int result n n
• return result
• Visibility modifier - public
• Return type - int
• Method Name - squareNumber
• List of parameters an int named n

46
More on the structure of an application

• Another Example
• private static void printSum(double x, double y)
• double result x y
• System.out.println(result)
• Visibility modifier - private
• Return type - void
• Method Name - printSum
• List of parameters a double named x and a
  double named y

47
Demo CircleCalculations

• Write a program with a class variable named pi
  equal to 3.14.
• Write a method (dont put it in main!) that asks
  the user to enter a radius (double) and returns
  this value.
• Write a method that accepts a radius (double) as
  a parameter and calculates the area of the circle
  with that radius. It should return the area. (
  a pi r2 )
• Write a method that accepts a radius (double) as
  input and prints to the screen the circumference
  of a circle given the radius. The return type
  should be void. ( c 2 r pi )
• In the main method call these three methods
  appropriately.

48
Method Overloading

• Methods expect the proper input. The compiler
  will object if the wrong type or number of
  variables are supplied as arguments.
• An overloaded method has several different
  definitions, all with the same method name, but
  different types and/or number of inputs.

49
Method Overloading

• Signature
• The ordered list of types of parameters in the
  header of a method is called a signature.
• As long as they have different signatures, we can
  define as many methods as we want with the same
  name. The compiler will choose the correct
  method based on the methods signature.

50
Demo PrintNames

• Write a method called printName that accepts a
  String called lastName as a parameter. It should
  print to the screen the name provided.
• EX if the input is Smith, it should print out
  Mr. Smith.
• Write a method with the same method name. This
  method should take two Strings as parameters,
  firstName and lastName.
• EX if the inputs are John and Smith, it
  should print out John Smith
• Write a third method with the same method name.
  This method should take two Strings as arguments
  and one char.
• EX if the inputs are John, A, and Smith,
  the method should print out John A. Smith.
• Write a main method to test the overloaded
  methods.

51
Objects

• You have already used built-in classes
• String and Scanner for example
• You must now learn to write your own.
• A class should contain two distinct pieces of
  information
• Properties The state of the object
• Given by the data members or instance variables
• Behaviors The capabilities of the object
• Given by the methods or instance methods

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Objects

• Anatomy of a class
• public class ClassName
• //instance variables
• //constructor(s)
• //methods(s)

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Objects

• Instance Variables
• Typically, instance variables are not directly
  accessible. All your instance variables should
  be declared private.
• public any program can see (and perhaps
  modify!) this data
• private only visible to methods of the class
• EX
• public class Rectangle
• private int width
• private int height
• .
• Any object you create from the Rectangle class
  will have its own width and height.

54
Objects

• Constructor
• Constructors are a special type of method. They
  are often used to initialize the data members of
  a class when you create objects. They are
  automatically called upon declaration of an
  object.
• They should be declared public
• They should have exactly the same name of the
  class
• Like any other method, they may or may not have
  parameters

55
Objects

• Constructor cont
• public Rectangle(int w, int h)
• width w
• height h
• In this example the constructor takes two
  parameters, both ints, and assigns their values
  to the instance variables of the class.

56
Objects

• Constructor cont..
• To call the constructor and create an object of
  the Rectangle class
• Rectangle r new Rectangle(2,3)
• Assuming our Rectangle class is complete, this
  will create Rectangle object with a width of 2
  and a height of 3.

57
Objects

• Instance methods represent the operations that
  can be performed by objects in the class
• The methods can also be public or private
• When deciding which to use, consider whether or
  not you want the operation available outside the
  class.
• Consider the methods on the following slides
• Notice the getter and setter methods
• Notice that the calculateArea method will be
  called on a specific Rectangle object and will
  return the area of a that specific Rectangle
  object.

58
Objects

• public void setWidth(int w)
• width w
• public void setHeight(int h)
• height h
• public int getWidth()
• return width

59
Objects

• public int getHeight()
• return height
• public int calculateArea()
• int area width height
• return area

60
Objects

• Static
• Notice that neither of the methods or instance
  variables in the Rectangle class are static.
• Static variables or methods belong to the class
  as a whole rather than to a particular object of
  the class.
• You do not need to declare an object from a class
  in order to use static methods
• Conversely, you must declare an object from a
  class in order to use non-static methods
• Non-static variables belong to a specific
  instance of the class, not the class as a whole.

61
Demo Rectangle

• Implement the Rectangle class as described in the
  notes.
• Create a second program called RectangleDriver
  that creates many Rectangle objects and
  thoroughly tests all the methods.
• Add to the rectangle class a static int that
  keeps count of how many Rectangle objects have
  been created.

62
Country and CalculatePopulations

• We have gone over many small examples together.
  Now I would like you to go over a larger example
  on your own.
• I will be happy to speak with you individually if
  you require help.
• Dont forget that tutors are available at CTI
  seven days a week and are happy to help you with
  your studies.

63
Country and CalculatePopulations

• Write a Class called Country
• It should contain the following instance
  variables
• A String for the name of the country
• An int for the population of the country (in
  millions)
• A double for the growth rate of the country
• It should contain the follows methods
• A constructor to initialize all the member
  variables
• Appropriate getter and setter methods.
• A method called calculatePopulationForYear that
  takes as a parameter a year (int) and using the
  population and growth rate of the country
  calculates the population for that year. Ill
  give you this formula.

64
Country and CalculatePopulations

• Write a class called CalculatePopulations
• It should create an array of 5 countries and
  instantiate the Country objects using the
  constructor of the Country class.
• See http//www.cia.gov/cia/publications/factbook/
  for a list of countries and the populations, etc.
• It should print to the screen the five countries
  (hint use a for loop) and label them 0 through
  4.
• It should then ask the user to input an int, 0
  through 4, and repeat until the user enters an
  appropriate number.

65
Country and CalculatePopulations

• CalculatePopulations cont..
• It should then ask the user to input a year
  greater than 2000. Repeat if necessary.
• Finally, it should print to the screen the
  population of the selected country in the
  selected year.