A%20Sorting%20Example

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A Sorting Example

• (a start for Lab 4 problem number 3)

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Alphabetizing Words

• How do we sort a list of words into alphabetic
  order?
• compare first letter, if different then done
• otherwise, compare 2nd letter
• repeat
• In general, this way of ordering is called
  lexicographic ordering

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To Start Alphabetizing Characters

• This is easy if everything is lower case
• for characters char1 and char2
• char1 lt char2 just in case char1 precedes char2
• It is just as easy if everything is upper case
• String s MixED uP cASe
• s s.toLowerCase()
• // resulting string is mixed up case
• After converting, then alphabetize

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Idea

• Given input strings s1 and s2 of equal length.
• boolean isBefore(String s1, String s2)
• if(s1.charAt(0)lts2.charAt(0))
• return true
• else if(s1.charAt(0)gts2.charAt(0))
• return false
• else // this means they are equal
• check charAt(1) .

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What About Different Lengths?

• To complete the solution
• What happens if s1 and s2 have different lengths?
• e.g. dump comes before dumpster
• After completing isBefore(s1,s2)
• How can you use it to sort an array of strings?

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Classes and Objects
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Problem Solving

• Solving a problem involves many activities
• understand the problem
• design a solution
• consider alternatives and refine the solution
• implement the solution
• test the solution
• These activities are not purely linear they
  overlap and interact

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Problem Solving

• Key point break problem into manageable pieces
• e.g. submethods, recursive calls
• When writing software, we design separate pieces
  that are responsible for certain parts of the
  solution
• An object-oriented approach lends itself to this
  kind of decomposition
• breaks problems into objects and classes

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Objects

• Informally bits of code that represent real
  objects in the world
• Example an object might represent an employee at
  a company
• Each employee object handles the processing and
  data management for that employee

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Objects

• An object has
• state descriptive characteristics
• behaviours
• what it can do
• what can be done to it
• The state includes fixed and variable
  characteristics
• Behaviours can change the state

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Objects

• Consider a bank account object
• state includes
• account balance (varies over time)
• account number (fixed)
• behaviours include
• deposit/withdrawal (change balance)
• opening/closing

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Classes

• Java uses the word class in two different ways
• collections of methods (like class libraries)
• descriptions of an object type
• So far, we have only really worked with the first
  kind of class

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Classes

• An object is defined by a class
• Think of a class as a blueprint for creating
  objects of a certain type
• Multiple objects can be created from the same
  class
• Instances of the class
• The class that contains the main method
  represents the whole program

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Objects and Classes
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Inheritance

• One class can be used to derive another via
  inheritance
• Classes can be organized into hierarchies

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Containment

• Sometimes an object contains objects of a
  different type
• This is not the same as inheritance
• For example, a class Bicycle may involve
• 2 Wheel objects
• 1 Chain object
• 1 Frame object
• A Bicycle itself might inherit from Vehicle
• A Bicycle might belong to a Rider

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Containment

• A graphical contains-a relationaship

Bicycle
Wheel
Wheel
Chain
Frame
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Creating Objects

• Generally, we use the new operator to create an
  object

title new String ("Java Software Solutions")
This calls the String constructor, which is a
special method that sets up the object

• Creating an object is called instantiation

• An object is an instance of a particular class

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Creating Classes

• Instantiating the class with new creates an
  object in memory
• Each class has one or more special methods called
  constructors
• called when the object is created
• used to set initial state parameters
• sometimes arguments are passed
• define our own next class

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Example String Objects

• We have already seen the String class
• //constructor
• String s new String(example)
• //String methods
• char c s.charAt(3)
• // c m
• String s1 s.concat(123)
• //sexample s1example123

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String Class Constructors

• String s1 //create reference no constructor
• s1 new String()
• //initialize to empty string
• String s2 new String(blah blah)
• // initialize from a string blah blah
• char chars a,b,c
• String s3 new String(chars)
• //initialize from an array of characters

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A Reminder about References

• For primitive types

num2 num1
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A Reminder about References

• For object references

"Steve Jobs"
name1
Before
"Steve Wozniak"
name2
name2 name1
"Steve Jobs"
name1
After
name2
"Steve Wozniak"
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Garbage Collection

• When there are no longer any references to an
  object, it can no longer influence the running of
  the program
• e.g. the string Steve Wozniak
• In this case, Java automatically returns this
  memory to the system
• This is called garbage collection

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Class Variables

• The state of an object is given by class
  variables or data members
• the BankAccount class may have a class variable
  called AccountBalance
• Typically these are declared private (later)
• Methods can use and change these variables
• getBalance()
• deposit(500)

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Getters and Setters

• The state of an object is usually manipulated
  with methods
• Instead of accessing data members
• Assures the state remains consistent
• e.g. getName() setName()
• Methods that read and write data members are
  called accessors and mutators
• or getters and setters

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Using static

• Two types of methods in a class definition
• code library vs. object description
• How do we tell which is which?
• Using static to differentiate
• static code library
• non-static object method

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Static Methods

• Wont be copied with each new object created
• Can be called as functions, no names needed
• x methodName()
• x ClassName.methodName()
• Need all values passed as parameters

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Non-static Methods

• Copied into each object instance
• Must be called with object reference
• MyClass o new MyClass()
• x o.MethodName()
• Implicitly use the object that they are part of
  as a parameter
• Used in classes that describe an object type

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Enumerated Types

• Simple types that allow you to enumerate all
  possible values
• The values are identifiers of your own choosing
• Any number of values can be listed
• Example
• enum Season winter, spring, summer, fall

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Enumerated Types

• Once an enumerated type is defined, variables of
  that type can be declared
• Season time
• time Season.fall
• Only admissible identifiers are allowed

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Why?

• Just a simple way to specify a simple type
• This could easily be done with full class
  declarations
• Could also be done with integer arrays
• (internally, types are stored as integers)
• Can not assign numeric identifiers
• We really wont use these much in class

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Wrapper Classes

• The java.lang package contains wrapper classes
  that correspond to each primitive type

Primitive Type Wrapper Class
byte Byte
short Short
int Integer
long Long
float Float
double Double
char Character
boolean Boolean
void Void
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Wrapper Classes

• The following declaration creates an Integer
  object which represents the integer 40 as an
  object
• Integer age new Integer(40)
• An object of a wrapper class can be used in any
  situation where a primitive value will not
  suffice
• For example, some objects serve as containers of
  other objects
• Primitive values could not be stored in such
  containers, but wrapper objects could be

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Wrapper Classes

• Wrapper classes also contain static methods that
  help manage the associated type
• For example, the Integer class contains a method
  to convert an integer stored in a String to an
  int value
• num Integer.parseInt(str)
• The wrapper classes often contain useful
  constants as well
• For example, the Integer class contains MIN_VALUE
  and MAX_VALUE which hold the smallest and largest
  int values

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Autoboxing

• Autoboxing is the automatic conversion of a
  primitive value to a corresponding wrapper
  object
• Integer obj
• int num 42
• obj int
• The assignment creates the appropriate Integer
  object
• The reverse conversion (called unboxing) also
  occurs automatically as needed

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How To Make a Wrapper

• class IntValue
• private int iValue
• public int getValue()
• return iValue
• public void setValue(int i)
• iValue I

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How To Make a Wrapper

• You dont need to do this
• Just use Integer
• It is better in that it has many more methods
• But this code shows why these are called wrapper
  classes
• Its like wrapping an integer inside a package
• Essentially the only time we use wrapper classes
  is for holding primitive types