CSE 501N Fall 06 03: Introduction to Objects

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CSE 501NFall 0603 Introduction to Objects

• 11 Sep 2006
• Rohan Sen

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Lecture Outline

• Administrative
• Lab 1 Questions
• Textbook TA
• Boolean expressions
• Objects
• Methods
• Packages
• Scopes

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TA

• Tom Mooney will be a TA for this class
• tbmooney_at_artsci.wustl.edu
• Office Hours
• 600 PM - 800 PM on Tuesdays
• Help with labs and answer questions

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Expressions

• An expression is a combination of one or more
  operators and operands
• Boolean expressions compute truth results and
  make use of the boolean operators

AND OR NOT ! XOR
Examples on the board
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Objects What is an object

• An object is a specific instance of a class
• Objects have an internal state
• Objects have methods that manipulate the internal
  state

Bank Account int balance 0
withdraw(int amt)
deposit(int amt)
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Objects

• Objects support encapsulation
• The internal state is hidden
• Done via access modifiers on its internal state
• Access to / modification of internal state is
  strictly via provided methods
• Prevents rogue programmer from compromising the
  object
• Access modifiers ensure only the right people
  can access the methods

Bank Account int balance 0
withdraw(int amt)
balance 100,000,000
deposit(int amt)
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ObjectsAccess Modifiers

• Access modifiers are used for internal state and
  methods
• Java provides four types of access modifiers
• public
• Anyone
• protected
• All other instances of this class and their
  subclasses
• default or Package
• All other classes in the package
• private
• Only instances of this class can

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ObjectsAccess Modifiers in practice
Bank Account private int balance 0
public withdraw(int amt)
default addInterest(int amt)
public deposit(int amt)
Why or why not does this make sense?
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Methods

• Methods support abstraction
• Abstraction is a process of hiding detail
• Methods specify what inputs it needs (parameters)
• Methods specify what outputs it produces (return
  values)
• How it translates the inputs to outputs is hidden
  from the programmer

Square Root Method
a
Sqrt(a)
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Methods

• Methods can be used to achieve different kinds of
  purposes
• Read the value of the internal state of an object
  (accessors)
• Change the value of the internal state of an
  object (mutators)
• Control the behavior of an object
• Act as a black box for some complex operation

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Method Declarations

• A method declaration specifies the code that will
  be executed when the method is invoked (called)
• When a method is invoked, the flow of control
  jumps to the method and executes its code
• When complete, the flow returns to the place
  where the method was called and continues
• The invocation may or may not return a value,
  depending on how the method is defined

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Method Control Flow

• If the called method is in the same class, only
  the method name is needed

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Method Control Flow

• The called method is often part of another class
  or object

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Method Header

• A method declaration begins with a method header

char calc (int num1, int num2, String message)
method name
parameter list
The parameter list specifies the type and name of
each parameter The name of a parameter in the
method declaration is called a formal parameter
return type
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Method Body

• The method header is followed by the method body

char calc (int num1, int num2, String message)
int sum num1 num2 char result
message.charAt (sum) return result
sum and result are local data They are created
each time the method is called, and are destroyed
when it finishes executing
The return expression must be consistent with the
return type
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The return Statement

• The return type of a method indicates the type of
  value that the method sends back to the calling
  location
• A method that does not return a value has a void
  return type
• A return statement specifies the value that will
  be returned
• return expression
• Its expression must conform to the return type

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Parameters

• When a method is called, the actual parameters in
  the invocation are copied into the formal
  parameters in the method header

ch obj.calc (25, count, "Hello")
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Local Data

• As weve seen, local variables can be declared
  inside a method
• The formal parameters of a method create
  automatic local variables when the method is
  invoked
• When the method finishes, all local variables are
  destroyed (including the formal parameters)
• Keep in mind that instance variables, declared at
  the class level, exists as long as the object
  exists

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Constructors

• Note that a constructor has no return type
  specified in the method header, not even void
• A common error is to put a return type on a
  constructor, which makes it a regular method
  that happens to have the same name as the class
• The programmer does not have to define a
  constructor for a class
• Each class has a default constructor that accepts
  no parameters

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Creating Objects

• A variable holds either a primitive type or a
  reference to an object
• A class name can be used as a type to declare an
  object reference variable
• String title
• No object is created with this declaration
• An object reference variable holds the address of
  an object
• The object itself must be created separately

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Creating Objects

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

title new String (Hello World")
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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Invoking Methods

• Once an object has been instantiated, we can use
  the dot operator to invoke its methods
• count title.length()
• A method may return a value, which can be used in
  an assignment or expression
• A method invocation can be thought of as asking
  an object to perform a service or to manipulate
  its state

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References

• Note that a primitive variable contains the value
  itself, but an object variable contains the
  address of the object
• An object reference can be thought of as a
  pointer to the location of the object
• Rather than dealing with arbitrary addresses, we
  often depict a reference graphically

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Assignment Revisited

• The act of assignment takes a copy of a value and
  stores it in a variable
• For primitive types

num2 num1
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Reference Assignment

• For object references, assignment copies the
  address

name2 name1
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Aliases

• Two or more references that refer to the same
  object are called aliases of each other
• That creates an interesting situation one object
  can be accessed using multiple reference
  variables
• Aliases can be useful, but should be managed
  carefully
• Changing an object through one reference changes
  it for all of its aliases, because there is
  really only one object

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Garbage Collection

• When an object no longer has any valid references
  to it, it can no longer be accessed by the
  program
• The object is useless, and therefore is called
  garbage
• Java performs automatic garbage collection
  periodically, returning an object's memory to the
  system for future use
• In other languages, the programmer is responsible
  for performing garbage collection

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The String Class

• Because strings are so common, we don't have to
  use the new operator to create a String object
• title "Java Software Solutions"
• This is special syntax that works only for
  strings
• Each string literal (enclosed in double quotes)
  represents a String object

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String Methods

• Once a String object has been created, neither
  its value nor its length can be changed
• Thus we say that an object of the String class is
  immutable
• However, several methods of the String class
  return new String objects that are modified
  versions of the original

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String Indexes

• It is occasionally helpful to refer to a
  particular character within a string
• This can be done by specifying the character's
  numeric index
• The indexes begin at zero in each string
• In the string "Hello", the character 'H' is at
  index 0 and the 'o' is at index 4

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Class Libraries

• A class library is a collection of classes that
  we can use when developing programs
• The Java standard class library is part of any
  Java development environment
• Its classes are not part of the Java language per
  se, but we rely on them heavily
• Other class libraries can be obtained through
  third party vendors, or you can create them
  yourself

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Packages

• The classes of the Java standard class library
  are organized into packages
• Some of the packages in the standard class
  library are

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The import Declaration

• When you want to use a class from a package, you
  could use its fully qualified name
• java.util.Scanner
• Or you can import the class, and then use just
  the class name
• import java.util.Scanner
• To import all classes in a particular package,
  you can use the wildcard character
• import java.util.

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The import Declaration

• All classes of the java.lang package are imported
  automatically into all programs
• It's as if all programs contain the following
  line
• import java.lang.
• That's why we didn't have to import the System or
  String classes explicitly in earlier programs
• The Scanner class, on the other hand, is part of
  the java.util package, and therefore must be
  imported

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The Random Class

• The Random class is part of the java.util package
• It provides methods that generate pseudorandom
  numbers
• A Random object performs complicated calculations
  based on a seed value to produce a stream of
  seemingly random values

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The Math Class

• The Math class is part of the java.lang package
• The Math class contains methods that perform
  various mathematical functions
• These include
• absolute value
• square root
• exponentiation
• trigonometric functions

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The Math Class

• The methods of the Math class are static methods
  (also called class methods)
• Static methods can be invoked through the class
  name no object of the Math class is needed
• value Math.cos(90) Math.sqrt(delta)
• Variables can be declared static too
• In such cases, the variable is shared across all
  instances of the class
• Changing the value in one instance will change it
  in all others
• Usually used to capture a property of a class
  that will be common across all instances

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Scopes

• A scope of a variable defines the context in
  which it is accessible
• state or instance variables are accessible from
  any method in the class
• If declared public / default / protected,
  accessible from other classes as well
• local variables are accessible only within the
  method in which they are declared
• Examples on board

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Conclusion

• Lab 1 due next week