Review

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Review
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Memory

• Memory holds
• programs
• data for the computer to process
• the results of intermediate processing.
• two kinds of memory
• main memory
• auxiliary memory

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Main memory

• working memory used to store
• the current program
• the data the program is using
• the results of intermediate calculations
• usually measured in megabytes (e.g. 256 megabytes
  of RAM)
• RAM is short for random access memory
• a byte is a quantity of memory

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Auxiliary Memory

• also called secondary memory
• disk drives, diskettes, CDs, etc.
• more or less permanent (nonvolatile)
• usually measured in gigabytes (e.g. 50 gigabyte
  hard drive)

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Bits, Bytes, and Addresses

• A bit is a digit with a value of either 0 or 1.
• A byte consists of 8 bits.
• Each byte in main memory resides at a numbered
  location called its address.

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Addresses
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Storing Data

• Data of all kinds (numbers, letters, strings of
  characters, audio, video, even programs) are
  encoded and stored using 1s and 0s.
• When more than a single byte is needed, several
  adjacent bytes are used.
• The address of the first byte is the address of
  the unit of bytes.

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Programming Languages

• High-level languages are relatively to write and
  to understand.
• Java, Pascal, FORTRAN, C, C, BASIC, Visual
  Basic, etc.
• Unfortunately, computer hardware does not
  understand high-level languages.
• Therefore, a high-level language program must be
  translated into a low-level language.

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Compilers

• A compiler translates a program from a high-level
  language to a low-level language the computer can
  run.
• You compile a program by running the compiler on
  the high-level-language version of the program
  called the source program.
• Compilers produce machine- or assembly-language
  programs called object programs.

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Compilers, cont.

• Most high-level languages need a different
  compiler for each type of computer and for each
  operating system.
• Most compilers are very large programs that are
  expensive to produce.

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Java Byte-Code

• The Java compiler does not translate a Java
  program into assembly language or machine
  language for a particular computer.
• Instead, it translates a Java program into
  byte-code.
• Byte-code is the machine language for a
  hypothetical computer (or interpreter) called the
  Java Virtual Machine.

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Java Byte-Code, cont.

• A byte-code program is easy to translate into
  machine language for any particular computer.
• A program called an interpreter translates each
  byte-code instruction, executing the resulting
  machine-language instructions on the particular
  computer before translating the next byte-code
  instruction.

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Compiling, Interpreting, Running

• Use the compiler to translate the Java program
  into byte-code (done using the compile command).
• Use the byte-code interpreter for your computer
  to translate each byte-code instruction into
  machine language and to run the resulting
  machine-language instructions (done using the run
  command).

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Portability

• After compiling a Java program into byte-code,
  that byte-code can be used on any computer with a
  byte-code interpreter and without a need to
  recompile.
• Byte-code can be sent over the Internet and used
  anywhere in the world.
• This makes Java suitable for Internet
  applications.

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Class Loader

• A Java program typically consists of several
  pieces called classes.
• Each class may have a separate author and each is
  compiled (translated into byte-code) separately.
• A class loader (called a linker in other
  programming languages) automatically connects the
  classes together.

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Object-Oriented Programming

• Our world consists of objects (people, trees,
  cars, cities, airline reservations, etc.).
• Objects can perform actions which effect
  themselves and other objects in the world.
• Object-oriented programming (OOP) treats a
  program as a collection of objects that interact
  by means of actions.

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OOP Terminology

• Objects, appropriately, are called objects.
• Actions are called methods.
• Objects of the same kind have the same type and
  belong to the same class.
• Objects within a class have a common set of
  methods and the same kinds of data
• but each object can have its own data values.

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OOP Design Principles

• OOP adheres to three primary design principles
• encapsulation
• polymorphism
• inheritance.

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Errors

• An error in a program is called a bug.
• Eliminating errors is called debugging.
• three kinds or errors
• syntax errors
• runtime errors
• logic errors

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Syntax Errors

• grammatical mistakes in a program
• the grammatical rules for writing a program are
  very strict
• The compiler catches syntax errors and prints an
  error message.
• example using a period where a program expects a
  comma

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Runtime Errors

• errors that are detected when your program is
  running, but not during compilation
• When the computer detects an error, it terminates
  the program an prints an error message.
• example attempting to divide by 0

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Logic Errors

• errors that are not detected during compilation
  or while running, but which cause the program to
  produce incorrect results
• example an attempt to calculate a Fahrenheit
  temperature from a Celsius temperature by
  multiplying by 9/5 and adding 23 instead of 32

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Compiling a Java Program or Class

• A Java program consists of one or more classes,
  which must be compiled before running the
  program.
• You need not compile classes that accompany Java
  (e.g. System and Scanner).
• Each class should be in a separate file.
• The name of the file should be the same as the
  name of the class.

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Compiling and Running

• Use an IDE (integrated development environment)
  which combines a text editor with commands for
  compiling and running Java programs.
• When a Java program is compiled, the byte-code
  version of the program has the same name, but the
  ending is changed from .java to .class.

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Compiling and Running, cont.

• A Java program can involve any number of classes.
• The class to run will contain the words
• public static void main(String args)
• near the beginning of the file.

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Primitive Types

• four integer types (byte, short, int, and long)
• int is most common
• two floating-point types (float and double)
• double is more common
• one character type (char)
• one boolean type (boolean)

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Primitive Types, cont.
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Assignment Statements

• An assignment statement is used to assign a value
  to a variable.
• answer 42
• The equal sign is called the assignment
  operator.
• We say, The variable named answer is assigned a
  value of 42, or more simply, answer is assigned
  42.

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Assignment Statements, cont.

• Syntax
• variable expression
• where expression can be another variable, a
  literal or constant (such as a number), or
  something more complicated which combines
  variables and literals using operators (such as
  and -)

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Simple Screen Output

• System.out.println(The count is count)
• outputs the string literal The count is
  followed by the current value of the variable
  count.

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Simple Input

• Sometimes the data needed for a computation are
  obtained from the user at run time.
• Keyboard input requires
• import java.util.
• at the beginning of the file.

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Simple Input, cont.

• Data can be entered from the keyboard
  using Scanner keyboard
• new Scanner(System.in)
• followed, for example, by
• eggsPerBasket keyboard.nextInt()
• which reads one int value from the keyboard and
  assigns it to eggsPerBasket.

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Assignment Compatibilities

• A value of one type can be assigned to a variable
  of any type further to the right
• byte --gt short --gt int --gt long
• --gt float --gt double
• but not to a variable of any type further to the
  left.
• You can assign a value of type char to a variable
  of type int.

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Type Casting

• A type cast temporarily changes the value of a
  variable from the declared type to some other
  type.
• For example,
• double distance
• distance 9.0
• int points
• points (int)distance
• (illegal without (int))

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The Division Operator

• The division operator (/) behaves as expected if
  one of the operands is a floating-point type.
• When both operands are integer types, the result
  is truncated, not rounded.
• Hence, 99/100 has a value of 0.

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The mod Operator

• The mod () operator is used with operators of
  integer type to obtain the remainder after
  integer division.
• 14 divided by 4 is 3 with a remainder of 2.
• Hence, 14 4 is equal to 2.
• The mod operator has many uses, including
• determining if an integer is odd or even
• determining if one integer is evenly divisible by
  another integer.

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Concatenation of Strings

• Two strings are concatenated using the
  operator.
• String greeting Hello
• String sentence
• sentence greeting officer
• System.out.println(sentence)
• Any number of strings can be concatenated using
  the operator.

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Classes

• A class is a type used to produce objects.
• An object is an entity that stores data and can
  take actions defined by methods.
• An object of the String class stores data
  consisting of a sequence of characters.
• The length() method returns the number of
  characters in a particular String object.
• int howMany solution.length()

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Objects, Methods, and Data

• Objects within a class
• have the same methods
• have the same kind(s) of data but the data can
  have different values.
• Primitive types have values, but no methods.

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Screen Output

• Weve seen several examples of screen output
  already.
• System.out is an object that is part of Java.
• println() is one of the methods available to the
  System.out object.

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Keyboard Input

• Java 5.0 has reasonable facilities for handling
  keyboard input.
• These facilities are provided by the Scanner
  class in the java.util package.
• A package is a library of classes.

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Using the Scanner Class

• Near the beginning of your program, insert
• import java.util.
• Create an object of the Scanner class
• Scanner keyboard
• new Scanner (System.in)
• Read data (an int or a double, for example)
• int n1 keyboard.nextInt()
• double d1 keyboard,nextDouble()

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The if-else Statement

• A branching statement that chooses between two
  possible actions.
• syntax
• if (Boolean_Expression)
• Statement_1
• else
• Statement_2

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The if-else Statement, cont.

• example
• if (count lt 3)
• total 0
• else
• total total count

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Compound Boolean Expressions

• Boolean expressions can be combined using the
  and () operator.
• example
• if ((score gt 0) (score lt 100))
• ...
• not allowed
• if (0 lt score lt 100)
• ...

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Compound Boolean Expressions, cont.

• Boolean expressions can be combined using the
  or () operator.
• example
• if ((quantity gt 5) (cost lt 10))
• ...
• syntax
• (Sub_Expression_1) (Sub_Expression_2)

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Negating a Boolean Expression

• A boolean expression can be negated using the
  not (!) operator.
• syntax
• !(Boolean_Expression)
• example
• (a b) !(a b)
• which is the exclusive or

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Using

• is appropriate for determining if two integers
  or characters have the same value.
• if (a 3)
• where a is an integer type
• is not appropriate for determining if two
  floating points values are equal. Use lt and
  some appropriate tolerance instead.
• if (abs(b - c) lt epsilon)
• where b, c, and epsilon are floating point types

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Using , cont.

• is not appropriate for determining if two
  objects have the same value.
• if (s1 s2), where s1 and s2 refer to strings,
  determines only if s1 and s2 refer a common
  memory location.
• If s1 and s2 refer to strings with identical
  sequences of characters, but stored in different
  memory locations, (s1 s2) is false.

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Using , cont.

• To test the equality of objects of class String,
  use method equals.
• s1.equals(s2)
• or
• s2.equals(s1)
• To test for equality ignoring case, use method
  equalsIgnoreCase.
• (Hello.equalsIgnoreCase(hello))

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equals and equalsIgnoreCase

• syntax
• String.equals(Other_String)
• String.equalsIgnoreCase(Other_String)

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The switch Statement

• The switch statement is a multiple branch that
  makes a decision based on an integral (integer or
  character) expression.
• The switch statement begins with the keyword
  switch followed by an integral expression in
  parentheses and called the controlling expression.

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The switch Statement, cont.

• A list of cases follows, enclosed in braces.
• Each case consists of the keyword case followed
  by
• a constant called the case label
• a colon
• a list of statements.
• The list is searched for a case label matching
  the controlling expression.

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The switch Statement, cont.

• The action associated with a matching case label
  is executed.
• If no match is found, the case labeled default is
  executed.
• The default case is optional, but recommended,
  even if it simply prints a message.
• Repeated case labels are not allowed.

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The switch Statement, cont.

• The action for each case typically ends with the
  word break.
• The optional break statement prevents the
  consideration of other cases.
• The controlling expression can be anything that
  evaluates to an integral type.

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The switch Statement, cont.

• syntax
• switch (Controlling_Expression)
• case Case_Label
• Statement(s)
• break
• case Case_Label
• default

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the while Statement

• also called a while loop
• A while statement repeats until a controlling
  boolean expression becomes false.
• If the controlling boolean expression is false
  initially, the while loop is not executed.
• The loop body typically contains a statement that
  ultimately causes the controlling boolean
  expression to become false.

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the while Statement, cont.

• syntax
• while (Boolean_Expression)
• Body_Statement
• or
• while (Boolean_Expression)
• First_Statement
• Second_Statement

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The do-while Statement

• also called a do-while loop
• similar to a while statement, except that the
  loop body is executed at least once
• syntax
• do
• Body_Statement
• while (Boolean_Expression)
• dont forget the semicolon!

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The do-while Statement, cont.

• First, the loop body is executed.
• Then the boolean expression is checked.
• As long as it is true, the loop is executed
  again.
• If it is false, the loop is exited.
• equivalent while statement
• Statement(s)_S1
• while (Boolean_Condition)
• Statement(s)_S1

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The for Statement

• A for statement executes the body of a loop a
  fixed number of times.
• example
• for (count 1 count lt 3 count)
• System.out.println(count)
• System.out.println(Done)

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The for Statement, cont.

• syntax
• for (Initialization, Condition, Update)
• Body_Statement
• Body_Statement can be either a simple statement
  or a compound statement in .
• corresponding while statement
• Initialization
• while (Condition)
• Body_Statement_Including_Update

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The break Statement in Loops

• A break statement can be used to end a loop
  immediately.
• The break statement ends only the innermost loop
  or switch statement that contains the break
  statement.
• break statements make loops more difficult to
  understand.
• Use break statements sparingly (if ever).

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The break Statement in Loops

• A break statement can be used to end a loop
  immediately.
• The break statement ends only the innermost loop
  or switch statement that contains the break
  statement.
• break statements make loops more difficult to
  understand.
• Use break statements sparingly (if ever).

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Basic Terminology

• Objects can represent almost anything.
• A class defines a kind of object.
• It specifies the kinds of data an object of the
  class can have.
• It provides methods specifying the actions an
  object of the class can take.
• An object satisfying the class definition
  instantiates the class and is an instance of the
  class.

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Basic Terminology, cont.

• The data items and the methods are referred to as
  members of the class.
• We will call the data items associated with an
  object the instance variables of that object
  (i.e. that instance of the class).

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Method Definitions

• All method definitions belong to some class.
• All method definitions are given inside the
  definition of the class to which they belong.
• If the definition of the method begins with
  public void, it does not return a value.
• public indicates that use is unrestricted.
• void indicates that the method does not return a
  value.

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Method Definitions, cont.

• The parentheses following the method name contain
  any information the method needs.
• The first part of the method definition is called
  the heading.
• The remainder of the method is called the body,
  and is enclosed in braces .
• Statements or declarations are placed in the body.

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Method Definitions, cont.

• The parentheses following the method name contain
  any information the method needs.
• The first part of the method definition is called
  the heading.
• The remainder of the method is called the body,
  and is enclosed in braces .
• Statements or declarations are placed in the body.

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Local Variables

• A variable declared within a method is called a
  local variable.
• Its meaning is local to (confined to) the
  method definition.
• Variables with the same name declared within
  different methods are different variables.
• A local variable exists only as long as the
  method is active.

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The public and private Modifiers

• The instance variables of a class should not be
  declared public.
• Typically, instance variables are declared
  private.
• An instance variable declared public can be
  accessed and changed directly, with potentially
  serious integrity consequences.
• Declaring an instance variable private protects
  its integrity.

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The private Modifier

• The private modifier makes an instance variable
  inaccessible outside the class definition.
• But within the class definition, the instance
  variable remains accessible and changeable.
• This means that the instance variable can be
  accessed and changed only via the methods
  accompanying the class.

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Accessor and Mutator Methods

• Appropriate access to an instance variable
  declared private is provided by an accessor
  method which is declared public.
• Typically, accessor methods begin with the word
  get, as in getName.
• Mutator methods should be written to guard
  against inappropriate changes.

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Static Methods and Static Variables, cont.

• Static methods and static variables belong to a
  class and do not require any object.

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Static Methods

• Some methods have no meaningful connection to an
  object. For example,
• finding the maximum of two integers
• computing a square root
• converting a letter from lowercase to uppercase
• generating a random number
• Such methods can be defined as static.

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Static Methods, cont.

• A static method is still defined as a member of a
  class.
• But, the method is invoked using the class name
  rather than an object name.
• syntax
• return_Type Variable_Name
• Class_Name.Static_Method_Name
• (Parameters)

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Putting main in Any Class

• A class which contains a method main serves two
  purposes
• It can be run as a program.
• It can be used to create objects for other
  classes.

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Putting main in Any Class, cont.

• A programs main method must be static.
• A nonstatic method in the same class cannot be
  invoked unless an object of the class is created
  and used as a calling object for the nonstatic
  method.
• In general, dont provide a method main in a
  class definition if the class will be used only
  to create objects.

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The Math Class

• The predefined class Math provides several
  standard mathematical methods.
• All of these methods are static methods.
• You do not need to create an object to call the
  methods of the Math class.
• These methods are called by using the class name
  (Math) followed by a dot and a method name.
• Return_Value Math.Method_Name(Parameters)
  

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The Math Class, cont.

• Method round returns a number as the nearest
  whole number.
• If its argument is of type double, it returns a
  whole number of type long.
• Method floor (ceil) returns the largest
  (smallest) whole number that is less (greater)
  than or equal to its argument.

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Overloading

• Weve seen that different classes can have
  methods with the same names.
• Two or more methods in the same class class can
  be defined with the same name if the parameter
  list can be used to determine which method is
  being invoked.
• This useful ability is called overloading.

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Overloading, cont.

• The number of arguments and the types of the
  arguments determines which method average is
  invoked.
• If there is no match, Java attempts simple type
  conversions of the kinds discussed earlier.
• If there is still no match, an error message is
  produced.

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Constructors

• When you create an object of a class, often you
  want certain initializing actions performed such
  as giving values to the instance variables.
• A constructor is a special method that performs
  initializations.

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Defining Constructors

• New objects are created using
• Class_Name Object_Name
• new Class_Name (Parameter(s))
• A constructor is called automatically when a new
  object is created.
• Class_Name (Parameter(s)calls the constructor and
  returns a reference.
• It performs any actions written into its
  definition including initializing the values of
  (usually all) instance variables.

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Defining Constructors, cont.

• Each constructor has the same name as its class.
• A constructor does not have a return type, not
  even void.
• Constructors often are overloaded, each with a
  different number of parameters or different types
  of parameters.
• Typically, at least one constructor, the default
  constructor, has no parameters.

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Defining Constructors, cont.

• When a class definition does not have a
  constructor definition, Java creates a default
  constructor automatically.
• Once you define at least one constructor for the
  class, no additional constructor is created
  automatically.