Constants, Variables, String Manipulation, Data Manipulation

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Constants, Variables, String Manipulation, Data
Manipulation

• MINS 116
• Spring 2000
• Java Basics

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Topics

• Constants
• Variables
• Data manipulation
• String manipulation
• Constructors

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Constants

• A constant is a value which will not change
  throughout the entire lifetime of your program.
• Constants should be declared for any value
  describing
• physical constants (e.g. pi, gravitational
  constant, etc.)
• logical constants (e.g. array bounds, loop
  constraints, etc.)

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Constants

• In Java, a constant is declared using the keyword
  final.
• The final keyword informs the compiler that the
  element being defined will not change.
• Example
• private final int MAX_ENTRIES 100

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Constants

• The use of the final keyword causes a constant to
  be defined for the scope of the class in which it
  was defined.
• E.g.
• public class UserParser
• private final int MAX_USERS 100
• defines a constant which can only be seen by
  class UserParser and will only exist while
  UserParser is in memory. There will also be a
  separate copy of MAX_USERS for each instance of
  class UserParser created.

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Constants

• The addition of keyword static will define a
  constant which holds across all instances of the
  class and will only take up a single piece of
  program memory. So
• public class UserParser
• private static final int MAX_USERS 100
• defines a constant which can only be seen by
  class UserParser. There will only be a single
  MAX_USERS for all instances of class UserParser
  created. Basically, MAX_USERS is a global
  constant with controlled access.

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Variables

• A variable is used to hold information which is
  variable and will change throughout the life of
  the program.
• Ex
• public class UserParser
• ...
• private int charCount 0 // Number of
  characters processed so far
• ...

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Variables

• charCount can be used throughout the life of
  class UserParser.
• Variables can be
• Any of the primitive data types (e.g. int, float,
  char, etc.).
• Classes included in the Java support files (e.g.
  String, Int, Float, Vector, JFrame, etc.)
• Classes created by other programmers and classes
  you have created yourself.

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Variables - Primitive Data Types

• Java has a defined set of primitive data types.
• These are data types which are not considered
  classes and have no methods associated with them.

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Variables - Primitive Data Types

• Type Size in bits
  Value Range
• boolean 8 true or false
• char 16 \u0000 to
  \uFFFF
• byte 8 -128 to 127
• short 16 -32,768 to 32,767
• int 32 -2,147,438,648 to 2,147,438,647
• long 64 -9,223,372,036,854,775,808 to
• 9,223,372,036,854,775,808
• float 32 -3.40292347E38 to
• 3.40292347E38
• double 64 -1.79769313486231570 to
• 1.79769313486231570

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Data Manipulation

• Data Manipulation encompasses
• Arithmetic operations
• Functions provided by the Java support classes.
• Logical operations

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Data Manipulation - Arithmetic Operations

• Available Arithmetic Operators
• Assignment
• Precedence ( )
• Addition x ab
• Subtraction - x a-b
• Multiplication x ab
• Division / x a/b
• Modulus x ab

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Data Manipulation - Arithmetic Operations

• Arithmetic Expressions obey a set precedence for
  evaluation.
• ( ) Inside evaluated first. If nested,
  innermost
• first. Left to right on the same
  level.
• , / or Evaluated second. Left to right for
  several.
• or - Evaluated last. Left to right for
  several.

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Data Manipulation - Arithmetic Operations

• One thing nice about Java is it what is called
  Strongly Typed. A strongly typed language will
  not allow conversion between data types unless
  there is a well defined conversion routine.
• Java will, however, let you typecast certain
  variables. Doing this can produce erroneous
  results.

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Data Manipulation - Arithmetic Operations

• For instance
• private int iResult 0
• private int iVar 12
• private float fVar 10.59
• iResult iVar (int)fVar
• What is in iResult?

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Data Manipulation - Arithmetic Operations

• In addition to the standard arithmetic operators,
  arithmetic operation may be combined with the
  assignment operator
• operator use translation
• c 5 c c5
• - c - 5 c c-5
• c 5 c c5
• / c / 5 c c/5
• c 5 c c 5

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Data Manipulation - Arithmetic Operations

• And finally, there is the auto operators which
  cause a certain increment or decrement to happen
  either before or after an arithmetic evaluation
• operator use translation
• c c c1
• c (a) c a a a1
• c (a) a a1 c a
• -- c-- c c-1
• c (a--) c a a a-1
• c (--a) a a-1 c a

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Data Manipulation - Support Classes

• Be careful with data type conversions.
• Java provides some utility classes to help out
  with this. They are
• Byte Character Double
• Float Integer Long
• Number Short String
• BigDecimal BinInteger

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Data Manipulation - Logical Operations

• Logical operations can be performed to direct the
  control of a program. For instance, an if-else
  statement will rely on the evaluation of a
  logical operation to direct the logic of the
  program.

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Data Manipulation - Logical Operations

• Available Logical Operators
• Symbol Meaning Syntax
• lt Less Than a lt b
• lt Less Than or Equal a lt b
• gt Greater Than a gt b
• gt Greater Than or Equal a gt b
• Equal a b
• ! Not Equal a ! b
• Precedence of logical operators is last and based
  on a left to right evaluation.

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String Manipulation

• String manipulation in Java is special. Java
  Strings understand how to do many nice things
  implicitly.
• When dealing with Strings, the symbol becomes
  the concatenation operator.
• Ex
• private String firstString Java
• private String secondString MINS 116
• private String thirdString firstStringsecondStr
  ing
• What is stored in thirdString?

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String Manipulation

• This allows strings to be easily put together.
• Also, many of the data types implicitly know how
  to be represented as a string.
• Ex
• private int firstVar 66
• private String secondVar Route
• private String output secondVar
  firstVar
• What is stored in variable output?

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Constructors

• Every Java class has a constructor associated
  with it.
• A constructor is automatically executed upon
  creation of a Java object.
• Constructors should contain code which is
  required to successfully initialize a class upon
  creation.

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Constructors

• Example
• public class eCustomer
• private Vector productList
• public eCustomer( )
• // Perform necessary initializations
• productList new Vector( ) //
  Create a new product list
• ...

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Constructors

• Every time a new eCustomer is created, a new
  product list will be allocated.
• New objects are created using the new
  operation.
• thisCustomer new eCustomer( )
• The new operation causes the allocation of
  memory for the object and the execution of the
  objects constructor after the memory has been
  allocated.

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Constructors

• new does not need to be called for the
  primitive data types.
• String can be treated and initialized as though
  it were a primitive data type.
• E.g.
• private String name John E. Doe

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Summary

• Constants
• Variables
• Data manipulation
• String manipulation
• Constructors
• review them in the book!