JAVA Workshop

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JAVA Workshop

• Ankur Purwar

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Object, Classes, Packages

• Ankur Purwar
• purwar_at_iitk.ac.in

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Objects, Classes, and Packages

• Introduction to Classes
• Object Variables and Object References
• Instantiating Objects
• Using Methods in Objects
• Packages and Importing Classes
• Familiarize yourself with API

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Introduction to Classes

• A class defines the attributes and behavior of a
  specific type of object
• Attributes are variables declared in the class
• Behaviors are methods defined in the class
• Normally, we access an object by calling a method
  defined by its class
• We may sometimes access an attribute defined by
  its class, but this is discouraged

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Introduction to Classes

• A class has a name that we can use as if it were
  a data type when declaring a variable
• When we declare a variable with the name of a
  class as its type, we are creating an object
  variable (can contain a reference to an object)
• We access an objects methods / attributes using
  the object variable name and the . notation, e.g.
  
• ClassName objectName //object variable
• objectName.methodName() // Note The ( )
• objectName.variableName // Note No ( )

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Example of a Class Definition

• We can draw a diagram of a class to outline its
  important features before writing code its
  name, attributes, and behaviors

StateMachine
Class Name

• state int
• . . .

List of its Variables
setState (int input) void . . .
List of its Methods
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Example of a Class Definition

• public class StateMachine
• // an attribute or variable
• private int state
• // a behavior or method
• public void setState (int input)
• state input

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Example of a Class Definition

• Declaring a StateMachine object variable
• StateMachine myStateMachine
• Accessing a myStateMachine method
• myStateMachine.setState(1)
• Why cant we just do this?
• myStateMachine.state 1
• Notice the word private in the declaration of the
  variable state?

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Prototype for a Class Definition

• We use the Java reserved word private to prevent
  access to a variable or method from code that is
  written outside the class
• We use the Java reserved word public to allow
  access to a variable or method from code that is
  written outside the class
• Normally, we declare variables to be private
• Normally, we declare methods to be public
• We will see some valid exceptions later

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Creating Objects, e.g. String class

• String is a commonly used class
• To declare a variable as an object reference, to
  a String, we use the class name of the object as
  the type name
• String title
• This declaration does not create an object
• It only creates a variable that can hold a
  reference to a String object.

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Creating Objects

• We use the new operator to create an object
• Creating an object is called instantiation
• An object is an instance of a particular class
• title is assigned a reference to a String object
  that contains (encapsulates) the string Java
  workshop

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

• Once an object has been instantiated, we can use
  the dot operator to invoke or call any of the
  objects methods
• int count title.length()
• A method may return a value which can be used in
  an assignment or expression
• The value of count will be set to 13
• A method invocation can be thought of as asking
  an object to perform a service

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References

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

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num1
Java Workshop"
title
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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

Java Workshop"
title
Before
Anant lab"
title2
title2 title
Java Workshop"
title
After
title2
Garbage See later slide
Anant Lab"
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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 more than one reference
  variable
• Aliases can be useful, but should be managed
  carefully
• Changing an object via one reference variable
  changes it for all of its aliases, because there
  is really only one object

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Garbage Collection

• When there are no longer any variables containing
  a reference to an object (e.g. Anant Lab on the
  earlier slide), the program can no longer access
  it
• The object is useless and is considered garbage
• Periodically, Java performs automatic garbage
  collection and returns an object's memory to the
  system for future use
• In other languages such as C/C, the programmer
  must write explicit code to do the garbage
  collection

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Garbage Collection

• Setting object variables value equal to null,
  makes the object garbage (unavailable)

Before
Java Workshop"
title
title null
null
title
After
No object
Garbage now
Java Workshop"
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Garbage Collection

• If an object variables value is equal to null,
  any reference to an attribute or method of that
  object will cause your program to fail.

String title new String(Java
Workshop) System.out.println(title.length())
// prints 13 title null System.out.println(ti
tle.length()) // fails
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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 Workshop"
• This special syntax 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
• See the list of String methods in API

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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 Java Workshop", the character J'
  is at index 0 and the W' is at index 5

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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
• Various classes we've already used (System,
  Scanner, String) are part of the Java standard
  class library (Look them up in API!)
• 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 packages in the standard class library are

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The import Declaration

• When you want to use a class contained in a
  package, you can use its fully qualified name
• java.util.Scanner scan ...
• Or you can import the package containing the
  class and just use the class name Scanner
• import java.util.Scanner
• Scanner scan ...
• 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, so that class must be
  imported as part of its package

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Formatting, Wrapper Classes

• Ankur Purwar
• purwar_at_iitk.ac.in

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Formatting, Wrapper Classes

• Random and Math Class
• Formatting Output
• Wrapper Classes and Autoboxing

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The Random Class

• The Random class is part of the java.util package
• It provides methods that generate a sequence of
  pseudorandom numbers
• A Random object performs complicated calculations
  based on a seed value to produce a stream of
  seemingly random values
• See RandomNumbers.java (Ref Lab3)

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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
• We already covered use of the Math class methods
  in solving quadratic equations

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Formatting Output

• When performing calculations with float or double
  types, we may have more precision in the result
  than is valid or than we need
• When we solved quadratic equations, the result
• had 16 digits after the decimal point
• That would be too many digits if the data we were
  displaying was dollars and cents where we would
  want only two digits after the decimal point

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Formatting Output

• The Java standard class library contains classes
  that provide formatting capabilities
• The NumberFormat class allows you to format
  values as currency or percentages
• The DecimalFormat class allows you to format
  values based on a pattern
• Both are part of the java.text package

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Formatting Output

• The NumberFormat class has static methods that
  return a formatter object
• getCurrencyInstance()
• getPercentInstance()
• Each formatter object has a method called format
  that returns a string with the specified
  information in the appropriate format

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Number Format

• // Format Locale locale Locale.CANADA
• String string NumberFormat.getPercentInstance(lo
  cale).format(123.45) // 12,345

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Formatting Output

• The DecimalFormat class can be used to format a
  floating point value in various ways
• For example, you can specify that the number
  should be truncated to three decimal places
• The constructor of the DecimalFormat class takes
  a string that represents a pattern for the
  formatted number

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• formatter new DecimalFormat(".")
• s formatter.format(-1234.567) // -1234.6
• formatter new DecimalFormat(".")
• s formatter.format(-1234.567) // -1234.567
• formatter new DecimalFormat(".")
• s formatter.format(-1234.567) // -1234.567
  formatter new DecimalFormat(".000000")
• s formatter.format(-1234.567) // -1234.567000

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Leading Blanks for Numbers

• There is no Java library mechanism to put leading
  blanks on digit strings to achieve right hand
  alignment of column of numbers
• Need to write nested conditional code
• System.out.println( "Number is "
• (n
• (n
• (n
• n))))

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Wrapper Classes

• The java.lang package contains a wrapper class
  that corresponds to each primitive type

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Wrapper Classes

• The following declaration creates an Integer
  object which is a reference to an object with the
  integer value 40
• Integer age new Integer(40)
• An object of a wrapper class is used in
  situations 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 may 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)
• Wrapper classes often contain useful constants
• For example, the Integer class contains MIN_VALUE
  and MAX_VALUE for 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 num
• The assignment creates the appropriate Integer
  object wrapping a value of 42
• The reverse conversion (called unboxing) also
  occurs automatically as needed

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Topic for next classGraphical User Interfaces

• Ankur Purwar
• purwar_at_iitk.ac.in