Static Methods Wrapper Class

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Static MethodsWrapper Class

• Dr. Hairong Zhao
• hairong_at_calumet.purdue.edu
• http//ems.calumet.purdue.edu/mcss/zhao
• Purdue University Calumet

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Static Methods

• Static methods belong to a class and do not
  require any object.
• The method can be invoked using the class name

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Predefined Classes with Static Methods

• Math
• part of the java.lang package
• The Math class contains methods that perform
  various mathematical functions
• Math.pow(2.0, 3.0) Math.sqrt(6.0)
  Math.PI Math.max(2.0, 3.0)
• Random
• part of the java.util package
• provides methods that generate pseudorandom
  numbers
• System
• Character

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Static Methods in Class Character
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Write the class Circle1

• instance variables
• private double radius
• methods
• public void setRadius(double newRadius)
• public double getRadius()
• public static double area(double radius)

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Write the class Circle1
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Write the test class Circle1Demo

• public class Circle1Demo
• public static void main(String args)
• Circle1 myCircle new Circle1( )
• myCircle.setRadius(2)
• double myArea myCircle.area(myCircle.get
  Radius())
• System.out.println("If my circle has
  radius " myCircle.getRadius()
  ", then its area is " myArea)
• System.out.println()
• System.out.println("Please choose a new
  radius for me")
• Scanner keyboard new Scanner(System.in)
  
• double newRadius keyboard.nextDouble(
  )
• myCircle.setRadius(newRadius)
• System.out.print("Now my circle has a new
  radius "
• newRadius)
• System.out.println(" and its area is "
  Circle1.area(newRadius))

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Using an Object to Call a Static or non-static
Method

• An object of the class can be used to call a
  static method of the class even though it is more
  common to use the class name to call the static
  method.
• You cannot invoke a nonstatic method, or refer to
  instance variables within a static method unless
  you create and use a calling object for the
  nonstatic method.

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Mixing Static and Nonstatic Methods

• public void showArea( )
• System.out.println("Area is "
  this.area(radius))
• public static void areaDialog( )
• Scanner keyboard new Scanner(System.in)
• System.out.println("Enter the radius of a
  circle")
• double newRadius keyboard.nextDouble( )
• Circle2 c new Circle2( )
• c.setRadius(newRadius)
• c.showArea( )

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Test Methods showArea and areaDialog
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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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Putting main in Circle Class
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Static Variables

• A class can have static variables and constants
  as well as static methods.
• public static final double PI 3.14159
• public static int numberOfInvocations 0
• The value of a static variable can be changed by
  any method that can access the variable.

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Static Variables, cont.

• Like instance variables, static variables
  generally are declared private.
• They should be read only by accessor methods.
• They should be changed only by mutator methods.
• Every object of the class has access to the
  static variable(s) via the (public) accessor and
  mutator methods.

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Static Variables, cont.

• Static variables are also called class variables.
• The primary purpose of static variables (class
  variables) is to store information that relates
  to the class as a whole
• The number of objects that have been created
• A constant that applies for all objects, PI,
  interestRate

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public class StaticDemo private static int
numberOfInvocations 0 public static
void main(String args) int i
StaticDemo object1 new StaticDemo( )
for (i 1 i lt10 i)
object1.outPutCountOfInvocations( )
StaticDemo object2 new StaticDemo( )
for (i 1 i lt10 i)
object2.justADemoMethod( )
System.out.println("Total number of invocations
" numberSoFar( )) public
void justADemoMethod( )
numberOfInvocations //In a real
example, more code would go here.
public void outPutCountOfInvocations( )
numberOfInvocations
System.out.println(numberOfInvocations)
public static int numberSoFar( )
numberOfInvocations return
numberOfInvocations
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Wrapper Classes

• Sometimes a primitive value needs to be passed in
  as an argument, but the method definition
  requires an object as the corresponding formal
  parameter.
• The java.lang package contains wrapper classes
  that correspond to each primitive type

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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, cont.

• To retrieve the integer value
• int i n.intValue()
• primitive type wrapper class extraction method
• int Integer intValue
• long Long longValue
• float Float floatValue
• double Double doubleValue
• char Character charValue

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Shorthand in Java 5.0

• Wrapping is done automatically in Java 5.0.
• Integer n 42
• which is equivalent to
• Integer n new Integer(42)
• Similarly
• int i n
• is equivalent to
• int i n.intValue

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Automatic Boxing and Unboxing

• Converting a value of a primitive type to an
  object of its corresponding wrapper class is
  called boxing.
• Integer n new Integer(42)
• Java 5.0 boxes automatically.
• Integer n 42

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Automatic Boxing and Unboxing, cont.

• Converting an object of a wrapper class to a
  value of the corresponding primitive type is
  called unboxing.
• int i n.intValue
• Java 5.0 unboxes automatically.
• int i n

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Automatic Boxing and Unboxing, cont.

• Automatic boxing and unboxing also apply to
  parameters.
• A primitive argument can be provided for a
  corresponding formal parameter of the associated
  wrapper class.
• A wrapper class argument can be provided for a
  corresponding formal parameter of the associated
  primitive type.

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Useful Constants

• Wrapper classes contain several useful constants
  and static methods such as
• Integer.MAX_VALUE
• Integer.MIN_VALUE
• Double.MAX_VALUE
• Double.MIN_VALUE

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Type Conversions

• Static methods in the wrapper classes can be used
  to convert a string to the corresponding number
  of type int, long, float, or double.
• String theString 199.98
• double doubleSample
• Double.parseDouble(theString)
• or
• Double.parseDouble(theString.trim())
• if the string has leading or trailing whitespace.

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Type Conversions, cont.

• Methods for converting strings to the
  corresponding numbers
• Integer.parseInt(42)
• Long.parseLong(42)
• Float.parseFloat(199.98)
• Double.parseDouble(199.98)

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Type Conversions, cont.

• Methods for converting numbers to the
  corresponding strings
• Integer.toString(42)
• Long.toString(42)
• Float.toString(199.98)
• Double.toString(199.98)

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Static Constants in Class Boolean

• The constants in wrapper class Boolean include
• Boolean.TRUE
• and
• Boolean.False
• but the keywords true and false are much easier
  to use.