Enhancing Classes

1
Enhancing Classes

• Now we can explore various aspects of classes and
  objects in more detail
• Chapter 5 focuses on
• object references and aliases
• passing objects references as parameters
• the static modifier
• wrapper classes
• nested classes and inner classes

2
References

• Recall from Chapter 2 that an object reference
  variable holds the memory address of an object
• Rather than dealing with arbitrary addresses, we
  often depict a reference graphically as a
  pointer to an object
• ChessPiece bishop1 new ChessPiece()

3
The null Reference

• An object reference variable that does not
  currently point to an object is called a null
  reference
• The reserved word null can be used to explicitly
  set a null reference
• name null
• or to check to see if a reference is currently
  null
• if (name null)
• System.out.println ("Invalid")

4
The null Reference

• An object reference variable declared at the
  class level (an instance variable) is
  automatically initialized to null
• The programmer must carefully ensure that an
  object reference variable refers to a valid
  object before it is used
• Attempting to follow a null reference causes a
  NullPointerException to be thrown
• Usually a compiler will check to see if a local
  variable is being used without being initialized

5
The this Reference

• The this reference allows an object to refer to
  itself
• That is, the this reference, used inside a
  method, refers to the object through which the
  method is being executed
• Suppose the this reference is used in a method
  called tryMe
• If tryMe is invoked as follows, the this
  reference refers to obj1
• obj1.tryMe()
• But in this case, the this reference refers to
  obj2
• obj2.tryMe()

6
The this reference

• The this reference can also be used to
  distinguish the parameters of a constructor from
  the corresponding instance variables with the
  same names

public Account (Sring name, long acctNumber,
double balance) this.name
name this.acctNumber acctNumber
this.balance balance
7
Assignment Revisited

• The act of assignment takes a copy of a value and
  stores it in a variable
• For primitive types
• num2 num1

8
Reference Assignment

• For object references, assignment copies the
  memory location
• bishop2 bishop1

9
Aliases

• Two or more references that refer to the same
  object are called aliases of each other
• One object (and its data) can be accessed using
  different reference variables
• Aliases can be useful, but should be managed
  carefully
• Changing the objects state (its variables)
  through one reference changes it for all of its
  aliases

10
Testing Objects for Equality

• The operator compares object references for
  equality, returning true if the references are
  aliases of each other
• bishop1 bishop2
• A method called equals is defined for all
  objects, but unless we redefine it when we write
  a class, it has the same semantics as the
  operator
• bishop1.equals(bishop2)
• We can redefine the equals method to return true
  under whatever conditions we think are appropriate

11
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

12
Objects as Parameters

• Parameters in a Java method are passed by value
• This means that a copy of the actual parameter
  (the value passed in) is stored into the formal
  parameter (in the method header)
• Passing parameters is therefore similar to an
  assignment statement
• When an object is passed to a method, the actual
  parameter and the formal parameter become aliases
  of each other

13
Passing Objects to Methods

• What you do with a parameter inside a method may
  or may not have a permanent effect (outside the
  method)
• See ParameterPassing.java (page 277)
• See ParameterTester.java (page 279)
• See Num.java (page 280)
• Note the difference between changing the
  reference and changing the object that the
  reference points to

14
ParameterPassing.java

• public class ParameterPassing
• public static void main (String args)
• ParameterTester tester new
  ParameterTester()
• int a1 111
• Num a2 new Num (222)
• Num a3 new Num (333)
• System.out.println ("Before calling
  changeValues")
• System.out.println ("a1\ta2\ta3")
• System.out.println (a1 "\t" a2 "\t"
  a3 "\n")
• tester.changeValues (a1, a2, a3)
• System.out.println ("After calling
  changeValues")
• System.out.println ("a1\ta2\ta3")
• System.out.println (a1 "\t" a2 "\t"
  a3 "\n")

15
ParameterTester.java

• public class ParameterTester
• public void changeValues (int f1, Num f2, Num
  f3)
• System.out.println ("Before changing the
  values")
• System.out.println ("f1\tf2\tf3")
• System.out.println (f1 "\t" f2 "\t"
  f3 "\n")
• f1 999
• f2.setValue(888)
• f3 new Num (777)
• System.out.println ("After changing the
  values")
• System.out.println ("f1\tf2\tf3")
• System.out.println (f1 "\t" f2 "\t"
  f3 "\n")

16
Num.java

• public class Num
• private int value
• // Sets up the new Num object, storing an
  initial value.
• public Num (int update)
• value update
• // Sets the stored value to the newly
  specified value.
• public void setValue (int update)
• value update
• // Returns the stored integer value as a
  string.
• public String toString ()
• return value ""

17
The static Modifier

• In Chapter 2 we discussed static methods (also
  called class methods) that can be invoked through
  the class name rather than through a particular
  object
• For example, the methods of the Math class are
  static
• Math.sqrt (25)
• To write a static method, we apply the static
  modifier to the method definition
• The static modifier can be applied to variables
  as well
• It associates a variable or method with the class
  rather than with an object

18
Static Variables

• Static variables are also called class variables
• Normally, each object has its own data space, but
  if a variable is declared as static, only one
  copy of the variable exists
• private static float price
• Memory space for a static variable is created
  when the class in which it is declared is loaded
• All objects created from the class share static
  variables
• Changing the value of a static variable in one
  object changes it for all others

19
Static Methods
Because it is static, the method can be invoked
as
value Helper.triple (5)
20
Static Methods

• The order of the modifiers can be interchanged,
  but by convention visibility modifiers come first
  
• Recall that the main method is static it is
  invoked by the system without creating an object
• Static methods cannot reference instance
  variables, because instance variables don't exist
  until an object exists
• However, a static method can reference static
  variables or local variables

21
The static Modifier

• Static methods and static variables often work
  together
• See CountInstances.java (page 284)
• See Slogan.java (page 285)

22
CountInstances.java

• public class CountInstances
• public static void main (String args)
  
• Slogan obj
• obj new Slogan ("Remember the Alamo.")
• System.out.println (obj)
• obj new Slogan ("Don't Worry. Be Happy.")
  
• System.out.println (obj)
• obj new Slogan ("Live Free or Die.")
• System.out.println (obj)
• obj new Slogan ("Talk is Cheap.")
• System.out.println (obj)
• obj new Slogan ("Write Once, Run Anywhere.")
  
• System.out.println (obj)
• System.out.println()
• System.out.println ("Slogans created "
  Slogan.getCount())

23
Slogan.java

• public class Slogan
• private String phrase
• private static int count 0
• public Slogan (String str)
• phrase str count
• public String toString()
• return phrase
• public static int getCount ()
• return count

24
Wrapper Classes

• A wrapper class represents a particular primitive
  type
• For example
• Integer ageObj new Integer (20)
• uses the Integer class to create an object which
  effectively represents the integer 20 as an
  object
• This is useful when a program requires an object
  instead of a primitive type

25
Wrapper Classes

• There is a wrapper class in the java.lang package
  for each primitive type

Primitive Type Wrapper Class
byte Byte
short Short
int Integer
long Long
float Float
double Double
char Character
boolean Boolean
void Void
26
Wrapper Classes

• Wrapper classes 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)
• The wrapper classes often contain useful static
  constants as well
• For example, the Integer class contains MIN_VALUE
  and MAX_VALUE which hold the smallest and largest
  int values

27
Keyboard Input Revisited

• The Keyboard class was introduced in Chapter 2 to
  facilitate capturing input from the keyboard
• Recall that the Keyboard class was written by the
  authors of the book
• The Keyboard class hides various aspects of Java
  input processing
• But it is important to understand how that
  processing works

28
Reading Keyboard Input

• Java I/O is accomplished using objects that
  represent streams of data
• A stream is an ordered sequence of bytes
• The System.out object represents a standard
  output stream, which defaults to the monitor
  screen
• Reading keyboard input is more complicated
• The input stream is made up of multiple objects
• BufferedReader in new BufferedReader (
• new InputStreamReader (System.in))

29
Reading Keyboard Input

• The System.in object is used to create an
  InputStreamReader object
• The InputStreamReader object is used to create a
  BufferedReader object
• This creates an input stream that treats input as
  characters and buffers them so that input can be
  read a line at a time
• The readLine method of the BufferedReader class
  reads an entire line of input as a String
• Wrapper class methods can be used to convert
  numeric input

30
Reading Keyboard Input

• Problems that arise in reading or converting a
  value manifest themselves as exceptions
• The throws clause of a method header indicates
  what exceptions it may throw
• See Wages2.java (page 289)
• I/O and exceptions are explored further in
  Chapter 8

31
Wages2.java

• import java.io.
• import java.text.NumberFormat
• public class Wages2
• public static void main (String args) throws
  IOException
• BufferedReader in new BufferedReader
  (new InputStreamReader (System.in))
• String name
• int hours
• double rate, pay
• System.out.print ("Enter your name ")
• name in.readLine ()
• System.out.print ("Enter the number of
  hours worked ")
• hours Integer.parseInt (in.readLine())
• System.out.print ("Enter pay rate per hour
  ")
• rate Double.parseDouble (in.readLine())
• System.out.println ()
• pay hours rate
• NumberFormat fmt NumberFormat.getCurrencyI
  nstance()

32
Nested Classes

• In addition to containing data and methods, a
  class can contain other classes
• A class declared within another class is called a
  nested class

33
Nested Classes

• A nested class has access to the variables and
  methods of the enclosing class, even if they are
  declared private
• In certain situations this makes the
  implementation of the classes easier because they
  can share information easily
• Furthermore, the nested class can be protected by
  the enclosing class from external use
• This is a special relationship and should be used
  with care

34
Nested Classes

• A nested class produces a separate bytecode file
• If a nested class called Inside is declared in an
  outer class called Outside, two bytecode files
  are produced
• Outside.class
• OutsideInside.class
• Nested classes can be declared as static, in
  which case they cannot refer to instance
  variables or methods

35
Inner Classes

• A nonstatic nested class is called an inner class
• An inner class is associated with each instance
  of the enclosing class
• An instance of an inner class can exist only
  within an instance of an enclosing class
• See TestInner.java (page 291)
• See Outer.java (page 292)

36
TestInner.java

• public class TestInner
• public static void main (String args)
• Outer out new Outer()
• System.out.println (out)
• System.out.println()
• out.changeMessages()
• System.out.println (out)

37
Outer.java

• public class Outer
• private int num
• private Inner in1, in2
• public Outer()
• num 9876
• in1 new Inner ("Half of the problem is
  90 mental.")
• in2 new Inner ("Another
  deadline. Another miracle.")
• public void changeMessages()
• in1.message "Life is uncertain. Eat
  dessert first."
• in2.message "One seventh of your life
  is spent on Mondays."
• public String toString() return in1 "\n"
  in2
• private class Inner
• public String message
• public Inner (String str) message
  str

38
Summary

• Chapter 5 has focused on
• object references and aliases
• passing objects references as parameters
• the static modifier
• wrapper classes
• nested classes and inner classes