Java Programming

1
Java Programming

• CHAPTER 3 5
• Extending Classes and Nested Classes

2
Contents

• Extending a Class
• Constructors
• Overloading and Overriding methods
• Class Compatibility
• The Object Class
• Cloning of Objects
• Shallow and Deep Copy
• Nested Types

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Introduction

• Polymorphism means that an object of a given
  class can behave as its own class or any class it
  extends
• A subclass extends its superclass
• The class contract is
• the collection of methods and fields that are
  accessible from outside a class, together with
• the description of how those members are expected
  to behave
• Class extension provides 2 forms of inheritance
• inheritance of contract or type
• inheritance of implementation
• Specialization is when the subclass defines new
  behavior and so becomes a specialized version of
  its superclass

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An Extended Class

• Every class in Java is extended from another
  class
• When the class directly extends Object then it is
  not necessary to specify that
• Only public class Attr is enough
• The Object class is the root of the class
  hierarchy
• Object defines methods that can be used by any
  class, e.g. toString

• import java.util.StringBuffer
• public class Attr extends Object
• private final String name
• private Object value // set to null by default
• public Attr(String name)
• this.name name
• public Attr(final String n,final Object v)
• name n
• value v
• public String getName()
• return name
• public Object getValue()
• return value
• public String toString() // more efficient

Object
Attr
5
Constructors in Extended Classes

• The Extend class contains the fields and inherits
  the methods from the Base class
• Constructors are not like other methods and they
  are not inherited
• Constructors belong to the class that has defined
  them
• Each class must define the constructors it needs
  to initialize its state
• Constructors can be invoked from other
  constructors by a this or super call

• class Base
• String str
• Base()
• str not set yet
• Base(String s)
• str s
• public String toString()
• return str
• class Extend extends Base
• Double dbl
• Extend(Double d)
• dbl d
• str set in Extend
• Extend(String s,Double d)

6
Constructor Order Dependencies

• When an object is created, then
• Memory is allocated for all its fields
• Each field is set to a default initial value
• Invoke a superclass constructor
• Initialize the fields using their initializers
  and any initialization blocks
• Execute the body of the constructor

• class XX
• int xa 10
• int xb
• XX() xb 5
• class YY
• int ya 20
• YY() xb ya

• YY y new YY()
• allocate memory for 3 int fields
• set fields to 0
• YY constructor invoked
• XX constructor invoked
• Object constructor invoked
• xa set to 10
• in XX constructor xb set to 5
• ya set to 20
• in YY constructor xb set to 25

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Overloading and Overriding Methods

• Overloading a method is defining more than one
  method with the same name
• Overriding a method is replacing the superclass
  method with an own

• class XX
• void method()
• void method(String s)
• class YY
• void method(String s)

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Accessing Inherited Members

• When invoking a method via an object reference,
  the actual class of the object governs what
  implementation is used
• When accessing a field via an object reference,
  the declared type of the reference is used

• import static java.lang.System.
• class XX
• String str X
• void show()
• out.println(XX.show str)
• class YY extends XX
• String str Y
• void show()
• out.println(YY.show str)
• public static void main(String args)
• final YY ext new YY()

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Accessibility and Overriding

• A method can be overridden only if accessible
• If the method is not accessible then it is not
  inherited, and it cannot be overridden
• If a subclass defines a method that has the same
  signature and return type as the superclass
  private method they are completely unrelated
• In that case the subclass method does not
  override the superclass private method

• import static java.lang.System.
• class XX
• private void show()
• out.println(XX.show)
• class YY extends XX
• public void show()
• out.println(YY.show)
• public static void main(String args)
• final YY ext new YY()
• final XX sup ext

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The super Keyword

• Like this, super is available in all nonstatic
  methods of a class
• super is used to refer to the superclass part of
  the current object
• super.method always picks the method from the
  superclass
• super.field always picks the field from the
  superclass part

• import static java.lang.System.
• class XX
• int x 5
• public void show()
• out.println(XX.show x)
• class YY extends XX
• int x 6
• public void show()
• super.show()
• out.println(YY.show super.x)
• public static void main(String args)
• final YY ext new YY()
• final XX sup ext

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Compatibility

• assignment compatibility YY is a subtype of XX
• but XX is not a subtype of YY
• XX is higher in the hierarchy and thus a wider or
  less specific type
• YY is a narrower or more specific type
• null can be assigned to a variable of any type
• explicit type casting
• XX sup (YY) ext
• YY ext (XX) sup // must do!

• class XX
• public void show()
• class YY extends XX
• public void show()
• public static void main(String args)
• YY ext new YY()
• XX sup ext // widening
• ext ? sup
• ext (YY)sup // narrowing

Object
higher
XX
lower
YY
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Testing for Type

• the instanceof operator is used to test for type
• if the a instanceof B expression evaluates to
  true then a is assignment compatible with type B
• if (null instanceof aType) always evaluates to
  false

• class XX
• public void show()
• class YY extends XX
• public void show()
• public static void main(String args)
• YY ext
• XX sup1 new YY()
• XX sup2 new XX()
• if (sup1 instanceof YY)
• ext (YY) sup1
• else if (sup2 instanceof XX)

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What protected Really Means?

• protected means that any class that extends this
  class or belongs to the same package as this
  class can also access its fields and methods
• Therefore, if Extend would take an object of
  class Base and use it as a Base then Extend
  cannot access its protected members even though
  it inherits from Base because Base belongs to a
  different package

java.lang
packageX
packageY
Object
Base
protected
Extend
access
no access
access
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final Methods and Classes

• A final class cannot be extended
• A final method cannot be overridden
• final methods simplify optimizations when
  compiling the code this is known as inlining
• inlining makes code run faster

• final class XX
• class YY extends XX
• class XX
• final void method ()
• class YY extends XX
• void method ()

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Inlining

• class XX
• int i 4
• int get () return i
• int value new XX().get()
• class YY
• int i 8
• final int get return i
• int value new YY().get()

• After compilation it looks as if the source is
• int value new XX().get()
• int value new YY().i // this method invocation
  is replaced with its body

16
Abstract Classes and Methods

• An abstract class defines only part of its
  implementation
• Abstract classes cannot be instantiated
• concrete class is the opposite of abstract
• A concrete class implements all the methods
• A concrete method from the superclass can be
  overridden into an abstract

• abstract class XX
• abstract void method()
• XX xx new XX() // error
• class YY extends XX
• void method()
• YY yy new YY()

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The Object Class

• The Object class is the root of the class
  hierarchy
• Every class directly or indirectly inherits from
  Object
• Therefore we can cast (a widening) any type into
  an Object

• Methods defined by Object
• public boolean equals(Object)
• protected Object clone()
• public final Classlt?gt getClass()
• public String toString()

18
Cloning Objects

• The Object.clone method makes an exact copy of
  the object that has invoked the method
• This is a shallow copy
• The object to be cloned must implement the
  Cloneable interface

• import static java.lang.System.
• class XX implements Cloneable
• public int i
• public static void main(String args)
• final XX orig new XX()
• orig.i 5
• XX clone orig.clone()
• out.println(clone.i clone.i)
• orig.i 6
• out.println(clone.i clone.i)
• ? clone.i 5

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Shallow and Deep Copy

• import static java.lang.System.
• class XX implements Cloneable
• public int i new int1
• public static void main(String args)
• final XX orig new XX()
• orig.i0 5
• final XX clone orig.clone()
• out.println(clone.i clone.i0)
• orig.i0 6
• out.println(clone.i clone.i0)

• import static java.lang.System.
• class XX implements Cloneable
• public int i new int1
• protected Object clone()
• try
• XX clone (XX)super.clone()
• clone.i new int1
• clone.i0 i0
• catch (CloneNotSupportedException )
• public static void main(String args)
• final XX orig new XX()
• orig.i0 5

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Extending Classes How and When

• The ability to extend a class is at the core of
  object-oriented approach
• By extending we can add new fields and methods
  while reusing the rest from the super class
• IsA relationship the extension creates a new
  kind of object that is a kind of the original
  class
• HasA relationship one object uses another
  object to store state or do work, i.e. it has a
  reference to that object

• class XX // XX IsA Object
• int i
• void method()
• class YY extends XX
• // YY IsA XX
• class ZZ // ZZ IsA Object
• String str // ZZ HasA String
• void method()
• str store data

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Nested Classes

• A class can be defined inside another class
• Benefits
• to structure and scope members
• to connect logically related objects
• A nested class is considered a part of its
  enclosing class
• They share a trust relationship, i.e. everything
  is mutually accessible
• Nested types could be
• static allows simple structuring of types
• nonstatic defines a special relationship
  between a nested object and an object of the
  enclosing class

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Static Nested Types

• Acts like any top-level class
• It can extend any class including the class it is
  a member of
• It can be declared abstract or final
• A static nested class can be extended by any
  class to which it is accessible, but
• The extended class does not inherit the
  privileged access that the nested class has to
  the enclosing class

• public class BankAccount
• private long number
• private static long bankID
• public Permissions permissionsFor(Person who)
• Permissions p new Permissions()
• p.method(this)
• return p
• public static class Permissions
• public boolean canDeposit
• public boolean canWithdraw
• void method(BankAccount acc)
• long bid bankID
• long num acc.number

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Inner Classes

• Inner classes are associated with instances of a
  class
• Inner classes cannot have static members
  including static nested types
• Inner classes can have final static fields that
  are constant
• Inner classes can extend any other class,
  implement any interface, or be extended
• When an inner class object is created inside a
  method of the enclosing class the current object
  this is automatically associated with it
• qualifiedthis

• public class BankAccount
• private long number
• private static long bankID
• public Permissions permissionsFor(Person who)
• Permissions p new Permissions()
• // equal to this.new Permissions()
• p.method()
• return p
• public class Permissions
• public boolean canDeposit
• public boolean canWithdraw
• void method()
• long bid bankID
• long num number

in scope
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Extended Inner Classes

• An inner class can be extended
• Objects of the extended class must remain
  associated with objects of the original enclosing
  class or a subclass

• class Outer
• class Inner
• class ExtendOuter extends Outer
• class ExtendInner extends Inner
• Inner ref new ExtendInner()
• class Unrelated extends Outer.Inner
• Unrelated(Outer ref)
• ref.super()

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Local Inner Classes

• Can be defined in code blocks and they are local
  and accessible to that block
• They do not have access modifiers (e.g. private,
  public) and cannot be static
• They are not members of the class to which the
  block belongs
• A local inner class can access all the variables
  in that block, and static variables, but a local
  variable or method parameter must be declared
  final

• import static java.lang.System.
• public class EnclosingClass
• int x 55
• public void enclosingMethod(final int a)
• int z 44
• final int q 32
• class LocalInner
• private int x 17
• public void innerMethod()
• out.println(x) // ? 17
• out.println(q) // ? 32
• out.println(a) // ? 99
• out.println(this.x) // ? 17
• out.println(EnclosingClass.this.x) // ?
  55

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Anonymous Inner Classes

• Extend a class or implement an interface
• They are defined at the same time when
  instantiated with new, as part of a statement
• They cannot have explicit constructors because
  they have no name

• import static java.lang.System.
• public class EnclosingClass
• int x 55
• public void anotherMethod2()
• out.println(new Object()
• private int x 17
• public String toString()
• return "Inner x " x
• " Enclosing x " EnclosingClass.this.x
• )
• ? Inner x 17 Enclosing x 55

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Inner Class Categories
Inner Classes
Local Inner Classes
Member Inner Classes
Anonymous Local Inner Classes
Nonstatic Member Inner Classes
Named Local Inner Classes
Static Member Inner Classes