Java 1.1 Extensions

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Java 1.1 Extensions

• Nested classes
• static public/protected/private

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Nested classes

• Static Top-level Classes and Interfaces
• Like ordinary package member except for its name.
• May use static members of its enclosing class.
• Member Classes
• Each instance has an enclosing instance.
• May use members of its enclosing classes.
• Local Classes
• Defined in a block of code.
• May also use final local variables and
  parameters.
• Anonymous classes
• One instance only.

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• public class LinkedList
• // static by default
• public interface Linkable
• public Linkable getNext()
• public void setNext(Linkable node)
• Linkable head
• public void insert(Linkable node) ...
• public void remove(Linkable node) ...

4

• class LinkableInt implements
• LinkedList.Linkable
• int i
• public LinkableInt(int _i)i _i
• LinkedList.Linkable next
• public LinkedList.Linkable
• getNext() return next
• public void setNext(LinkedList.Linkable node)
• next node

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Member Class

• Every instance of a member class is internally
  associated with an instance of the enclosing
  class.
• The methods of a member class can implicitly
  refer to the fields defined within the member
  class, as well as those defined by any enclosing
  class (including private fields.)
• static final members permitted.

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• public class BankAccount
• private long number
• private int balance
• private Action lastAct
• public class Action
• private String act
• private int amount
• Action(String a, int amt)
• act number a amount
  amt
• public void deposit(int amt)
• balance amt
• lastAct new Action(deposit,amt)
• public void withdraw(int amt)
• balance - amt
• lastAct new Action(withdraw,amt)

7

• public class BankAccount
• private long number
• private int balance
• private Action lastAct
• public class Action
• public void deposit(int amt)
• public void withdraw(int amt)
• public void transfer(BankAccount ac, int amt)
• ac.withdraw(amt)
• deposit(amt)
• lastAct new Action(transfer,amt)

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Scope of Fields

• public class A
• public String name"a"
• public class B
• public String name"b"
• public class C
• public String name"c"
• public void printNames()
  
• public static void main(String a)

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• public void printNames() // class C
  System.out.println(name) // c
  System.out.println(this.na
  me) // c
  System.out.println(B.this.name) // b
  System.out.println(A.this.name) //
  a
• public static void main(String a) //class A
• A a new A()
• B b a.new B()
• A.B.C c b.new C()
• c.printNames()

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javap -p ABC

• public class A.B.C extends java.lang.Object
• / ACC_SUPER bit NOT set /
• private final A.B this1
• public java.lang.String name
• public A.B.C(A.B)
• public void printNames()

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javap -p A.B

• public class A. B extends java.lang.Object
• private final A this0
• public java.lang.String name
• public A.B(A)
• static A access0(A.B)
• // related to accessing private
• ...A.B.C...

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Scope vs Inheritance

• Top-level class can extend a member class.
• Class hierarchy
• superclass to subclass
• Containment hierarchy
• containing class to nested class
• Whenever there is a name conflict between an
  inherited member and a member of an enclosing
  class, Java requires explicit disambiguation.
• The name of a member class must differ from the
  name of the enclosing package or class.

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Pathological cases

• Static subclass of a member class legal
• class A int i 0
• class B int i 1
• static class C extends B
• int i 2
• C(A a) a.super()
• public static void main(String args)
• A a new A()
• B b a.new B()
• C c new C(a)

• Cyclic Inheritance illegal

class A extends A.B int i 0 class
B int i 1
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Local Class (vs Member class)

• A local class is similar to a member class except
  that its methods can also access final local
  variables, final formal method parameters, and
  final exception parameters.
• An instance of a local class has private fields
  to hold values of final local variables, final
  exception parameter, and final method parameters,
  in addition to a private reference to the
  enclosing class instance.

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• Enumeration myEnumerate(final Object arr)
• class E implements Enumeration
• int count 0
• public boolean hasMoreElements()
• return count lt arr.length
• public Object nextElement()
• return arrcount
• return new E()
• Scope of arr is the body of myEnumerate function.
• Lifetime of the array instance referenced by arr
  should be at least as long as the Enumeration
  object.

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• import java.util.
• class XYZ
• int i
• Enumeration myEnumerate(final Objectarr)
• class E implements Enumeration
• int count i
• public boolean hasMoreElements()
• return count lt arr.length
• public Object nextElement()
• return arrcount
• return new E()

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javap -p XYZ1E

• class XYZ1E extends java.lang.Object implements
  java.util.Enumeration
• private final java.lang.Object valarr
• int count
• XYZ1E(java.lang.Object, XYZ)
• public boolean hasMoreElements()
• public java.lang.Object nextElement()

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• public class Weird // cf. closure
• interface IntHolder int getValue()
• public static void main(String args)
• IntHolder holders new IntHolder10
• for(int i0 ilt10 i)
• final int fi i
• class MyIntHolder implements IntHolder
• public int getValue() return fi
• holdersi new MyIntHolder()
• // MyIntHolder invisible here
• for(int i0 ilt10 i)
• System.out.println(holdersi.getValue())

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Anonymous Class

• Local class without a name.
• Combines class definition and instantiation.
• No constructor.
• Can implement an interface (by implicitly
  extending class Object).
• new ltclass-namegt ( ltarg-listgt )
• ltclass-bodygt
• new ltinterface-namegt ( )
• ltclass-bodygt

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• import java.applet.
• import java.awt.
• import java.awt.event.
• public class Scribble extends Applet
• public void init()
• this.addMouseMotionListener(
• new MouseMotionAdapter()
• public void mouseDragged(MouseEvent e)
• Graphics g getGraphics()
• int x e.getX(), y e.getY()
• g.drawLine(0, 0, x, y)
• )

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• class LocalVarThreads
• public static void main(final String args)
• for (int i 0 i lt args.length i)
  
• final int ii i
• Runnable r new Runnable()
  
• public int count
• public void run()
  System.out.println("ltlt"ii"gtgt
• "count)
• new Thread(r).start()

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Other new features

• Instance Initializers
• similar to class initializers
• serves as a constructor in anonymous classes
• Blank finals
• assign only once prior to use instead of in decl.
• Anonymous arrays
• int a a new int 1,2,3,4
• Class literals
• String.class, Integer.class, Integer.TYPE, etc
• In Java 1.1, class is a keyword and a field.

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Reflection

• Introspection
• Meta-programming

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What does Reflection API do?

• Java Runtime Environment maintains an
  immutable Class object that contains information
  about a class. Reflection API enables a program
  to
• Determine the class of an object.
• Get information about a class's modifiers,
  fields, methods, constructors, and superclasses.
• Find out what constants and method declarations
  belong to an interface.

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• Create an instance of a class whose name is not
  known until runtime.
• Get and set the value of an object's field, even
  if the field name is unknown to your program
  until runtime.
• Invoke a method on an object, even if the method
  is not known until runtime.
• Create a new array, whose size and component type
  are not known until runtime, and then modify the
  array's components.

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Applications Construction of development tools.

• Tools that need to discover and use public
  members of a target object based on its class.
• GUI builders
• selecting and creating a component.
• determining properties, methods, events, etc
  supported by a JavaBean.
• customizing a component to running a method.

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(contd)

• Tools that need to discover and use members
  declared in a given class.
• Class browsers
• requires information about classes to display
  their fields, methods, constructors, etc.
• Debuggers
• accessing and modifying field values.
• Interpreters

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Java Core Reflection APIs

• package java.lang.reflect
• new interface Member
• new classes Field, Method, Constructor.
• new class Array
• new utility class Modifier
• package java.lang
• new wrapper classes Byte, Short
• updated class Class
• new objects instances of class Class to
  represent primitive types.