Reflection

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Reflection

• CSC335
• By Craig Barber

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What is it?

• Reflection
• A means of extracting information about classes
  and objects during runtime
• A means of using functionality from foreign
  classes during runtime
• A means of having references (pointers) to
  methods, for arbitrary use during runtime

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How does it work?

• java.lang.Class
• a Class object is an object which represents a
  type in java
• It allows you to get information about a certain
  class during run time
• general info, variables, methods
• It allows you to construct instances of objects
  within the class and thereby allowing you to use
  methods/fields within that class

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java.lang.Class

• methods in Class
• public static Class forName( String )
• returns a reference to the class at the specified
  package address
• public String getName( )
• returns the full name of a class (package
  included) ie. java.util.ArrayList
• public int getModifiers( )
• returns an int flag set with information about
  the class
• abstract, interface, etc.
• public Object newInstance( )
• returns a new instance of the object in the
  class. Can only be used if constructor takes no
  arguments

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java.lang.Class contd

• public Class getClasses( )
• returns an array of all the inner classes within
  a class
• public Constructor getConstructor( Class
  params)
• returns a constructor object defined by params
  for the class
• public Field getField( String name )
• returns a Field object for the instance variable
  with the given name in the class
• public Field getFields( )
• returns an array of the fields for the instance
  variables in a class
• public Method getMethod( String name, Class
  params)
• returns a reference to the method in the class
  with the given name and parameter types
• public Method getMethods( )
• returns an array of all of the methods within the
  class

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java.lang.reflect

• Constructor
• public Object newInstance( Object args )
• creates a new instance of a class by calling the
  constructor from a class, using the given
  arguments (args)
• Method
• public Object invoke( Object source, Object
  args )
• calls a method from the given object( source),
  using the given arguments (args)

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Getting class references

• To get a class from an existing object
• Object object new Object( )
• Class aClass object.getClass()
• To get a class statically
• Class stringClass String.class
• To get a class using Class.forName( String )
• Class aClass Class.forName( class name )
• To get a class from an arbitrary file system
• Use Class Loader
• ClassLoader cl ClassLoader.getSystemClassLoade
  r( )

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Getting info about classes

• public class Bob
• public void bob1( int n)
• public void bob2( )
• public void printMethods( )
• try
• Class aClass Class.forName( Bob )
• System.out.println( aClass.getName( ) )
• Method methods aClass.getMethods( )
• for ( int i 0 i lt methods.length i )
• System.out.println( methodsi )
• catch( Exception e )

• Printed result
• Bob
• public void Bob.bob1(int)
• public void Bob.bob2()
• public native int java.lang.Object.hasCode()
• .

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Using foreign classes at run time

• public class Bob
• String name
• public Bob( String value ) name value
• public void printNum( int n )
• System.out.println( name n )
• Public class Test
• try
• Class aClass Class.forName( Bob )
• Class params new Class String.class
• Constructor cst aClass.getConstructor(
  params)
• Object args new Object Joe
• Bob bob (Bob)cst.newInstance( args )
• bob.printNum( 2 )
• catch( Exception e )

• Printed Result
• Joe 2

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Using Method References (function pointers)

• Normal ActionListener
• public void actionPerformed( ActionEvent e )
• String command e.getActionCommand( )
• if ( command.equals( Redo) )
• performRedo( )
• else if( command.equals( Undo ) )
• performUndo( )
• else if
• else if
• .

• ActionListener with Reflection
• Public void actionPerformed( ActionEvent e )
• String command e.getActionCommand( )
• String methodName perform command
• Method method
• method getClass( ).getMethod( methodName,
  null )
• method.invoke( this, null )

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Working Examples of Reflection

• JUnit
• public void test()
• Eclipse
• field/method listing pop-up boxes
• real-time compiler error checking
• Runtime Plug-ins
• Parsers