Reflection

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Reflection
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What is Reflection?

• Reflection is a way that an object can find out
  about itself, or about other objects and classes.
• What is the name of this class?
• What attributes does it have?
• What methods does it have?
• What are the parameters and return type of
  methods?

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Getting a Class by Name

• Create a "Class" object.
• You must use the fully qualified name (String).

Class cl null try cl Class.forName(
"java.lang.Math" ) catch(ClassNotFoundException
e) return null // couldn't find this
class return cl // return reference to this
class
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Getting a Class's Methods

• getDeclaredMethods( ) methods of this class only.
• getMethods( ) gets all methods, incl. inherited
  ones.

// get the methods of Class referred to by
cl Method methods cl.getDeclaredMethods() fo
r( Method m methods ) // get the name of the
method out.println( m.getName() ) // get
parameters Class params m.getParameterTypes(
) // get return type Class ret
m.getReturnType( )
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Invoking a Class's Methods

• Simple example invoke Math.sqrt( 0.5 )

// get a class object for Math class Class cl
Class.forName("java.lang.Math") if ( cl null
) return // class object for the Double
class Class doubleCl Class.forName("java.lang.Dou
ble") // get the "sqrt" method with one "double"
param Method m cl.getMethod("sqrt", doubleCl)
try Double argsDbl new Double 0.5
Double ans (Double) m.invoke(null,
argsDbl) return ans.doubleValue() catch(
Exception e ) ... handle the exception ...
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Invoking a Class's Methods

• Create an array of parameter values.
• Use the "invoke" method.

/ invoke method m with arguments args / static
double eval(Method m, double args) throws
ParseException // TODO verify correct number
of parameters Double argsDbl new
Doubleargs.length for(int k0 kltargs.length
k) argsDblk argsk try Double d
(Double) m.invoke(null, argsDbl) return
d.doubleValue() catch( Exception e ) ...
handle the exception ...
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Exceptions from invoke

• Hey, can we use "invoke" to invoke private
  methods?
• This would bypass the access privilege system!

Method invoke throws IllegalAccessException,
IllegalArgumentException, InvocationTargetExce
ption
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Applications of Reflection

• JavaBeans uses reflection to discover properties
  of beans.
• JUnit uses reflection to discover methods to test.

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Reflection in Calculator

• Use reflection to add all the functions in
  java.lang.Math to your calculator.
• Only required add functions that accept one
  double argument.
• If your calculator can only handle functions with
  0, 2, ... parameters, then add those functions,
  too.
• Use the invoke method of Method class to call a
  method when the user inputs it.
• More later...