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Reflection In Java
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For every loaded class, the Java Runtime Environment
(JRE) maintains an associated Class object
The Class object “reflects” the class it represents
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The primitive Java types are also represented as Class objects
Reflection can be used to:
 Determine the class of an object.
 Get information about a class's modifiers, fields, methods, interfaces,
constructors, and superclasses.
 Create an instance of a class whose name is not known until runtime.
 Get and set the value of an object's field.
 Invoke a method on an object.
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Main Java reflection Classes
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Class ( java.lang.Class )
 Instances of the class Class represent classes and interfaces in a
running Java application, every object is represented by a Class
object.
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Member
 An Interface that reflects identifying information about a single
member (a field or a method) or a constructor.
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Method ( java.lang.reflect.Method )
 Implements Member Interface
 provides information about, and access to, a single method on a
class or interface.
 Represents instance methods and class methods ( static )
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Field
 Implements Member Interface
 provides information about, and dynamic access to, a single field (
also for static fields )
 Provides access and modification ( set, get ) methods.
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Main Java reflection Classes
 Constructor
 provides information about, and access to, a single
constructor for a class.
 Package
 Package objects contain version information about the
implementation and specification of a Java package
 Modifier
 provides static methods and constants to decode class and
member access modifiers. The sets of modifiers are
represented as integers with distinct bit positions
representing different modifiers
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Accessing the Class object
To get the Class object for an object mystery:
Class c = mystery.getClass();
Or, using the class name:
Class c = Class.forName(“mysteryClass”);
Getting the superclass of MysteryClass:
Class s = c.getSuperclass();
Getting the class name:
String s = c.getName();
Discovering the interfaces implemented by a class:
Class[] interfaces = c.getInterfaces();
Discovering the fields of a class:
Field[] fields = c.getFields();
Discovering the methods of a class:
Method[] methods = c.getMethods();
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Example
static void showMethods(Object o) {
Class c = o.getClass();
Method[] theMethods = c.getMethods();
for (int i = 0; i < theMethods.length; i++) {
String methodString = theMethods[i].getName();
System.out.println("Name: " + methodString);
String returnString = theMethods[i].getReturnType().getName();
System.out.println(" Return Type: " + returnString);
Class[] parameterTypes = theMethods[i].getParameterTypes();
System.out.print(" Parameter Types:");
for (int k = 0; k < parameterTypes.length; k ++) {
String parameterString = parameterTypes[k].getName();
System.out.print(" " + parameterString);
}
System.out.println();
}
}
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Example – Cont.
Output for a call of the form:
Polygon p = new Polygon();
showMethods(p);
Name: equals
Return Type: boolean
Parameter Types: java.lang.Object
Name: getClass
Return Type: java.lang.Class
Parameter Types:
Name: intersects
Return Type: boolean
Parameter Types: double double double double
.
.
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A Java Reflection Example
 Illustrates Four Issues:
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Runtime type Information (RTTI)
Introspection
Invoking Method Objects
Dynamic Instantiation
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Back to our Employee Example…
Employee
Only
number
partial
view of the
classes…
level
print()
MonthlyEmployee
HourlyEmployee
print()
print()
public final class MonthlyEmployee extends Employee {
public void print() {
System.out.println(“I’m a Monthly Employee");
}
}
public final class HourlyEmployee extends Employee {
public void print() {
System.out.println(“I’m a Hourly Employee");
}
}
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Reflection and Dynamic Binding
What is the output of the following test code:
Employee e;
e = new MonthlyEmployee();
Class c = e.getClass();
System.out.println("class of e = " + c.getName());
e = new HourlyEmployee();
c = e.getClass();
System.out.println("class of e = " + c.getName());
class of e = MonthlyEmployee
class of e = HourlyEmployee
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SuperClass
What is the output of the following test code:
c = c.getSuperclass();
System.out.println("base class of e = " +
c.getName());
c = c.getSuperclass();
System.out.println("base of base class of e = "
+ c.getName());
base class of e = Employee
base of base class of e = java.lang.Object
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Getting all fields
Field fields[] = c.getFields();
for(int i = 0; i < fields.length; i++) {
System.out.print(fields[i].getName() + "= ");
System.out.println(fields[i].getInt(e));
}
e is an instance of
Employee or its
subtype !
The output produced:
number= 111
level= 12
Non public
fields are not
printed!
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Setting a Field
Let’s promote an employee…
Field fields[] = c.getFields();
for(int i = 0; i < fields.length; i++) {
if(fields[i].getName() == “Level”)
fields[i].setInt(e, (fields[i].getInt(e))++);
}
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Examining Modifiers
What is the output of the following test code:
int m = c.getModifiers();
if (Modifier.isPublic(m))
System.out.println("public");
if (Modifier.isAbstract(m))
System.out.println("abstract");
if (Modifier.isFinal(m))
System.out.println("final");
public final
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Invoking Method Objects
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We can ask a method object to invoke the method
it represents
we must provide it with the implicit and explicit
arguments
Employee e = new HourlyEmployee();
Class c = e.getClass();
Method m = c.getMethod(“print”, null);
m.invoke(e, null);
the output produced:
I’m a Hourly Employee
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Dynamic Instantiation
The universal printer gets the employee type and invokes the print method..
class UniversalPrinter {
public void print(String empType) {
Class c = Class.forName(empType);
Employee emp = (Employee ) c.newInstance();
emp.print();
}
}
-for calling other constructors( with arguments ), use the
Constructor class
- Constructor c = ...
- Object newObject = c.newInstance( Object[] initArguments )
What is the output of the following code?
UniversalPrinter p = new UniversalPrinter();
String empType;
empType = “HourlyEmployee";
p.print(empType);
empType = “MonthlyEmployee";
p.print(empType);
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Reflection - What’s missing
 Reflection is ‘read only’
 Cant add / modify fields, methods
 Non public members (fields, methods) are not
accessible
 Implementation is not available
 Program logic is not reflected
 Major performance impact
 Much slower then doing the same operations directly…
 Complex code
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