Reflection

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Reflection basics

• java.lang.reflect package contains classes for examining code at runtime
• It is NOT to be used by the likes of You (OR Me!!)
• allows the manipulation of code at runtime (Run Away Now!)
• only Coding Super Heros use it for arcane and dangerous purposes
• No, I am not kidding.

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Reflection basics

• java.lang.reflect package contains classes for examining code at runtime
• Constructor, Method, and Field classes represent constructors, methods, and fields
• Class objects provide getFields(), getMethods() and getConstructors() methods

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The Class Class

• Well, a class has to be in the Java data model, yeah?
• Used for modeling classes
• A blueprint for other Classes
• Contains info about classes

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A Class Object

• One for every class in your program
• Used for creating objects
• Contains info about the class of object it Constructs

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Class Object Creation

• Created by the ClassLoader
• Right Here There Be Dragons
• Created on first reference of a static member of the class

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Getting a Class object

• Object.getClass() on an instance of the desired class
• Class literal (eg: Object.class or String.class)
• Class.forName(String className) eg: Class.forName("java.lang.String")

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Checking the class of an object

• instanceof keyword - requires name of class at compile time
• isInstance() method - allows dynamic checking
• class object equality (== or .equals())

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Example

String str = "Hello";

If used in a conditional:

( str instanceof String ) //true
( Object.class.isInstance(str) ) // true
( str.getClass() == String.class )// true
( str.getClass() == Object.class ) // false
( str.getClass().equals(Object.class) ) //false

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Useful Class object methods

• getName(), getSimpleName(), getCanonicalName()
• newInstance()
• cast(), getSuperclass()

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Resources

• Class class documentation