Difference between revisions of "Metaclasses"
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* e.g. getMethods return objects which are of type Method class. Instances of java.lang.reflect.Method are methods. | * e.g. getMethods return objects which are of type Method class. Instances of java.lang.reflect.Method are methods. | ||
| − | * The method class defines behavior of all methods, for e.g. getReturnType(), getModifiers() etc. | + | * The method class defines behavior of all methods, so it is a meta-method. for e.g. getReturnType(), getModifiers() etc. |
* java.lang.Class is not a complete class, since it is declared as final. We cannot add behavior to it or specialize it. | * java.lang.Class is not a complete class, since it is declared as final. We cannot add behavior to it or specialize it. | ||
| + | * This whole meta thing is '''reflection''' - a program can reflect on itself. | ||
| + | * Useful for tools. e.g. Eclipse. Automatic method drop down possibly reflects on the Class object and gets all its declared methods. | ||
[[Category:OODE]] | [[Category:OODE]] | ||
Revision as of 06:54, 17 December 2011
- Each object belongs to a class.
- But, Everything is an object.
- So, classes are objects apart from being classes.
String s = "Hello";
Class stringClassObj = s.getClass();
- So the class object (stringClassObj) is an instance of which class ?
- It is an instance of java.lang.Class.
- java.lang.Class extends java.lang.Object since classes are objects.
- So java.lang.Class is a metaclass. Instances of java.lang.Class are classes.
- A class defines the behaviour of its instances. Similarly, a metaclass defines the behavior of the classes that are its instances.
- E.g. java.lang.Class has methods that defined the behavior of its instances.
- e.g. getName(), getDeclaredFields(), getMethods(),
- e.g. getMethods return objects which are of type Method class. Instances of java.lang.reflect.Method are methods.
- The method class defines behavior of all methods, so it is a meta-method. for e.g. getReturnType(), getModifiers() etc.
- java.lang.Class is not a complete class, since it is declared as final. We cannot add behavior to it or specialize it.
- This whole meta thing is reflection - a program can reflect on itself.
- Useful for tools. e.g. Eclipse. Automatic method drop down possibly reflects on the Class object and gets all its declared methods.
