Review the reflection mechanism in Java

The use of class classes

Any class is an instance object of class, and this instance object has three ways of expressing it.

/** *  *//** *  @author  jacky 2014 December 27  */class Foo {  Void print ()  {  system.out.println ("foo");  }}public class reflect {  public static void main (String[] args)  {  Foo foo1 =  New foo ();  //  Any class is an instance object of class, and there are three representations of this instance object   //  The first expression: actually tells us that each class has an implicit static member class  class c1 = foo.class;  //  Second expression: The object of the class is known to get   class c2 = foo1.getclass () through the GetClass method;  /**    *  everything is objects   classes are objects, instance objects of class classes    */  //  regardless of c1 or  C2 all represent the class type of the Foo class, and a class can only be an instance object of Class   system.out.println (C1 == C2);  //  The third expression: the object of this class is known to get   Class c3 = null;  try {    by the GetClass method Com.jacky.reflect--> Package Name    c3 = clasS.forname ("Com.jacky.reflect.Foo");  } catch  (classnotfoundexception e)  {    // todo auto-generated catch block   e.printstacktrace ();   }  system.out.println (C2 == C3);  //  We can create an instance object of the class from the class type of the class---> by C1  RO C2 OR C3 Create instance object for Foo   try {   Foo foo =  ( Foo)  c1.newinstance ();//  requires a parameterless construction method    foo.print ();  } catch  ( Instantiationexception e)  {   // todo auto-generated catch block    e.printstacktrace ();  } catch  (illegalaccessexception e)  {    // todo auto-generated catch block   e.printstacktrace ();   } }}

Output:

Truetruefoo

Dynamic Load Class

First say static loading:

The new creation object is a statically loaded class that needs to load all the possible classes at compile time

As follows: The Excel class is not implemented, and if static loading requires all classes to be implemented. I can use dynamic loading to solve this problem so that the program can still run.

Class Office {public static void main (string[] args) {//new creates an object is a static load class that needs to load all possible classes at compile time//by dynamically adding            Download can resolve the problem if ("word". Equals (Args[0])) {Word w = new word ();        W.start ();            } if ("Excel". Equals (Args[0])) {Excel e = new Excel ();        E.start (); }    }}

Dynamic loading:

Class officebetter{    public static void main (String[] args)      {        try         {            //dynamic load class, at run time             class c = class.forname (Args[0]);             //create the class object through the class type              OfficeAble oa =  (officeable) C.newinstance ();             oa.start ();         }        catch  ( Exception e)         {             e.printstacKtrace ();        }             }}

Parameter Args[0] Specifies the class to load at run time. If we just implemented Excel, and for the implementation of the word class, we can specify that loading Excel

As follows:

Officeable.java:

Class Word implements officeable{public void Start () {System.out.println ("Word start"); }}

Interface officeable

Interface officeable{public void start ();

The Execute command argument is: Word

Results:

Word start

Get method information, construct etc.

Classutil.printclassmessage (Object obj)
Basic data type, void keyword exists class type

Classdemo2.java:

Public class classdemo2 { public static void main (String args[])   Class type of {   class c1 = int.class;//int     class c2 = Class type of  String.class;//String     Class c3 = double.class;    Class c4 = Double.class;   Class c5 = void.class;      system.out.println (C1.getname ());    system.out.println (C2.getName ());    system.out.println (C2.getsimplename ());    system.out.println (C3.getName ());    system.out.println (C4.getname ());    system.out.println (C5.getName ());     String hello =  "Xixi";  int i = 1;   Classutil.printclassmethodmessage (i);   classutil.printclassconmessage ("Hello");   Classutil.printclassconmessage (new iNteger (1));  }} 

Classutil.java:

public class classutil { /**  *  prints information about a class, including member functions of a class, member variables (get member functions only)   *    *  @author  jacky 2014 December 27   */ public static void  printclassmethodmessage (object obj)  {  //  to get information about a class, first get the class type of the class   class  c = obj.getclass (); //  is the object of which child class is passed, and C is the class type of the subclass   system.out.println ("Class name is:"  + c.getname ());   /**   * getmethods () gets all the public functions, including inherited by the parent class     * getdeclaredmethods () Gets all the methods of the class declaration, without asking for access rights &NBSP;&NBSP;&NBSP;*/&NBSP;&NBSP;METHOD[]&NBSP;MS  = c.getmethods ();  for  (int i = 0; i < ms.length;  i++)  {   //  gets the class type of the return value type of the method    Class returnType =  Ms[i].getreturntype ();    system.out.println ("Return value type:"  + returntype.getname ());    system.out.print (Ms[i].getname ()  +  "(");   //  get the parameter list, get the class type of the parameter list type    class[]  paramtypes = ms[i].getparametertypes ();   for  (Class class1 :  paramtypes)  {    system.out.print (Class1.getsimplename ()  +  ",");    }   system.out.println (")");  } } /**  *  Get member variables   *   *  @author  jacky 2014 December 27   */ public  static void printclassfieldmessage (object obj)  {  /**   * The  java.lang.reflect.field field class encapsulates the operation of a member variable    * getfields () The    * getdeclaredfields method gets the information for the member variable of all public () Gets the information of the member variable declared by the class itself    */  } /**  *  Print constructor Information   */ public static void  Printclassconmessage (object obj) &NBSP;{&NBSP;&NBSP;CLASS&NBSP;C&NThe Bsp;= obj.getclass ();  /**   *  constructor is also an object   Java.lang.Constructor encapsulates information about constructors    * getconstructors get constructors for all public      * getdeclaredconstructors () gets all the constructors    */  constructor[] cs =  c.getdeclaredconstructors ();  for  (CONSTRUCTOR&NBSP;CONSTRUCTOR&NBSP;:&NBSP;CS)  {    system.out.print (Constructor.getname ()  +  "(");   class  Paramtypes[] = constructor.getparametertypes ();   for  (Class class1 :  paramtypes)  {    system.out.print (Class1.getname ()  +  ",");    }   system.out.println (") \ n");   } }}

Method of Reflection Method.invoke (object, parameter list)

/** *  @author  jacky 2014 December 27  */public class classdemo { public  static void main (string args[])  {  //  get the Print (Int,int) method   a  a1 = new a ();   class c = a1.getclass ();  /**    *  get method, name and argument list determines  getmethod gets the method that the public method  getdeclaredmethod itself declares    */   try {   method m = c.getmethod ("Print", int.class,  Int.class) Reflection operation of the;   //  method    // a1.print (10,20)   method reflection using M-object for method invocation    //  If no return value is returned, NULL is returned, the return value type returns a specific return value of    m.invoke (A1, 10, 20);       method m1 = c.getmethod ("Print", new class[]{});    m1.invoke (a1, null);  } catch  (exception e)  {     todo autO-generated catch block   e.printstacktrace ();  } }}class a  { public void print () {  system.out.println ("Hello world"); }   Public void print (int a, int b)  {  system.out.println (a + b) ;  } public void print (string a, string b)  {   System.out.println (A.touppercase ()  + b.tolowercase ());  }}

The nature of generics

Reflected operations are post-compilation operations
The generics of the collection after compilation are de-generalized
Generics in Java are only prevented from being incorrectly entered and only valid during the compilation phase
Bypassing compilation is not working.

/** *  @author  jacky 2014 December 27  */public class classdemo { public  static void main (string args[])  {  List list = new  ArrayList ();  list<string> list1 = new arraylist<string> ();   list1.add ("Xixi");   class c1 = list.getclass ();   class c2 =  list1.getclass ();   system.out.println (C1&NBSP;==&NBSP;C2);  //true  // The actions that are reflected are compiled operations     /**   *  instructions after compilation The generics of the collection are de-generics    *  Generics in a collection in Java, just to prevent error input, only valid at compile time    *  bypass compilation Invalid    *  Validate: Method's reflection to operate, bypass compilation     */    try {   method m = c2.getmethod ("Add" ,  object.class);    m.invoke (list1,20);    system.out.println (List1);   } catch  (ExceptIon e)  {   // TODO Auto-generated catch block    E.printstacktrace ();   } }}

Review the reflection mechanism in Java