基于Java class對象說明、Java 靜態(tài)變量聲明和賦值說明(詳解)

java.lang.Object
  java.lang.Class<T>
  
Instances of the class Class represent classes and interfaces in a running Java application. An enum is a kind of class and an annotation is a kind of interface. Every array also belongs to a class that is reflected as a Class object that is shared by all arrays with the same element type and number of dimensions. The primitive Java types (boolean, byte, char, short, int, long, float, and double), and the keyword void are also represented as Class objects.

Class has no public constructor. Instead Class objects are constructed automatically by the Java Virtual Machine as classes are loaded and by calls to the defineClass method in the class loader.

The following example uses a Class object to print the class name of an object:

   void printClassName(Object obj) {
     System.out.println("The class of " + obj +
              " is " + obj.getClass().getName());
   }
 
It is also possible to get the Class object for a named type (or for void) using a class literal.

  System.out.println("The name of class Foo is: "+Foo.class.getName());

在一個運行著的JAVA應(yīng)用中，類的任何實例都可以用Class對象來指代，Class可以指代所有的類和接口。枚舉屬于類，注解屬于接口，均可以用Class指代。每個數(shù)組均屬于反射的Class對象，數(shù)組中的每個元素和維度也同樣擁有Class對象。Java基本類型（boolean, byte, char, short, int, long, float, and double）以及關(guān)鍵字void也都可以用Class指代。

Class類不存在構(gòu)造函數(shù)，當(dāng)類被加載過程中由JVM通過調(diào)用類加載器中的defineClass方法自動構(gòu)造。

下面的例子是將一個對象通過Class對象打印出類名。
   void printClassName(Object obj) {
     System.out.println("The class of " + obj +
              " is " + obj.getClass().getName());
   }
通過class關(guān)鍵字指定類型也是可以得到Class對象的
  System.out.println("The name of class Foo is: "+Foo.class.getName());

/**
 * 
 * 旨在測試Class對象和Instance之間的關(guān)系；
 * 旨在測試靜態(tài)變量的聲明和賦值過程；
 * @author zzy
 *
 */
public class ObjClass {
  private enum tmpEnum {A, B, C};
  public static void main(String[] args){
    int[] tmpArray = {1,2,3};
    Class classType;
    
    try {
      // 通過類名直接獲取Class對象，JVM中沒有加載。
      classType = InClass.class;
      System.out.println(".class: " + classType);
      System.out.println(".class finish.");
      
      // Java加載類
      classType = Class.forName("InClass");
      System.out.println("Class.forName: " + classType);
      System.out.println("Class.forName: finish.");

      
      // 創(chuàng)建實例
      InClass newClassType = new InClass();
      classType = newClassType.getClass();
      System.out.println("new Object.getClass: " + classType);
      System.out.println("new Object.getClass: finish.");

      // 數(shù)組對象
      classType = tmpArray.getClass();
      System.out.println("Array.getClass:" + classType.getSimpleName());
      System.out.println("Array.getClass: finish.");
      
      // 枚舉對象
      classType = tmpEnum.class;
      System.out.println("enum.class:" + classType);
      System.out.println("enum.class: finish.");

    } catch (ClassNotFoundException e) {
      // TODO Auto-generated catch block
      e.printStackTrace();
    }      
  }  
}
class InClass{
  // 對靜態(tài)變量聲明之前賦值 
  {
    staticPara = 10;
  }
  public static int staticPara;
  
  // 構(gòu)造函數(shù)
  public InClass(){
    System.out.println("construction...");
  }
  
  // 靜態(tài)代碼塊
  static {
    System.out.println("static function...");
  }
  
  // 靜態(tài)變量賦值
  {  
    staticPara = 20;
  }
  
  {
    System.out.println("normal function, staticPara：" + staticPara);
  }
  
  // 靜態(tài)變量賦值
  {
    staticPara = 30;
  }  
}

.class: class InClass
.class finish.
static function...
Class.forName: class InClass
Class.forName: finish.
normal function, staticPara：20
construction...
new Object.getClass: class InClass
new Object.getClass: finish.
Array.getClass:int[]
Array.getClass: finish.
enum.class:class ObjClass$tmpEnum
enum.class: finish.