2017年9月16日 new Class()和getInstance()的区别

public static Calendar getInstance()
{
return createCalendar(TimeZone.getDefault(), Locale.getDefault(Locale.Category.FORMAT));
}

 1 package day12.am;
 2 
 3 import java.io.Serializable;
 4 import java.util.Calendar;
 5 import java.util.Date;
 6 import java.util.GregorianCalendar;
 7 import java.util.Locale;
 8 import java.util.TimeZone;
 9 
10 import org.omg.Messaging.SyncScopeHelper;
11 
12  abstract class A{
13      public A(){         
14      }
15      
16     abstract void aa();
17     public static A getInstance() {
18         System.out.println("我被创建了");
19         return null;
20     }
21 };
22 public class ClassLoader {
23  public static void main(String[] args) throws  ClassNotFoundException, InstantiationException, IllegalAccessException {
24      
25      /*Date类可以直接new()
26       * public class Date implements java.io.Serializable, Cloneable, Comparable<Date>{}
27       * */
28     Date date =new Date();
29     System.out.println(date);
30     /* Calendar类只能用getInstance()方法来实现创建对象
31      * 查看底层发现public abstract class Calendar implements Serializable, Cloneable, Comparable<Calendar>();
32      * Calendar类属于抽象类,是不能被直接创建的,而Calendar的底层有getInstance()的静态方法：
33      *  public static Calendar getInstance(){
34         return createCalendar(TimeZone.getDefault(), Locale.getDefault(Locale.Category.FORMAT)); }   
35      * */
36     Calendar c =Calendar.getInstance();
37     //Calendar c2 =new Calendar();
38     System.out.println(c);
39     System.out.println(c.getClass());
40     //子类 GregorianCalendar()去实现 父类Calendar()也是可以的
41     Calendar c2= new GregorianCalendar();
42     System.out.println(c2);
43   //因为newInstance()就是实例化对象，相当于还是new(),而get是写的一个方法，不属于new 对象,通过发射创建还是会报错
44 //    Calendar c2=(Calendar) Class.forName("java.util.Calendar").newInstance();
45 //    System.out.println(c2);
46     
47    //抽象类A,用getInstance()实例化
48     A a1=A.getInstance();
49     System.out.println(a1);
50     //通过反射然后用newInstance()
51 //    A a2=(A) Class.forName("day12.am.A").newInstance();
52 //    System.out.println(a2);
53     
54 }
55 }