单例模式

 1 public class A{
 2     
 3     private static final A a= new A();
 4     
 5     private A(){};
 6     
 7     public static A getInstance(){
 8         return a;
 9     }
10     
11 }

 1 public class B {
 2 
 3     private static B b = null;
 4     
 5     private B(){};
 6     
 7     public static B getInstance(){    
 8         if (b == null) {
 9             b = new B(); // 如果线程A执行到次行，但还没有获取到对象，这时第二个线程B也在执行，执行到（b == null）判断，那么线程B获得的条件为真，于是继续运行下去，这样线程A和B各自获得了一个对象，在内存中就会出现两个对象！
10         }            
11         return b;
12     }
13     
14 }

 1 public class B {
 2 
 3     private static B b = null;
 4     
 5     private B(){};
 6     
 7     public static synchronized B getInstance(){
 8         if (b == null) {
 9         }            b = new B();
10 
11         return b;
12     }
13     
14 }

 1 public class C {
 2 
 3     /**
 4      * 上限数量
 5      */
 6     private static final int MAX = 2;
 7     
 8     /**
 9      * C对象的“名字”list
10      */
11     private static List<String> names = new ArrayList<String>();
12     
13     /**
14      * C对象list
15      */
16     private static List<C> models = new ArrayList<C>();
17     
18     /**
19      * 当前序列号
20      */
21     private static int curNum = 0;
22     
23     
24     static {
25         for (int i = 0; i < MAX; i++) {
26             models.add(new C("C对象" + i));
27         }
28     }
29     
30     private C() {
31     }
32 
33     /**
34      * 建立一个带"名字"的C对象
35      * @param name
36      */
37     private C(String name){
38         names.add(name);
39     }
40     
41     public static C getInstance(){
42         Random random = new Random();
43         
44         curNum = random.nextInt(MAX);
45         
46         return models.get(curNum);
47     }
48     
49     public void say(){
50         System.out.println(names.get(curNum));
51     }
52     
53 }

 1 public class Main {
 2 
 3     public static void main(String[] args) {
 4         int max = 5;
 5         for (int i = 0; i < max; i++) {
 6             System.out.print("第" + i + "个C对象：");
 7             C.getInstance().say();
 8         }
 9     }
10     
11 }