Question

我必须创建一个单例类，除此之外我必须确保不超过它创建的11个实例的10个实例创建它应该抛出异常的11个实例，我已经想出了下面的设计请指出它是正确的te问题在我看来是它会抛出异常的地方，请建议。

public final class LazySingleton {
    private static volatile LazySingleton instance = null;


     private int counter = 0; // use to track the count of instance 

    // private constructor
    private LazySingleton() {
    }

    public static LazySingleton getInstance() {
        if (instance == null || counter<=10) {
            synchronized (LazySingleton.class) {
                instance = new LazySingleton();
                counter ++;
            }
        }
        return instance;
    }
}

Answer 1

我认为您正在寻找multiton pattern。在Java中，这可能类似于

Multiton UML from Wikipedia entry

public final class LazySingleton {
    private static final int MAX = 10;
    private static LazySingleton[] instances = new LazySingleton[MAX];
    static {
        for (int i = 0; i < MAX; i++) {
            instances[i] = new LazySingleton(i);
        }
    }
    private int count;

    private static int counter = 0; // use to track the count of instance

    // private constructor
    private LazySingleton(int count) {
        this.count = count;
    }

    public static synchronized LazySingleton getInstance() {
        if (counter < MAX) {
            return instances[counter++];
        }
        throw new RuntimeException("Out of new instances.");
    }
}

Answer 2

将计数器设为static变量。这样它就可以与所有其他实例共享。如果在一个实例中你增加变量，其他人也会看到它。

将增量放在构造函数中，最后用if语句检查，如果counter大于10，如果是，则抛出异常

Answer 3

将counter变为static变量，以便LazySingleton的所有实例共享相同的值。然后在LazySingleton的构造函数中，您可以检查

if (counter > 10) {
    //throw desired exception
}

同时确保在构造函数中增加counter。

Answer 4

我认为这不再是单身，这是一个工厂类，因为你无法访问其他最后一个对象，你只能参考当前的对象。

购买方式，你必须设置counter static，也将if语句更改为

synchronized (LazySingleton.class) {
    if (instance == null || counter < 10) {
            instance = new LazySingleton();
            counter ++;
         }
    else{
            throw new yourCustomException();
    }
}

Answer 5

这个怎么样::

你要制作单身的班级。

private static SingleTon _inst = null;
private static Map<SingleTon, Integer> map = new HashMap<SingleTon, Integer>();


private SingleTon()
{

}

public static synchronized SingleTon _instance()throws Exception
{


         if(map.size()>0)
         {
             System.out.println("Okay Object is there");
         if(!map.containsValue(10))
         {
           int temp = map.get(_inst)+1;
            map.put(_inst, temp);
            return   _inst;

         }else
         {
             throw new Exception("you reached limit");
         }
    }else
    {
        _inst = new SingleTon();
        map.put(_inst,1);
        return _inst;
    }


}

调用线程..尝试使用不同的线程。

    for(int i=0;i<11;i++)
        {
            System.out.println("*** Calling NUmber "+i+" Object ::: "+SingleTon._instance());
        }

输出

             *** Calling NUmber 0 Object ::: com.comparison.test.SingleTon@1befab0
               Okay Object is there
              *** Calling NUmber 1 Object ::: com.comparison.test.SingleTon@1befab0
              Okay Object is there
              *** Calling NUmber 2 Object ::: com.comparison.test.SingleTon@1befab0
               Okay Object is there
              *** Calling NUmber 3 Object ::: com.comparison.test.SingleTon@1befab0
             Okay Object is there
             *** Calling NUmber 4 Object ::: com.comparison.test.SingleTon@1befab0
            Okay Object is there
             *** Calling NUmber 5 Object ::: com.comparison.test.SingleTon@1befab0
            Okay Object is there
            *** Calling NUmber 6 Object ::: com.comparison.test.SingleTon@1befab0
            Okay Object is there
            *** Calling NUmber 7 Object ::: com.comparison.test.SingleTon@1befab0
           Okay Object is there
          *** Calling NUmber 8 Object ::: com.comparison.test.SingleTon@1befab0
         Okay Object is there
        *** Calling NUmber 9 Object ::: com.comparison.test.SingleTon@1befab0
      Okay Object is there
             Exception occuredyou reached limit
      java.lang.Exception: you reached limit
                at com.comparison.test.SingleTon._instance(SingleTon.java:31)
                at com.comparison.test.Hell.main(Hell.java:97)

java中的单例类设计仅用于创建10个实例

5 个答案: