Singleton 204

Question

Singleton is a most widely used design pattern. If a class has and only has one instance at every moment, we call this design as singleton. For example, for class Mouse (not a animal mouse), we should design it in singleton.

You job is to implement a getInstance method for given class, return the same instance of this class every time you call this method.

Solution

单例模式，要考虑并发情况。

为了防止多个线程创建多个实例，要在new之前加一个锁，即synchronized。同时为了防止多个线程在判断实例为null之后先后进入synchronized依然创建多个实例，可以在synchronized之后double check一下实例是否为null。更多讨论见下：

Reference

代码如下：

class Solution {
    /**
     * @return: The same instance of this class every time
     */
     //考察静态变量
    public static Solution s = null;
    public static Solution getInstance() {
        // write your code here
        if(s == null){
            synchronized (Solution.class){
                if(s == null){
                    s = new Solution();
                }
            }
        }

        return s;
    }
};