单例Singleton

//恶汉式，可以在多线程环境下使用。但是会过早地创建实例，从而减低内存的使用效率

class Solution {
    /**
     * @return: The same instance of this class every time
     */
    private Solution(){};
    public static Solution instance = new Solution();
    public static Solution getInstance() {
        // write your code here
        return instance;
    }
};

//懒汉式，只能在单线程环境下使用。

class Solution {
    /**
     * @return: The same instance of this class every time
     */
    private Solution(){};
    private static Solution instance = null;
    public static Solution getInstance() {
        // write your code here
        if(instance == null){
            instance = new Solution();
        }
        return instance;
    }
};

//懒汉式改进1。在getInstance方法上加同步。每次通过属性Instance得到单例都会去同步。而同步非常耗时。

class Solution {
    /**
     * @return: The same instance of this class every time
     */
    private Solution(){};
    private static Solution instance = null;
    public static synchronized Solution getInstance() {
        // write your code here
        if(instance == null){
            instance = new Solution();
        }
        return instance;
    }
};

//懒汉式改进2。双重检查锁定。只需要在单例还没有被创建之前加锁。如果实例已经创建，则不需要进行加锁同步。

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