缓存控制单例个数

public class OneExtend {
    // 用于控制当前是第几个实例，达到最大数目，返回从1开始
    private static int num = 1;
    private static Map map = new HashMap<>();
    // 允许创建的最大数量
    private static final int MAX_NUMS = 3;

    // 私有化构造函数
    private OneExtend() {
    };

    // 缓存的key
    private static String DEFAULT_KEY = "cache";

    public static OneExtend getInstance() {
        String key = DEFAULT_KEY + num;
        OneExtend oneExtend = map.get(key);
        if (oneExtend == null) {//同样需要保证线程安全，使用双重检测锁的方法
            synchronized (OneExtend.class) {
                if (oneExtend == null) {
                    oneExtend = new OneExtend();
                    map.put(key, oneExtend);
                }
            }
        }
        num++;
        if (num > MAX_NUMS) {
            num = 1;
        }
        return oneExtend;
    }
}