寫個單例

public class Singleten {
    /**
     * 關鍵點 1.靜態函數以及靜態對象
     *      2.私有構造函數
     *      3.雙重檢查
     */
    private static volatile Singleten s;

    public static Singleten getInstance() {
        if (s == null) {
            synchronized (Singleten.class) {
                if (s == null) {
                    s = new Singleten();
                    return s;
                } else return s;
            }
        } else
            return s;
    }

    private Singleten() {
    }
}