单列模式

  

public class PreferenceHelper {
          //懒汉式-----速度有点慢，不用一开始占资源
           private PreferenceHelper() {}//构造器私有
       private static PreferenceHelper preManager;
           //synchronized 同步
        public static synchronized PreferenceHelper instance() {
            if (preManager == null)
                preManager = new PreferenceHelper();
            return preManager;
        }

      //饿汉式    -----速度快，一开始占资源
  private static PreferenceHelper  preManager = null;
           
   public static  PreferenceHelper instance() {
             
            return preManager;
        }

    //还可以这样写  -----速度快，一开始占资源

 private static PreferenceHelper preManager = null; 

public static PreferenceHelper instance() { 

    if (preManager == null)｛ 

           synchronized（PreferenceHelper.this）｛ 

                if (preManager == null)
                       preManager = new PreferenceHelper();
            ｝ 
     ｝
 return preManager;
 }