public class DataBaseContext {    private static DataHelper dataHelper;    private static Object     INSTANCE_LOCK = new Object();    public static DataHelper getInstance(Context context) {            synchronized (INSTANCE_LOCK) {                   if (dataHelper == null) {              dataHelper = new DataHelper(context);            }                   return dataHelper;        }    }}

/*  懒汉式单例 */public class Singleton {        private static Singleton uniqueInstance = null;        private Singleton() {}        public static synchronized Singleton getInstance ( ) {           if ( uniqueInstance == null ) {           uniqueInstance = new Singleton();       }           return uniqueInstance;    }}/*  饿汉式单例 */public class Singleton {        private static Singleton uniqueInstance =  new Singleton();        private Singleton() {}       public static  Singleton getInstance ( ) {        return uniqueInstance;    }}