//类加载时，对象已经创建
class Single{
private static Single s = new Single();
private Single(){}
public static Single getInstance(){
return s;
}
}
//类加载时，未创建对象，在调用 getInstance方法时，才会创建对象
//延迟类的加载
class Single{
private static Single s = null;
private Single(){}
public static Single getInstance(){
if(s==null){
s=new Single();
}
return s;
}
}
//考虑线程，以及优化 延迟加载类型
class Single{
private static Single s = null;
private Single(){}
public static Single getInstance(){
if(s==null){ //加判断，解决效率问题：避免产生对象后，每次判断锁，影响程序效率
synchronized(Single.class){ //加锁，解决线程安全问题
if(s==null){
s=new Single();
}
}
}
return s;
}
}