单例模式

饿汉单例模式：

class Student{

private static Student s = new Student();

private Student(){

}

public static Student getInstance(){

return  s;

}

}

//懒汉单例模式

class Person{

private static Person p;

private person(){

}

public static Person getinstance(){

if(p==null){

p=new Person();

}

return p;

}

}