单态设计模式（Singleton pattern）

单例设计模式：
在内存中对象只有一个存在。
*/
//饿汉式
class Student
{
    private Student(){}

    private static Student s = new Student();

    public static Student getInstance()
    {
        return s;
    }
}
//懒汉式
class Teacher
{
    private Teacher(){}

    private static Teacher t;

    public static Teacher getInstance()
    {
        if(t==null)
        {
            t = new Teacher();
        }
        return t;
    }
}

class SingletonDemo 
{
    public static void main(String[] args) 
    {
        /*
        Student s1 = new Student();
        Student s2 = new Student();
        System.out.println(s1==s2);
        */

        Student s1 = Student.getInstance();
        Student s2 = Student.getInstance();
        System.out.println(s1==s2);

        Teacher t1 = Teacher.getInstance();
        Teacher t2 = Teacher.getInstance();
        System.out.println(t1==t2);
    }
}