어떻게 대상의 유일성을 보증합니까

/*
          ：
   1.        new       。
   2.           。
   3.                       。
*/
public class Test{
    public static void main(String[] args){
       
	   //Subject sub = Subject.oSub;//       ，       
	   Subject sub = Subject.getInstance();//            ，             ，        ！
	   new Student().show(sub);
	   new Teacher().show(sub);
   }
}

class Subject{
    public  String sub;
	public static Subject oSub = new Subject();
	private Subject(){
	    sub = "        ";
	}
	
	public static Subject getInstance(){
	    //oSub = new Subject();
		//if(****)    if               
		return oSub;
	}
	
	public String getSubject(){
		return sub;
	}
	
	public void setSubjcet(String name){
	    sub = name;
	}
}

/*
                         ：
         ：
                         ！（    ，          ，          ？    ？
           ，     ！            ，               ）             ，     
                          ，                  ！
 class Subject{
    public  String sub;
	//public Subject ss = new Subject();
	public static Subject oSub = new Subject();//   
        //public static Subject oSub = null;//   
	private Subject(){
            //oSub = new Subject();//oSub     ，         oSub         !
	    sub = "        ";
	}
	
	public static Subject getSubject(){        
		return oSub;
                //return oSub = new Subject();
	}
}
*/

class Student{
    public void show(Subject sub){
	    System.out.println(sub.sub);
	}
}

class Teacher{
    public void show(Subject sub){
	    System.out.println(sub.sub);
	}
}