gistfile1.java

package all;


public class FDList<T> {
   public class Element{
	   T val1;
	   Element next;
	   Element prev;
	   public Element(T a, Element b, Element c){
		   val1=a;
		   next=b;
		   prev=c;
	   }
	   public T value(){
		   return this.val1;   
   }
   }
   
   public class Cursor{
	   private Element curr; 
	   private Writer writer(){
		  Writer a =new Writer();
		  return a;
	   }
	   public Cursor( Element x ) { curr = x; }
	   public Element curr() { return this.curr ;}
	   public void prev() {curr= this.curr.prev; }
	   public void next() { curr=this.curr.next; }
   }
   
   public class Writer{//where is the cursor
	   public boolean insertBefore(T val) { 
		   Element a=new Element(val, cu.curr,cu.curr.prev);
		   cu.curr.prev=a;
		   if(cu.curr.next==cu.curr)
			   cu.curr.next=a;
		   if(cu.curr.next.next==cu.curr)
			   cu.curr.next.next=a;
		   return true;
		   }
	   
		   public boolean insertAfter(T val) { 
			Element a=new Element(val,cu.curr.next,cu.curr);
			cu.curr.next=a;
			if(cu.curr.prev.prev==cu.curr)
				cu.curr.prev.prev=a;
			if(cu.curr.prev==cu.curr){
				cu.curr.prev=a;
		    
			}
			return true;
		   }
		   
		   public boolean delete() { //when the list is null, throw an error;when delete an element, the pointer will point to its previous element
			   if(cu.curr.next==cu.curr.prev && cu.curr.next==cu.curr){//only one element in the list
				   System.out.println("delete "+cu.curr.val1);
				   System.out.println();
				   throw new Error("the list is empty");
			   }
			   if(cu.curr.prev==cu.curr.next.next){//three elements in the list
				   cu.curr.next.prev=cu.curr.prev;
				   cu.curr.prev.next=cu.curr.next;
			   }
			   if(cu.curr.next==cu.curr.prev && cu.curr.next!=cu.curr){//two elements in the list
				   cu.curr.next.next=cu.curr.prev;
				   cu.curr.next.prev=cu.curr.prev;
			   }
			   System.out.println("delete "+ cu.curr.val1);
			   cu.curr.next.prev=cu.curr.prev;
			   cu.curr.prev.next=cu.curr.next;
			   Element a=cu.curr.prev;
			   cu.curr=a;
			   return true;
   }
}
		   
		   private Cursor cu ;
		   private Element head;

		   public Element head() { return this.head; }
		   public Cursor reader(T a) {
			  Element temp=cu.curr;
			  if(cu.curr==null)
				  throw new Error("no such element existing");
			  else{
				   do{
					  System.out.println(temp.val1);
					  if(temp.val1.equals(a)){
						  Cursor v=new Cursor(temp);
						  return v;
					  }
					  temp=temp.next;
				  }while(temp!=cu.curr);
			  
				  throw new Error("no such element exiting");
			  }
			  } 
		   public FDList (T val1){
			 Element a1=new Element(val1,null,null);
			 a1.next=a1;
			 a1.prev=a1;
			 cu=new Cursor(a1);
			 head=a1;
			  
		   }
		   
		   
		   
   public static void main(String[]   args){
     
     
     FDList<String> f=new FDList<String> ("int");
     f.cu.writer().insertAfter("bye");
     f.cu.writer().insertBefore("hi");
     f.cu.writer().insertAfter("good");
     
     f.cu.writer().delete();
     f.cu.writer().delete();
     f.cu.writer().delete();
     f.cu.writer().delete();
     f.cu.writer().delete();
     
     
     
     
   }
}