ช่วยหน่อยนะครับพอดีติดปัญหา เรื่อง linkedlist java

import java.util.Random;
public class testlistt {

	public static void main(String[] args) {
	 lllist s1 = new lllist(99);
	 Random read = new Random();
		for (int i = 0; i < 50; i++  ){
	         s1.InsertFirst(read.nextInt(100));
	         s1.Display();
		}
	         System.out.println("--------- removeFirst -------------");
	 		s1.removeFirst();
	 		s1.Display();
	 		s1.removeFirst();
	 		s1.Display();
	 		System.out.println("--------- removeLast -------------");
	 		s1.removeLast();
	 		s1.Display();
	 		s1.removeLast();
                        s1.Display();
}
}
class ที่ 2
public class lllist {
	private int data;
	private lllist next;
	private lllist head = null;
	
	
public lllist(int value)
{
	data = value;
	next = null;
	head = this;
	
}
public void init(lllist value){
	head = value;
}
public void Display()
{
	lllist temp = head;
	
		while(temp != null)
		{
			System.out.print(temp.data + "  ");
			temp = temp.next;
		}
		System.out.println("\n\n");
}
		public void InsertFirst(int value)
		{
			lllist node = new lllist(value);
					node.next = head;
					head = node;
		}
		
		public void InsertLast(int value)
		{
		lllist node = new lllist(value);
				
		node.next = null;
			 }	
				
		
					public void removeFirst()
					{		
						 lllist node = head;
						 head = node.next;  // can be NULL here
						 node = null;
					}
					
					public void removeLast()
					{
						lllist temp = head;
						lllist tempNode;
						while(temp.next.next != null)
						{
							temp = temp.next;
						}
						tempNode = temp.next;
						tempNode = null;
						temp.next = null;
						}
	public void check()
	{
		
		lllist temp = head;
		head = temp.next;
		
	
		
			while(temp.data <=49 )
			{
				System.out.print(temp.data + " 50 ");
				temp = temp.next;
				
			}
		}