errors when using own implemented linked list

Question

I tried to use my own implemented LinkedList.

 public class LinkedList<O> {
       private Node<O> first,last;
       private int count;
       public LinkedList(){}

       public Node getfirst(){
        if(first==null) return null;
        else return first;
       }
       public Node getLast(){
        if(first==null) return null;
        else return last;
       }
    public int getSize(){
        return count;
    }
    public void addFirst(Object x){
        if(first==null)first=last=new Node(x);
        else{
            Node temp =new Node(x);
            temp.next=first;
            first=temp;
        }
        count++;
    }
    public void addLast(Object x){
        if(first==null)first=last=new Node(x);
        else{
            last.next= new Node(x);
            last=last.next;
        }
        count++;
    }
    public void add(Object x,int index){
        if(index==0)addFirst(x);
        else if(index>=getSize())addLast(x);
        else{
            Node current=first;
            for(int i=0; i<index-1;i++)
                current=current.next;
            Node temp = new Node(x);
            temp.next=current.next;
            current.next=temp;
            count++;
        }
    }
    public boolean removeFirst(){
        if(first==null)return false;
        else if(first==last){
            first=last=null;
            count--;
            return true;
            }
        else{
            first=first.next;
            count--;
            return true;
            }
    }
    public boolean removeLast(){
        if(first==null)return false;
        else if(first==last){
            first=last=null;
            count--;
            return true;
            }
        else{
            Node current=first;
            for(int i=0;i<getSize()-2;i++)
                current=current.next;
            last=current;
            last.next=null;
            count--;
            return true;
            }
    }
    public boolean remove(int index){
        if(index==0)return removeFirst();
        else if(index==getSize()-1)return removeLast();
        else{
            Node current=first;
            for(int i=0;i<index-1;i++)
                current=current.next;
            current.next=(current.next).next;
            count--;
            return true;
            }
    }
}




public class Node<E> {
   E element;
   Node<E> next;
   public Node(E element) {
      this.element = element;
    }
}

when I used :

 for(Book b:books){
        System.out.println(b);
 }

it gave me an error: Can only iterate over an array or an instance of java.lang.Iterable

So, I tried to use :

for(Book current=books.getFirst(); current !=null; current=current.next){
    System.out.println(current);
    }

It print :

project11.Node@139a55
project11.Node@1db9742
project11.Node@106d69c

and when I used collection.sort

Collections.sort(books,new Comparator<Book>()){
public int compare(Book book1, Book book2) {
    return book1.getTitle().compareToIgnoreCase(book2.getTitle());
    }
    }

It gave me: The method sort(List, Comparator) in the type Collections is not applicable for the arguments (LinkedList, Stock.MyTitleComp)

Can someone explain these errors please and how to fix them.

its all worked out, but now it gives me java.lang.NullPointerException at Collections.sort(books,new Comparator()) — Nicky, Apr 04 '16 at 18:53
before using collections.sort it works just fine, there is no null values. — Nicky, Apr 04 '16 at 19:02

score 2 · Accepted Answer · answered Apr 04 '16 at 18:21

The first and third errors you are seeing is since you created a list of your own, but you did not implement the List interface, so methods that work on lists (or Iterable, this one is a super-interface of List) does not work on it. Once you'll implement the interface, it will work

The second error is simply missing toString() implementation of your class, so you are getting this toString implementation from Object, which gives a not so readable print, override it with how you want your class to be printed

score 1 · Answer 2 · edited May 23 '17 at 12:24

For #1, it means what it says. You can only use for-each loops over implementors of the java.lang.Iterable interface or arrays.

For #2, you're running into Why does the default Object.toString() include the hashcode?.

For #3, your list still doesn't implement the java.util.List interface, so Collections.sort can't do anything with it for the same reason #1 didn't work. You also messed up the parentheses, it seems.

errors when using own implemented linked list

2 Answers2