Linkedlist queue

public class LinkedQueue{
    private int size;
    private Node head;

    public LinkedQueue(){
        head=new Node(null);
        size=0;
    }

    public void add(Object data){
        Node temp=head;
        while(temp.next!=null){
            temp=temp.next;
        }
        temp.next=new Node(data);
        size++;

    }

    public void delete(){
        if(size==0){
            System.out.println("Stack Under Flow");
        }
        else{
            size--;
            Node temp= head;
            temp.next=temp.next.next;
        }
    }

    public Object front(){
        if(isEmpty()){
            return null;
        }
        else{
            return head.next.data;
        }
    }
    public Object rear(){
        if(isEmpty()){
            return null;
        }
        else{
            Node temp=head;
            while(temp.next!=null){
                temp=temp.next;
            }
            return temp.data;
        }
    }

    public boolean isEmpty(){
        return (size==0);
    }

    public boolean isFull(){
        return false;
    }

    public String toString()
    {
        String output = "";
        if(!isEmpty()){
            Node temp= head.next;
            while(temp != null)
            {
                output += "[" + temp.data + "]";
                temp= temp.next;
            }
            return output;
        }
        else{
            return null;
        }

    }

    private class Node
    {
        protected Node next;
        protected Object data;

        public Node(Object _data)
        {
            next = null;
            data = _data;
        }

        public Node(Object _data, Node _next)
        {
            next = _next;
            data = _data;
        }

    }
}