java入门选手提问为什么一直报异常 我的想法就是单纯的头节点不存放元素

public class MySingleList implements MyList {
class Node {
E item;
Node next;

    Node(E item, Node<E> next) {
        this.item = item;
        this.next = next;
    }
}

private Node<E> head;

private int size;


@Override
public void add(E item) {
    Node<E> node = new Node<>(item, null);
    Node<E> tail = getTail();

    if (tail == null)
       this.head.next = node;
    else
        tail.next = node;
    this.size++;
}

private Node<E> getTail() {

    if (this.head==null) {
        return null;
    } else {
        Node<E> p = this.head;

        while (p.next != null) {
            p = p.next;
        }
        return p;
    }
}

@Override
public E get(int index) {
    return null;
}

@Override
public int size() {
    return 0;
}

@Override
public E remove(int index) {
    return null;
}


public static void main(String[] args) {
    MySingleList<String> msl = new MySingleList<>();

 msl.add("21");
    // msl.add("215");
    System.out.println(msl.head.item);
}

}

老是会在add函数那报空指针异常，真的难受

你head没有初始化，对Null调用.next自然会报空指针