Question

我只是想确保我从Eclipse收到的是一个错误，还是没关系？这是我的代码：

package prj2;

public class Sandbox {
    public static void main(String[] args) {
        NodeChain<Integer> myList = new NodeChain<Integer>();

        myList.add(1); 
        myList.add(2); 
        myList.add(0, -99); 
        myList.add(3, 45);
        myList.add(99);
        myList.add(4, 50);

        myList.remove(0);
        myList.remove(3);
        myList.remove(1);

        System.out.println(myList.contains(45));

        myList.print();
        System.out.println("myList has size "+myList.size());
    }

}

结果如下：

Exception in thread "main" java.lang.NullPointerException
    at prj2.NodeChain.contains(NodeChain.java:57)
    at prj2.Sandbox.main(Sandbox.java:18)

如果这是一个错误，请帮助解决问题吗？

NodeChain的来源：

package prj2;

public class NodeChain<E> implements ListADT<E>{
    private ListNode<E> head; // this is a dummy header 
    private ListNode<E> tail;
    private int numItems;

    public NodeChain() {
        head = new ListNode<E>(null);
        tail = head;
        numItems = 0;
    }

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

    public void add(E item) { 
        // Note the lack of a need for null checks
        tail.setNext(new ListNode<E>(item, tail.getNext()));
        tail = tail.getNext();
        numItems++;
    }

    public void add(int pos, E item) {
        // Handle the cases where an invalid pos is given
        if (pos < 0 || pos > numItems) throw new IndexOutOfBoundsException();

        // Handle the case where we're adding to the end of the list.
        if (pos == numItems) {
            add(item);
            return;
        }

        // No other special case handling required
        // Traverse the list until we get to the point of insertion and execute the insertion.
        ListNode<E> tmp = traverseTo(pos);
        tmp.setNext(new ListNode<E>(item, tmp.getNext()));

        // Increment numItems
        numItems++;
    }

    public int size() { return numItems; }

    public E remove(int pos) { // Left unimplemented
        return null;
    }

    public E get(int pos) {
        if (pos < 0 || pos >= numItems) throw new IndexOutOfBoundsException();
        ListNode<E> node = traverseTo(pos+1);
        return node.getData();
    }

    public boolean contains(E obj) {
        ListNode<E> tmp = head;
        while (tmp != null) {
            if (tmp.getData().equals(obj)) return true;
            tmp = tmp.getNext();
        }
        return false;
    }

    private ListNode<E> traverseTo(int pos) {
        if (pos < 0 || pos >= numItems) throw new IndexOutOfBoundsException();
        ListNode<E> tmp = head;
        for (int i = 0; i < pos; i++) 
            tmp = tmp.getNext();
        return tmp;
    }

    /* Extra method to facilitate debugging */
    public void print() {
        ListNode<E> tmp = head;
        while (tmp != null) {
            System.out.print(tmp.getData()+", ");
            tmp = tmp.getNext();
        }
        System.out.println();
    }
}

ListNode的来源：

package prj2;

public class ListNode<T> {
    private T data;
    private ListNode<T> next;

    public ListNode(T obj) {
        this(obj, null);
    }

    public ListNode(T obj, ListNode<T> ptr) {
        data = obj;
        next = ptr;
    }

    public void setData(T obj) { data = obj; }

    public T getData() { return data; }

    public void setNext(ListNode<T> n) { next = n; }

    public ListNode<T> getNext() { return next; }
}

ListADT的来源：

package prj2;

/**
 * A List is an ordered collection of items.
 */
public interface ListADT<E> {
    /**
     * Add item to the end of the List.
     *
     * @param item the item to add
     */
    void add(E item);

    /**
     * Add item at position pos in the List, moving the items
     * originally in positions pos through size()- 1 one place
     * to the right to make room.
     *
     * @param pos the position at which to add the item
     * @param item the item to add
     * @throws IndexOutOfBoundsException if pos is less than 0
     * or greater than size()
     */
    void add(int pos, E item);

    /**
     * Return true iff item is
     * item x in the List such
     *
     * @param item the item to
     * @return true if item is
     */
    boolean contains(E item);

    /**
     * Return the number of items in the List.
     *
     * @return the number of items in the List
     */
    int size();

    /**
     * Return true iff the List is empty.
     *
     * @return true if the List is empty, false otherwise
     */
    boolean isEmpty();

    /**
     * Return the item at position pos in the List.
     *
     * @param pos the position of the item to return
     * @return the item at position pos
     * @throws IndexOutOfBoundsException if pos is less than 0
     * or greater than or equal to size()
     */
    E get(int pos);

    /**
     * Remove and return the item at position pos in the List,
     * moving the items originally in positions pos+1 through
     * size() one place to the left to fill in the gap.
     *
     * @param pos the position at which to remove the item
     * @return the item at position pos
     * @throws IndexOutOfBoundsException if pos is less than 0
     * or greater than or equal to size()
     */
    E remove(int pos);
}

Answer 1

嗯，从输出中可以清楚地看出它是抛出异常，或者更确切地说Error。抛出的异常是java.lang.NullPointerException！

此外，错误所在的堆栈跟踪打印在Exception thrown行下面： -

at prj2.NodeChain.contains(NodeChain.java:57)
at prj2.Sandbox.main(Sandbox.java:18)

此外，从堆栈跟踪中可以清楚地看到System.out.println(myList.contains(45));处抛出异常。

在contains()类中检查NodeClass方法的代码后，您最好解决此错误！

这个输出是否正确？还是错误？

1 个答案: