显示链表：空指针异常错误

Question

我在尝试输出我的链表时一直收到空异常错误。

当我只打印条件名称和空罐测试值时，我没有得到那个错误!!!?不确定原因？

但是，当我尝试遍历整个链表以打印出来时，出现空异常错误

public class ItemLinkedList {

    private ItemInfoNode head;
    private ItemInfoNode tail;
    private int size = 0;

    public int getSize() {
        return size;
    }

    public void addBack(ItemInfo info) {
        size++;
        if (head == null) {
            head = new ItemInfoNode(info, null, null);
            tail = head;
        } else {
            ItemInfoNode node = new ItemInfoNode(info, null, tail);
            this.tail.next = node;
            this.tail = node;
        }
    }

    public ItemInfo removeFront() {
        ItemInfo result = null;
        if (head != null) {
            size--;
            result = head.info;
            if (head.next != null) {
                head.next.prev = null;
                head = head.next;
            } else {
                head = null;
                tail = null;
            }
        }
        return result;
    }


    public static void main(String[] args) {

        ItemLinkedList list = new ItemLinkedList();

        list.addBack( new ItemInfo("Bicipital Tendonitis", 1, 0, 1, 1) );
        list.addBack( new ItemInfo("Coracoid Impingement", 0, 1, 1, 1) );
        list.addBack( new ItemInfo("Supraspinatus Impingement", 1, 0, 0, 1) );
        list.addBack( new ItemInfo("Bicipital Tendonitis", 1, 0, 1, 1) );
        list.addBack( new ItemInfo("Glenohumeral Dislcation", 0, 0, 1, 1) );
        list.addBack( new ItemInfo("Clavicular Fracture", 1, 0, 1, 0) );
        list.addBack( new ItemInfo("Labral Tear", 1, 1, 0, 0) );     
        list.addBack( new ItemInfo("SubAcromial Bursitis", 1, 0, 0, 0) );


         while (list.getSize() > 0){

            System.out.println( "Condition Name " + list.removeFront().getCondName() );
            System.out.println( "\t Empy Can Test: " + list.removeFront().getEmptyCanTest() );
            System.out.println( "\t Speed's Test: " + list.removeFront().getSpeedsTest() );
            System.out.println( "\t Apprehension Test: " + list.removeFront().getApprehensionTest() );
            System.out.println( "\t Pain Provocation Test: " + list.removeFront().getpainProvocationTest() );
            System.out.println();
        }

    }

Answer 1

您的循环条件检查列表中是否至少有一个元素，但随后您尝试从列表中删除 5 个元素。

你应该在每次迭代中只调用一次 removeFront :

     while (list.getSize() > 0){
        ItemInfo item = list.removeFront();
        System.out.println( "Condition Name " + item.getCondName() );
        System.out.println( "\t Empy Can Test: " + item.getEmptyCanTest() );
        System.out.println( "\t Speed's Test: " + item.getSpeedsTest() );
        System.out.println( "\t Apprehension Test: " + item.getApprehensionTest() );
        System.out.println( "\t Pain Provocation Test: " + item.getpainProvocationTest() );
        System.out.println();
    }

显示链表：空指针异常错误

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