显示链接列表:Null指针异常错误

Question

当我试图输出我的链表时，我一直得到null异常错误。

当我只打印条件名称和空的can测试值时，我不会得到错误！！！？不确定原因是什么？

但是，当我试图在整个链表中进行迭代以将其打印出来时，我会得到Null Exception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

循环的条件检查列表中是否至少有一个元素，然后尝试从列表中删除5个元素。

您应该在每次迭代中只调用removeFront一次：

     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();
    }