具有单向链表到哈希表 Java 的空指针异常

Question

这是提供的初始类，我们无法修改

public class SLL {
    public class Node {
        private int data;
        private Node next;
        public Node() {
            data = 0;
            next = null;
        }
        public Node(int newData, Node linkValue) {
            data = newData;
            next = linkValue;
        }
        public int getData() {
            return data;
        }
        public Node getLink() {
            return next;
        }
    }// End of Node inner class
    private Node head;
    public SLL() {
        head = null;
    }
    public void addToStart(int itemData) {
        head = new Node(itemData, head);
    }
    public boolean contains(int item) {
        return (find(item) != null);
    }
    /**
     * Finds the first node containing the target item, and returns a reference
     * to that node. If target is not in the list, null is returned.
     */
    public Node find(int target) {
        Node position = head;
        int itemAtPosition;
        while (position != null) {
            itemAtPosition = position.data;
            if (itemAtPosition == target) {
                return position;
            }
            position = position.next;
        }
        return null; // target was not found
    }
    public void outputList() {
        Node position = head;
        while (position != null) {
            System.out.print(position.data + "  ");
            position = position.next;
        }
        System.out.println();
    }
}

这是我们应该完成以使测试器工作的 Set 类，并且我的 add 方法不断收到空指针异常，但是，它几乎与我在包括我们的教科书在内的其他代码中看到的完全相同。 任何见解将不胜感激，因为我的老师已经预先制作了Powerpoint，并且不会向寻求帮助的学生解释任何内容或提供任何建议。

public class Set {
    private SLL[] hashArray; // DO NOT MODIFY THIS LINE
    private int size = 10; // DO NOT MODIFY THIS LINE
    // DO NOT MODIFY THIS METHOD
    public Set() {
        hashArray = new SLL[size];
    }
    // DO NOT MODIFY THIS METHOD
    private int computeHash(int s) {
        return s % size;
    }
    // COMPLETE BELOW
        public void add(int x)
    {
        int hash = computeHash(x);  // Get hash value
        SLL list = hashArray[hash];
                if (!list.contains(x))
        {
            // Only add the target if it's not already
            // on the list.
            list.addToStart(x);/*replaced hashArray[hash] with list*/
        }               
        }
        public void output( )
        {
            System.out.println("I will work on this later");
        }     
}

最后，测试仪...

public class Tester{
    // Have this method to display your name, instead.
    static void displayName(){
        System.out.println("Program written by Tony.n");
    }
    // DO NOT MODIFY THE MAIN METHOD
    public static void main(String[] args){
        displayName();
        Set set1 = new Set();
        Set set2 = new Set();
        set1.add(3);
        set1.add(3);
        set1.add(13);
        set1.add(23);
        set1.add(4);
        set1.add(5);
        set2.add(15);
        set2.add(6);
        set2.add(6);
        System.out.println("Contents of set 'set1': ");
        set1.output();
        System.out.println("Contents of set 'set2': ");
        set2.output();
        System.out.println();
    }
}

Answer 1

我不想直接给出答案，因为这可能是家庭作业(如果我错了，请纠正我(。考虑第一次在新构造的集合上调用 add 方法。此时"hashArray"的所有索引中都有哪些值，这对添加方法中的局部变量"list"意味着什么？

Answer 2

这条线没有做你认为它正在做的事情。

hashArray = new SLL[size];

您需要实际创建每个SLL，一旦创建数组本身，这些将填充数组。