java - 为什么这不起作用？

Question

import java.util.*;

class HashingDemo {
    public static void main(String[] args) {
        Scanner keyboard = new Scanner(System.in);

        System.out.print("Please input the size of the hash table: ");
        int tableSize = keyboard.nextInt();

        LinkedListN[] hashTable = new LinkedListN[tableSize];

        // this works
        LinkedListN list = new LinkedListN();
        list.addToEnd(50);
        System.out.println(list);
        //

        System.out.print("Enter the number of keys to be hashed: ");
        int numberOfKeys = keyboard.nextInt();

        Random randomGenerator = new Random();

        for (int i = 0; i < numberOfKeys; i++) {
            int randomNumber = randomGenerator.nextInt(10000) + 1;

            int location = randomNumber % tableSize;

            hashTable[location].addToEnd(randomNumber);
        }
    }
}

LinkedListN 是一个自定义类(代码附在下面)，因为数组不能很好地处理泛型。

但是每次运行该程序时都会出现以下错误:

Please input the size of the hash table: 10
LinkedListN@5265a77f
Enter the number of keys to be hashed: 20
Exception in thread "main" java.lang.NullPointerException
    at HashingDemo.main(HashingDemo.java:30)

尽管正如我上面评论的那样，如果我只有一个 LinkedListN 并向其中添加数据，就没有问题。这是怎么回事？我一直在尝试解决这个问题，但我做不到。

Answer 1

LinkedListN[] hashTable = new LinkedListN[tableSize]; 仅分配数组，而不分配其中的对象。要克服 NullPointerException ，您必须为每个元素分配对象:

for (int i = 0; i < numberOfKeys; i++) {
    int randomNumber = randomGenerator.nextInt(10000) + 1;
    int location = randomNumber % tableSize;
    if(hashTable[location]==null) {
        hashTable[location] = new LinkedListN();
    }
    hashTable[location].addToEnd(randomNumber);
}

您错过了行hashTable[location] = new LinkedListN();