Question

我正在创建一个程序，它可以计算随机段落中每个单词的使用次数。它编译，但当我尝试运行它时，给我一个NullPointerException。

以下是代码：

import java.util.StringTokenizer; 

class Count
{
    int count;
    String name;

    void SetCount(int c, String n)
    {
        count = c;
        name = n;
    }

    void Show()
    {
        System.out.print("Name=" + name);
        System.out.print("Count=" + count);
    }
}

class Contains2 extends Count
{
    public static void main(String args[])
    {
        String s = "Once you have made it to the box office and gotten your tickets, you are confronted with the problems of the theater itself. If you are in one of the run-down older theaters, you must adjust to the musty smell of seldom-cleaned carpets. Escaped springs lurk in the faded plush or cracked leather seats, and half the seats you sit in seem loose or tilted so that you sit at a strange angle. The newer twin and quad theaters offer their own problems. Sitting in an area only one-quarter the size of a regular theater, moviegoers often have to put up with the sound of the movie next door. This is especially jarring when the other movie involves racing cars or a karate war and you are trying to enjoy a quiet love story. And whether the theater is old or new, it will have floors that seem to be coated with rubber cement. By the end of a movie, shoes almost have to be pried off the floor because they have become sealed to a deadly compound of spilled soda, hardening bubble gum, and crushed Ju-Jubes";
        int size, i, count = 0, j;

        size = s.length();
        String[] test = new String[size];

        Count[] c = new Count[size];

        StringTokenizer st = new StringTokenizer(s, " ");   

        while (st.hasMoreTokens())
        {
            for (i=0; i < size; i++)
            {
                test[i] = st.nextToken();
                c[i].SetCount(1, test[i]);
            }
        }

        for (i=0; i<size; i++)
        {
            for (j=0; j<size; j++)
            {
                if (c[i].name.equals(test[j]))
                    c[i].count+=1;
            }
        }

        for (i=0; i<size; i++)
        {
            c[i].Show();
        }
    }   
}

Answer 1

主要问题是，即使您已经创建了一个Count[]数组，但您还没有在数组中的每个位置初始化Count()对象。

Count[] c = new Count[size];

这会初始化数组本身，但它仍然不会在数组的每个位置放置一个初始化的Count()对象。您需要使用new Count()实际创建和分配这些新对象，如下所示：

for (int i=0; i<size; i++)
{
    c[i] = new Count();
}

另一个问题似乎在这里：

while (st.hasMoreTokens())
{
    for (i=0; i<size; i++)
    {
        test[i] = st.nextToken();
        c[i].SetCount(1, test[i]);
    }
}

您在st.hasMoreTokens()时循环播放，但随后您继续拨打st.nextToken() size次，结束了。

请改为尝试：

import java.util.StringTokenizer;


class Contains2 extends Count
{
    public static void main(String args[])
    {
        String s = "Once you have made it to the box office and gotten your tickets, you are confronted with the problems of the theater itself. If you are in one of the run-down older theaters, you must adjust to the musty smell of seldom-cleaned carpets. Escaped springs lurk in the faded plush or cracked leather seats, and half the seats you sit in seem loose or tilted so that you sit at a strange angle. The newer twin and quad theaters offer their own problems. Sitting in an area only one-quarter the size of a regular theater, moviegoers often have to put up with the sound of the movie next door. This is especially jarring when the other movie involves racing cars or a karate war and you are trying to enjoy a quiet love story. And whether the theater is old or new, it will have floors that seem to be coated with rubber cement. By the end of a movie, shoes almost have to be pried off the floor because they have become sealed to a deadly compound of spilled soda, hardening bubble gum, and crushed Ju-Jubes";
        int size, count = 0;


        StringTokenizer st = new StringTokenizer(s, " ");
        size = st.countTokens();

        Count[] c = new Count[size];
        String[] test = new String[size];

        while (st.hasMoreTokens())
        {
            String token = st.nextToken();

            for (int i=0; i<size; i++)
            {
                test[i] = token;

                c[i] = new Count();
                c[i].SetCount(1, token);
            }
        }

        for (int i=0; i<size; i++)
        {
            for (int j=0; j<size; j++)
            {
                if (c[i].name.equals(test[j]))
                    c[i].count+=1;
            }
        }

        for (int i=0; i<size; i++)
        {
            c[i].Show();
        }
    }
}

public class Count
{
    protected int count;
    protected String name;

    public void SetCount(int c, String n)
    {
        count = c;
        name = n;
    }

    public void Show()
    {
        System.out.println("Name=" + name);
        System.out.println("Count=" + count);
    }
}

Answer 2

执行c[i].SetCount(.....)后，c[i]处的对象尚未使用new Count()进行初始化

如何处理这个NullPointerException？

2 个答案: