Question

我正在制作一个项目，以检查文本文件并输出文件中每个字母的计数。

import java.util.*;
import java.io.*;
    public class frequencyAnalysis {

        private static String text;
        public static String alphabet;
        public static int Freq[];

    public frequencyAnalysis(String text) {
        this.text = text;
        int [] Freq = {0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0};  //array of ints to keep track of how many of each letter there is.
        alphabet = "abcdefghijklmnopqrstuvwxyz";  //point of reference for the program to know which number in the array should be increased
    }

    public static void freqAnalysis() throws IOException {

        String token = "";
        int index;

        File subset = new File(text);  //creates a new file from the text parameter
        Scanner inFile = new Scanner(subset);

        while(inFile.hasNext()) {
            token = inFile.next();
            index = alphabet.indexOf(token);
            if (index == -1) {  //makes sure that the character is a letter
                break;  
            } else {
                Freq[index]++;
            }
        }
        inFile.close();
    }
 }

这是应该通过给定文本文件并计算其中每个字母有多少的类。

import java.util.*;
import java.io.*;
public class tester {
    public static void main(String args[]) throws IOException {
        Scanner in = new Scanner(System.in);

        System.out.println("Please type the input file path: ");  //allows the user to specify a file
        String input = in.next();

        frequencyAnalysis Freq = new frequencyAnalysis(input);

        frequencyAnalysis.freqAnalysis();  //calls the method to run through the file

        for(int i = 0; i <= 25; i++){  //prints the alphabet and the Freq array
            System.out.println(frequencyAnalysis.alphabet.charAt(i) + ": " + frequencyAnalysis.Freq[i]);  //this is where the error is
        }

    }
}

这是实现类，它允许用户指定文件，然后运行freqAnalysis方法来调整静态Freq数组，然后将其打印出来。但是，当我运行程序时，它在指定的行上给了我java.lang.NullPointerException错误。我已经发现问题出在“ frequencyAnalysis.Freq [i]”中，而不是在“ frequencyAnalysis.alphabet.charAt（i）”中。但是，我不知道问题出在哪里或如何解决。

Answer 1

在构造函数中，您将创建一个初始化局部变量int [] Freq而不是类字段。代替：

int [] freq = {0, 0, 0, 0, 0, 0, ...

您应该拥有：

freq = {0, 0, 0, 0, 0, 0, ...

Answer 2

好的，所以您没有在frequencyAnalysis类中正确初始化数组。

public static int Freq[];

您认为您正在使用它进行初始化 int [] Freq = {0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0};

但实际上应该是： this.Freq = new int[]{0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0};

Answer 3

其他答案还不错，但是在编码风格上有缺陷。

首先，整数数组默认使用零初始化，因此您可以简化Freq的初始化。另外，您在Freq的构造函数中创建了局部变量 frequencyAnalysis，而不是设置了静态类变量。

您应该将构造函数更改为此：

    public frequencyAnalysis(String text) {
        this.text = text;
        frequencyAnalysis.Freq = new int[26];  //array of ints to keep track of how many of each letter there is.
        alphabet = "abcdefghijklmnopqrstuvwxyz";  //point of reference for the program to know which number in the array should be increased
    }

此外，类名应始终大写！（将其更改为FrequencyAnalysis）

Answer 4

为了通知您正在发生的事情，

public frequencyAnalysis（字符串文本）{ this.text =文字； int []频率= {0，0，0，0，0，0，0，0，0，0，0，0，0，0，0，0，0，0，0，0，0，0， 0，0，0，0}; //整数数组，以跟踪每个字母有多少个。字母=“ abcdefghijklmnopqrstuvwxyz”； //程序的参考点，以了解应增加数组中的哪个数字 }

执行此操作时，int [] Freq是一个新的局部变量，作用域仅限于构造函数，而另一方面，它是public static int Freq [];仍然为Null，如上文所述，指向相同的变量进行初始化

引用来自其他类的int数组时，java.lang.NullPointerException

4 个答案: