Question

在以下代码中，构造函数未初始化numFile Scanner。我将构造函数的内容添加到main方法以使其工作。如果我不这样做，则抛出java.lang.NullPointerException。有人会介意解释原因吗？另外，我是否需要在构造函数上抛出IOException？

感谢任何有用的建议。

贝

import java.io.*;
import java.util.Scanner;

public class CountPositiveIntegers {

    static Scanner numFile;
    static String fileName;     // the name of the file in which the integers are stored
    static int number;          // holds the current number being read
    static int counter;         // a counter used to sum the number of positive integers


    public CountPositiveIntegers() throws IOException { 
        fileName ="D:\\Java\\Source\\numFile.dat"; 
        System.out.println("File Name: " + fileName);   
        numFile = new Scanner(new FileReader(fileName)); 
        number = 0; 
        counter = 0; 
    }


    public static void main(String[] args) throws FileNotFoundException {
        // numFile is not being initializing in the constructor
        fileName = "D:\\Java\\Source\\numFile.dat";
        numFile = new Scanner(new FileReader(fileName));
        number = 0;
        counter = 0;

        if (numFile.hasNext()) {                    // check to see if there are any values in the file
            while (numFile.hasNextInt()) {      // reads in integers
                number = numFile.nextInt();
                if (number % 2 == 0 & number != 0) {
                    counter++;                  
                }
            }
            numFile.close();                            // close the file           

            // print to screen the number of even integers stored in the file
            System.out.println("There are " + counter
                    + " even numbers in this file");
        } else {
            System.out.println("The file is empty.");
        }   

        System.exit(0); // cleanly exit the program
    }
}

Answer 1

您必须显式调用构造函数才能使其正常工作。（你永远不会创建new CountPositiveIntegers()）。

Answer 2

实际上，您只使用静态变量，不会调用构造函数，并且此类对象没有非静态字段。面向对象编程的一个例子：

public class CountPositiveIntegers {

    Scanner numFile;
    String fileName;     // the name of the file in which the integers are stored


    public CountPositiveIntegers(String fname) throws IOException { 
        fileName = fname; 
        System.out.println("File Name: " + fileName);   
        numFile = new Scanner(new FileReader(fileName)); 
    }


    public static void main(String[] args) throws FileNotFoundException {
        try {
            CountPositiveIntegers obj = new CountPositiveIntegers("D:\\Java\\Source\\numFile.dat");
            int number = 0;          // holds the current number being read
            int counter = 0;         // a counter used to sum the number of positive integers

            if (obj.numFile.hasNext()) {                    // check to see if there are any values in the file
                while (obj.numFile.hasNextInt()) {      // reads in integers
                    number = obj.numFile.nextInt();
                    if (number % 2 == 0 & number != 0) {
                        counter++;                  
                    }
                }
                obj.numFile.close();                            // close the file           

                // print to screen the number of even integers stored in the file
                System.out.println("There are " + counter
                        + " even numbers in this file");
            } else {
                System.out.println("The file is empty.");
            }   

            System.exit(0); // cleanly exit the program
        } catch (IOException ex) {
            Logger.getLogger(CountPositiveIntegers.class.getName()).log(Level.SEVERE, null, ex);
        }
    }
}

Answer 3

我认为问题在于，当你的程序启动时，不创建一个执行main的类的新实例，而只是运行{中的代码{1}}。由于您的初始化代码在构造函数中，因此您从未实际运行，因为您没有创建主类的实例。

要解决此问题，我强烈建议让main创建main所在类的新实例，然后对该对象执行所有操作，而不是创建所有内容{{1}并直接在main执行操作。您当前的方法并不是特别好的设计。

希望这有帮助！

Answer 4

您的CountPositiveIntegers类对象在哪里。

您需要显式调用构造函数，否则编译器将调用默认构造函数。

new CountPositiveIntegers();

Answer 5

除非您实际调用它，否则不会调用构造函数。 main方法不会隐式调用构造函数，因为它是static方法。

构造函数未初始化Scanner对象

5 个答案: