Question

我是这个网站的新手，我是java的初学者。我正在制作一个程序，它需要5个数字并将它们吐出来。为什么这段代码不起作用？这是我的第一个.java。我把标记放在我认为问题出现的地方。

public class digitExtractor {
    int tNumber;
    String sNumber;

    // OP thinks the problem is here...
    public digitExtractor(int tNumber) {       


    }
    // ... and here
    public digitExtractor(String sNumber) {



    }

    public void returnInvertedOrderByMath() {
        int Integer1;
        int Integer2;
        int Integer3;
        int Integer4;
        int Integer5;
        Integer1 = (tNumber % 10);
        Integer2 = (tNumber % 100) / 10;
        Integer3 = (tNumber % 1000) / 100;
        Integer4 = (tNumber % 10000) / 1000;
        Integer5 = (tNumber % 100000) /10000;

        System.out.println(Integer1 + "\n" + Integer2 + "\n" + Integer3 + "\n" + Integer4 + "\n" + Integer5);
    }

    public void returnInvertedOrderByString() {
        char part1;
        char part2;
        char part3;
        char part4;
        char part5;
        part1 = sNumber.charAt(4);
        part2 = sNumber.charAt(3);
        part3 = sNumber.charAt(2);
        part4 = sNumber.charAt(1);
        part5 = sNumber.charAt(0);
        System.out.println(part1 + "\n" + part2 + "\n" + part3 + "\n" + part4 + "\n" + part5);

    }
}

Answer 1

我认为你想要的是初始化你的构造函数中的实例字段。

在public digitExtractor(int tNumber)中，你会有一个身体：

// assigning instance field with constructor's argument value
this.nNumber = tNumber;

在public digitExtractor(String sNumber)中，你会有一个身体：

// assigning instance field with constructor's argument value
this.sNumber = sNumber;

这样，您的returnInvertedOrder将引用已初始化的DigitExtractor字段，然后执行您要执行的操作。

请注意，这对于String字段尤其重要，因为如果在您的代码中引用它之前尚未初始化它，则会抛出NullPointerException。

如果未初始化，int将默认为0。

Answer 2

这应该工作时请记住，我在您的程序中添加了一个主要方法来测试您的代码并查看我添加到您的代码中的评论。

public class DigitExtractor {
    private int tNumber;
    private String sNumber;

    public static void main(String[] args){

        //this will work because i chose the right constructor
        DigitExtractor d = new DigitExtractor(10000);
        d.returnInvertedOrderByMath();

        //this wont work because sNumber is null
        //d.returnInvertedOrderByString
    }

    public DigitExtractor(int tNumber) {
        this.tNumber = tNumber;
    }
    public DigitExtractor(String sNumber) {
        this.sNumber = sNumber;
    }

    public void returnInvertedOrderByMath() {
        int Integer1 = (tNumber % 10);
        int Integer2 = (tNumber % 100) / 10;
        int Integer3 = (tNumber % 1000) / 100;
        int Integer4 = (tNumber % 10000) / 1000;
        int Integer5 = (tNumber % 100000) /10000;

        System.out.println(Integer1 + "\n" + Integer2 + "\n" + Integer3 + "\n" + Integer4 + "\n" + Integer5);
    }

    public void returnInvertedOrderByString() {
        char part1 = sNumber.charAt(4);
        char part2 = sNumber.charAt(3);
        char part3 = sNumber.charAt(2);
        char part4 = sNumber.charAt(1);
        char part5 = sNumber.charAt(0);

        System.out.println(part1 + "\n" + part2 + "\n" + part3 + "\n" + part4 + "\n" + part5);

    }
}

Answer 3

int tNumber; // This is an instance variable as it belongs to the class
String sNumber;

// This is a constructor which takes an integer
public digitExtractor(int tNumber) {  // tNumber here is a parameter which is not the same as tNumber above. You can change the name of this parameter to anything you like     
    // "this" keyword tells us we are referring to instance variable and assigning the value of tNumber parameter to instance one
    this.tNumber = tNumber; 
}

public digitExtractor(String sNumber) {
    if (sNumber == null){
        this.sNumber = ""; // if null, let's put an empty string as default
    } else {
        this.sNumber = sNumber;
    }
}

这怎么了？你如何构建对象和方法

3 个答案: