java - 我的复制构造函数出了什么问题？

Question

Java 初学者。我一直在试图弄清楚如何为 java 项目编写构造函数之一。我将包含大部分代码以提供一些上下文。此类将由另一个包含终端菜单方法的文件访问。

// java class for keyboard I/O
import java.util.Scanner;

// declaration of the class
public class NumberList
{
    //integer constant that determines the size of the array
    public static final int MAX_CAPACITY = 100;

    //array to store the numbers        
    private double [] numbers;  
    //the number of valid numbers currently in the NumberList
    private int length;

    //default constructor that initializes array to size MAX_CAPACITY and initializes all
    //array values to 0, sets length to 10
    public NumberList()
    {
        numbers = new double[MAX_CAPACITY];

        int i;

        for(i = 0; i < MAX_CAPACITY; i++)
            numbers[i] = 0;

            length = 10;
    }

    //outputs the numbers in the NumberList to the 
    //standard output screen    
    public void print() 
    {
        int i;

        for(i = 0; i < length-1; i++)
            System.out.println(numbers[i]+ ", ");

        System.out.println(numbers[i]);
    }



    //assignment constructor, initializes array to size 100,
    //initializes length to l and sets the first l values of the list to the     value n
    NumberList(int l, double n)
    {   
        numbers = new double[MAX_CAPACITY];

        length = l;

        int i;

        for(i = 0; i < MAX_CAPACITY; i++)
            numbers[i] = n;

    }


    //array constructor, initializes array to size 100, takes an array
    //as input and completes a deep copy (element to element copy) from the array
    //parameter to the numbers array 
    NumberList(final double[] a)
    {
        this.numbers = new double[a.length];
        for (int i = 0; i < a.length; ++i)
        {
            this.numbers[i] = a[i];
        }
    }

上面的一切都编译得很好。不确定我是否走在正确的轨道上，或者是否需要“for”循环。

    **//copy constructor, initializes array to size 100, 
    //creates a copy of parameter NumberList nl to the calling NumberList
    NumberList(final NumberList nl)
    {
        numbers = new double[MAX_CAPACITY];

        nl = new NumberList(MAX_CAPACITY);

    }**

    //returns the length of NumberList
    public int length()
    {
        int length = numbers.length;
        return length;
    }

    //returns the sum of the numbers in the NumberList
    public double sum()
    {
        double sum = 0;

        for (int i = 0; i < numbers.length; i++)
            {
                sum = sum + numbers[i];
            }
        return sum;
    }

感谢我能得到的任何提示/建议。

Answer 1

复制构造函数就是:一个构造函数。因此，您正在为您新创建的新实例上进行操作。您不想创建另一个实例，只需设置this即可。

例如，在您的情况下，将当前实例的 numbers 设为源实例的 numbers 的副本，并复制 length >:

NumberList(final NumberList nl)
{
    this.numbers = nl.numbers.clone();
    this.length = nl.length;
}

<小时/>

(感谢 Jorn Vernee 和 assylias 分别指出 Arrays.copyOf 和 clone。使用 clone.)