复制构造函数测试不工作 (Java)

Question

我正在做 CS 作业，运行遇到了问题。作业最后问的是使用拷贝构造函数。目标是 "make one Payroll object, instantiate it, make a second one, then print them both. Then, change values in the second Payroll object, and show that the changed values only appear in one and not both (that is, print out the original and the copy with slightly changed values)." 我尝试更改第二个 Payroll 对象中的值，但它也更改了第一个。我在下面列出了我的代码：

import java.util.Random;


public class Payroll {
private int[] employeeId;
private int[] hours;
private double[] payRate;

public Payroll(){
    this.employeeId = new int[0];
    this.hours = new int[0];
    this.payRate = new double[0];
}

public Payroll(Payroll obj){
    this.employeeId = obj.employeeId;
    this.hours = obj.hours;
    this.payRate = obj.payRate;
}

public Payroll(int i){
    this.employeeId = new int[i];
    this.hours = new int[i];
    this.payRate = new double[i];

}

public int getEmployeeIdAt(int index){
    return employeeId[index];   
}

public int getHoursAt(int index){
    return hours[index];
}

public double getPayRateAt(int index){
    return payRate[index];
}

public double getGrossPay(int index){
    double grossPay = hours[index] * payRate[index];
    grossPay = Math.round(grossPay * 100);
    return grossPay/100;
}

public void setEmployeeIdAt(int index, int id){
    this.employeeId[index] = id;
}

public void setHoursAt(int index, int hrs){
    this.hours[index] = hrs;
}

public void setPayRateAt(int index, double pr){
    this.payRate[index] = pr;
}

public void setHoursAt(int i){
    Random rand = new Random();
    int randHours = rand.nextInt((50 - 15) + 1) + 15;
    this.hours[i] = randHours;
}
}

import java.util.Scanner;


public class PayrollDriver {

public static void main(String[] args) {
    Payroll pr = new Payroll(5);
    Scanner scan = new Scanner(System.in);
    int empID = 1001;
    for(int i = 0; i < 5; i++){
        pr.setEmployeeIdAt(i, empID);
        empID++;
    }
    for(int i = 0; i < 5; i++){
        System.out.println("Enter the hourly pay rate for employee number "        + pr.getEmployeeIdAt(i) + ": ");
        double payRate = scan.nextDouble();
        if(payRate < 7.50){
            do{
                System.out.println("ERROR: Enter 7.50 or greater for pay rate: ");
                payRate = scan.nextDouble();
            } while(payRate < 7.50);
        }
        pr.setPayRateAt(i, payRate);
        pr.setHoursAt(i);
    }
    System.out.println("PAYROLL DATA");
    System.out.println("======================");
    for(int i = 0; i < 5; i++){
        System.out.println("Employee ID: " + pr.getEmployeeIdAt(i) + "       Hours: " + pr.getHoursAt(i) + "    Rate: " + pr.getPayRateAt(i) + 
                "    Gross Pay: $" + pr.getGrossPay(i));
    }
    System.out.println("Would you like to run the Copy Constructor Test? Enter 'y' (lowercase) if yes, enter any other letter if no: ");
    char copyTestVerify = scan.next().charAt(0);
    if(copyTestVerify == 'y'){
        CopyConstructorTest ct = new CopyConstructorTest();
        ct.CopyTest();
    }

    scan.close();
}

}

以下是我的 CopyConstructorTest class，它测试复制构造函数是否会更改原始对象的值：

public class CopyConstructorTest {
    public void CopyTest(){
        Payroll pay = new Payroll(5);
        pay.setEmployeeIdAt(0, 1001);

        Payroll payCopy = new Payroll(pay);
        System.out.println("Original: " + pay.getEmployeeIdAt(0));
        System.out.println("Copy: " + payCopy.getEmployeeIdAt(0));
        payCopy.setEmployeeIdAt(0, 5000);
        System.out.println("Original after changes: " +     pay.getEmployeeIdAt(0));
        System.out.println("Copy after changes: " + payCopy.getEmployeeIdAt(0));

    }
}

我对自己做错的事情并不乐观。非常感谢任何帮助或指导。

Answer 1

您只是在复制对数组的引用，而不是实际数据。因此，无论何时更改其中一个对象中的数据，都会在两个对象中看到更改，因为它们指向同一个数组。
复制数据的最简单方法可能是使用 System.arraycopy():

public Payroll(Payroll obj) {
    this.employeeId = new int[obj.employeeId.length];
    System.arraycopy(obj.employeeId, 0, this.employeeId, 0, obj.employeeId.length);
    ...

复制构造函数测试不工作 (Java)

Copy Constructor Test not Working (Java)

java

copy

object