我试图抓住我的教授在编程任务中向我提出的一些评论。作业是编写一个调用另一个类的程序。该计划旨在让2位投资者拥有不同的期初余额，并显示他们在15个月后会收取多少利息。 所以每个人都很清楚，我的作业已经被评分，所以任何批评或改变都不会帮助我完成我的作业，但只是为了帮助我理解我做错了什么以及如何为将来的作业修复它。我的成绩获得了90％的成绩。
以下评论是关于我的任务：
＆＃34;你的setInterest方法是一个类方法而不是一个实例 方法。此外，您的电话应该是
IWUnit4Ch13Investor.setInterest(rate);"
我并没有完全遵循我的错误或如何解决它。有人可以告诉我我做错了什么，并可能解释为什么它错了，以便我可以纠正我未来任务的习惯并掌握如何正确编写代码？
// IWUnit4Ch13.java
import java.util.*;
import java.util.Scanner;
// main class
public class IWUnit4Ch13 {
public static void main(String[] args) {
// Declares local variables
double rate, INTEREST_RATE;
// Instantiate investor1 & investor2 objects using a two parameter constructor
IWUnit4Ch13Investor investor1 = new IWUnit4Ch13Investor(1001, 2000);
IWUnit4Ch13Investor investor2 = new IWUnit4Ch13Investor(2001, 4000);
Scanner input = new Scanner(System.in);
// Receives the APR by the user
System.out.print("Please enter the APR in the form of .05 for 5%: ");
rate = input.nextDouble();
// Moves the decimal 2 places to the left (used later)
INTEREST_RATE = rate * 100;
// Sets the interest rate using a class variable
investor1.setInterestRate(rate);
investor2.setInterestRate(rate);
// Prints the header for the table
System.out.printf("Monthly balances for one year with %.2f annual interest:\n\n", rate);
System.out.println("Month Account # Balance Account # Balance");
System.out.println("----- --------- ------- --------- -------");
// Loop that prints each month on the table
// Uses class variables to add interest and get balance for display
for (int i = 1; i <= 15; i++) {
investor1.addInterest();
investor2.addInterest();
System.out.printf("%5d %9d %9.2f %9d %9.2f\n", i, investor1.getACCOUNT_NUMBER(), investor1.getBalance(), investor2.getACCOUNT_NUMBER(), investor2.getBalance());
}
// Prints the calculated interest earned by both investors
System.out.printf("\nInvestor1 earned : %.2f", investor1.getInterest());
System.out.printf(" interest in 15 months at %.2f", INTEREST_RATE);
System.out.print("%\n");
System.out.printf("Investor2 earned : %.2f", investor2.getInterest());
System.out.printf(" interest in 15 months at %.2f", INTEREST_RATE);
System.out.print("%\n\n");
} // end of internal main
} // end of main class
// Creates IWUnit4Ch13Investor.java which is used in IWUnit4Ch13.java
public class IWUnit4Ch13Investor extends IWUnit4Ch13 {
// All variables are declared private
private static double interestRate; // class variable
private final int ACCOUNT_NUMBER; // declare constant ACCOUNT_NUMBER
private double balance; // instance variable called balance
private double initialBalance; // to hold the initial balance
private double interest; // to count how much interest was made
private double INTEREST_RATE; // used to convert the float interestRate to int
// Two parameter constructor to initialize account number and balance
public IWUnit4Ch13Investor(int acctNum, double initialBalance) {
this.initialBalance = initialBalance;
this.ACCOUNT_NUMBER = acctNum;
balance = initialBalance;
}
// To return the balance to parent
public double getBalance() {
return balance;
}
// Class method to set the annual interest rate
public void setInterestRate(double rate) {
interestRate = rate;
}
// Method to add interest based on balance * interestRate / 12
public void addInterest() {
balance += balance * interestRate/12.0;
}
// To get account number in parent
public int getACCOUNT_NUMBER() {
return ACCOUNT_NUMBER;
}
// Used to get the amount of interested earned during 15 months
public double getInterest() {
interest = balance - initialBalance;
return interest;
}
} // end of class
提前感谢大家的帮助。
答案 0 :(得分：0)
首先，请不要混淆条款。变量与方法：
不同// A class
public class Dog {
// Member variable
private String name;
// Method
public void bark() {
// Variable
String textToBark = "Wuff";
// Call to a method
System.out.println(textToBark);
}
}
其他地方：
// Creating a new instance of class "Dog"
// and saving a reference to it inside a variable
Dog bello = new Dog("Bello");
您的老师说他希望setInterest成为类方法而不是实例方法。他想说的是它应该被声明为static，因此不是属于类的实例，而是属于类本身。以下是static对方法的含义：Java: when to use static methods
所以方法应如下所示：
// Method to set the annual interest rate
public static void setInterestRate(double rate) {
IWUnit4Ch13Investor.interestRate = rate;
}
其中interestRate还需要声明static（你已经做过）：
// To count how much interest was made
private static double interestRate;
要指示static变量的访问权限，应该先添加类名，所以请改写如下：
// Method to add interest based on balance * interestRate / 12
public void addInterest() {
balance += balance * IWUnit4Ch13Investor.interestRate / 12.0;
}
同样适用于调用static方法，而不是
investor1.setInterestRate(rate);
investor2.setInterestRate(rate);
这样做
IWUnit4Ch13Investor.setInterestRate(rate);
由于static的性质，您还需要设置一次，因为static变量共享 所有实例类。
答案 1 :(得分：0)
所有类的实例静态变量共享。
如果您希望所有投资者被迫分享相同的利率，请使用静态变量。
仅影响静态变量的方法应该（https://www.ietf.org/rfc/rfc2119.txt）声明为static：
public static void setInterestRate(double rate) {
interestRate = rate;
}
private static double interestRate;
然后，您可以通过指定类名而不是实例变量来调用静态方法。请注意，您甚至可以在拥有该类的任何实例之前调用静态方法，并且只需要调用一次：
Investor.setInterestRate(0.05);
Investor i1 = new Investor();
Investor i2 = new Investor();
double r1 = i1.calculateBalance();
double r2 = i2.calculateBalance();
如果您希望每个投资者能够拥有不同的利率，不使用静态变量，请使用成员变量和方法：
public void setInterestRate(double rate) {
interestRate = rate;
}
private double interestRate;
然后通过指定实例变量来调用该方法：
Investor i1 = new Investor();
Investor i2 = new Investor();
i1.setInterestRate(0.04);
i2.setInterestRate(0.06);
double r1 = i1.calculateBalance();
double r2 = i2.calculateBalance();
此外，由于输出错误，这一切都不重要：
double monthly_rate = pow(10.0, log10(1.0 + yearly_rate) / 12.0) - 1.0;
系统不会失败