我的数组超出范围，但它的设置高于 im 从它调用的值

Question

public class hw16 {
   
    public static void main(String[] args) {
        MonthlyRecord MonthRecord = new MonthlyRecord("January", 31);
        MonthRecord.Transaction(5, 600);
    
    }
    
}
    
public class MonthlyRecord {
    private String month;
    private int day;
    private double[] moneyDaily = new double[day];
    public MonthlyRecord() {
        this.month = "January";
        this.day = 0;
        this.moneyDaily[1] = 0;
    }
    public MonthlyRecord(String name, int day) {
       
        System.out.println("Hello");
        this.setMonthlyRecord(name);
        this.setDays(day);
    }
    public void setDays(int temp) {
        this.day = temp;
    }
    public void setMonthlyRecord(String temp) {
        this.month = temp;
    }
    public int setDays() {
        return this.day;
    }
    public String getMonthlyRecord() {
        return this.month;
    }
    public void Transaction(int daynum, int amount) {
        System.out.println("You've made $" + amount + " on the " + daynum + "th day");
        System.out.println(moneyDaily[daynum]);
    }
}

我收到错误：

您好，您在第 600 天赚了 5 美元 线程"main"中的异常java.lang.ArrayIndexOutOfBoundsException： 5 athwk16jzuramski1.MonthlyRecord.Transaction(MonthlyRecord.java：40( athwk16jzuramski1.Hwk16jzuramski1.main(Hwk16jzuramski1.java：11( Java结果：1

所以它说我的数组超出了我不明白的范围？如果我将 day 设置为数组的长度，为什么当数组长度应该是 31 时，它说它在 int 5 时越界？

Answer 1

我认为

正在发生的事情是，在类的顶部，您正在构造函数之外monthDaily[]实例化数组。由于 int 变量day仅在构造函数中初始化，因此您还应该在构造函数中实例化monthDaily[]变量。因此，代码的开头部分如下所示：

public class MonthlyRecord {
    private String month;
    private int day;
    private double[] moneyDaily;
    public MonthlyRecord() {
        this.month = "January";
        this.day = 0;
        this.moneyDaily = new double[1]; //variable is instantiated to have one slot as default
    }
    public MonthlyRecord(String name, int day) {
        System.out.println("Hello");
        this.setMonthlyRecord(name);
        this.setDays(day);
        moneyDaily = new double[days]; //array gets instantiated with number of slots equal to the value of the day variable
    }

Answer 2

您需要在调用构造函数时设置数组的大小：

public MonthlyRecord(String name, int day) {
    moneyDaily = new double[day];
    System.out.println("Hello");
    this.setMonthlyRecord(name);
    this.setDays(day);
}

否则，数组moneyDaily的大小将与最初当天的大小相同，即 0。