java - Java 中的变量作用域和可见性

Question

我正在做一名假期和假期司机。假期文件将作为我的蓝图，驱动程序将是创建假期实例的程序的交互部分。我一切都工作得很好，但是当我添加一个值时，我在假期驱动程序类第 47 行的打印语句中缺少了一个值，我破坏了程序。

我需要调用 numSales 的值，我认为该值是在第 42 行中声明的。当我在第 47 行的开头和输出中间输入 numSales 时，如图所示，我在下面看到一条红线，并且 Eclipse 告诉我“numSales 无法解析为变量”。我需要做什么才能让 numSales 的值在假期驱动第 47 行的 print 语句中主动输出？

这是假期类(class):

    package cbrownmod4;

    import java.text.NumberFormat;

    public class Vacation {

// money formatting
NumberFormat money = NumberFormat.getCurrencyInstance();

// instance variables
private String vacationName;
private int numSold;
private Double priceEach;

// empty constructor
public Vacation() {
}

// partial constructor
public Vacation(String n, int s, double e) {
    vacationName = n;
    numSold = s;
    priceEach = e = 0;
}

// updatSales method
public int updateSales() {
    int updateSales = 0;
    updateSales = updateSales + numSold;
    return updateSales;
}

// totalValue method
public double totalValue() {
    double totalValue = 0;
    totalValue = totalValue + (numSold * priceEach);
    return totalValue;
}

// toString method
public String toString() {
    return vacationName + " has been sold " + numSold + " times for " + money.format(priceEach) + 
            " each for a total value of " + money.format(numSold*priceEach);
}

// getters and setters
public String getVacationName() {
    return vacationName;
}

public void setVacationName(String vacationName) {
    this.vacationName = vacationName;
}

public int getNumSold() {
    return numSold;
}

public void setNumSold(int numSold) {
    this.numSold = numSold;
}

public Double getPriceEach() {
    return priceEach;
}

public void setPriceEach(Double priceEach) {
    this.priceEach = priceEach;
}

}

这是度假司机:

    package cbrownmod4;

   import java.text.NumberFormat;
   import java.util.Scanner;

   public class VacationDriver {

public static void main(String[] args) {

    Scanner input = new Scanner(System.in);
    NumberFormat money = NumberFormat.getCurrencyInstance();

    // ask for the number of vacations and read it into numVacations
    System.out.println("How many vacations are there?:");
        int numVacations = input.nextInt();

    // ask for the number of sales people and read it into numPeople
    System.out.println("How many sales people are there?: ");
        int numPeople = input.nextInt();

    // beginning of loop for number of vacations
    for(int i = 0; i < numVacations; i++) { 

        //ask for the name and price and read these into variables
        System.out.println("What is the name of vacation #" + Integer.toString(i+1) + "?:");
            String name = input.next();

        System.out.println("How much does " + name + " cost?: ");
            Double price = input.nextDouble();

        // create a Vacation instance using the data obtained above
        Vacation vacay = new Vacation (name, i, price); 

        // loop through the sales people    
        for(int j = 0; j < numPeople; j++) {

            // ask for the sales for the current vacation and read it in
            System.out.println("What are the sales for the current vacation by person #" + Integer.toString(j+1) + "?:");
                int numSales = input.nextInt(); // line 42

            // call updateSales with this number
            numSales = vacay.updateSales(); 

        } //end of inner loop
        //where line 47 begins  
        //print out this vacation info
        System.out.println(numSales + " trips to " + name + " were sold for " + money.format(price) + " for a total of " + money.format(numSales * price));

    } //end of outer for loop

} //end of main

}

由于要求提供此处不起作用的代码部分的片段，这证明了我的问题:

    //print out this vacation info
        System.out.println(numSales + " trips to " + name + " were 
    sold for " + money.format(price) + " for a total of " + 
    money.format(numSales * price));

注意:如果您取出假期驱动程序代码片段中的 numSales，程序会正确执行，但不会有正确的输出，因为它缺少必要的输出变量

清晰简洁的问题是 - 为什么当我像简短片段中所示使用 numSales 时它不起作用。我的问题又是 Eclipse 说“numSales 无法解析为变量。”

Answer 1

问题在于您在 for {} block 内声明了 numSales。

您需要在 for block 之前声明它:

    int numSales = 0;  // set initial value in case numPeople is 0 and the loop never runs

    // loop through the sales people    
    for(int j = 0; j < numPeople; j++) {

        // ask for the sales for the current vacation and read it in
        System.out.println("What are the sales for the current vacation by person #" + Integer.toString(j+1) + "?:");
        numSales = input.nextInt();  // line 42; this value is never used?  it is overwritten below

        // overwrite the never-used value from line 42?? 
        numSales = vacay.updateSales(); 

    } //end of inner loop

    // now numSales is still visible, because it was declared on this same 'level' and not in an inner block
    System.out.println("numSales: " + numSales);

第 42 行中设置的值从未使用过，并在第 45 行中被覆盖，因此您不妨调用 input.nextInt(); 而不将该值设置为 numSales。