Java noobie：for循环调用方法不起作用

Posted 2021-05-06

我知道这应该是基本的，但我无法弄清楚（我还是初学者）。我想要的代码是： / *用户输入3个国家的人口和土地面积。 *计算每个国家的人口密度（即人口/土地面积）， *输出所有3个国家的平均人口密度。 * /

由于某种原因，我的For循环调用方法不起作用。它只调用第一种国家方法（美国，加拿大或墨西哥），然后停止。任何帮助非常感谢！这是我的代码： （链接）jdoodle.com/a/oQY

（码）

import java.util.*;
public class Q8_PopAndLandArea2 {
    static String pplPerMile = " person/s per square mile.
";
    static Scanner x = new Scanner(System.in);
    public static void main (String[] args) {
        Q8_PopAndLandArea2 obj = new Q8_PopAndLandArea2();
        obj.CountryCaller();
}


String userSelected;
public String CountryCaller() {
    int i;
    for (i=0; i<3; i++) {
        System.out.println("Enter one of these 3 countries: 
1.USA 
2.Canada 
3.Mexico");
        userSelected = x.next();
        if (userSelected.equalsIgnoreCase("USA")) {
            return "The population density of the USA is " + USA();
        } else if (userSelected.equalsIgnoreCase("Canada")){
            return "The population density of the Canada is " + Canada();
        } else if (userSelected.equalsIgnoreCase("Mexico")){
            return "The population density of the Mexico is " + Mexico();
        } else {
            System.out.println("I don't understand. Care to try again?");
            String tryAgain2 = x.next();
            if (tryAgain2.equalsIgnoreCase("N")) {
                System.out.println("Goodbye!");
            } else {
                CountryCaller();
            }               
        }       
    } return "The average is"+ popDensity();    
}


    public int USA() {
        int calcUSA2 = 0;
        String calcUSAstr = "The population density of the USA is ";
        System.out.println("Enter the approximate population: ");
        int popUSA = x.nextInt();
        System.out.println("Enter the land area of the country (in sq miles): ");
        int areaUSA = x.nextInt();
         calcUSA2 = popUSA/areaUSA;
        System.out.println(calcUSAstr + calcUSA2 + pplPerMile + calcUSA2);
        return  calcUSA2;
        } 


    public int Canada() {
        int calcCanada2 = 0;
        String calcCanadaStr = "The population density of Canada is ";
        System.out.println("Enter the approximate population: ");
        int popCanada = x.nextInt();
        System.out.println("Enter the land area of the country (in sq miles): ");
        int areaCanada = x.nextInt();
        int calcCanada = popCanada/areaCanada;
        System.out.println(calcCanadaStr + calcCanada + pplPerMile);
        return calcCanada2;
    }

    public int Mexico() {
        int calcMexicio2 = 0;
        String calcMexiciostr = "The population density of Mexico is ";
        System.out.println("Enter the approximate population: ");
        int popMexico = x.nextInt();
        System.out.println("Enter the land area of the country (in sq miles): ");
        int areaMexico = x.nextInt();
        int calcMexicio = popMexico/areaMexico;
        System.out.println(calcMexicioStr + calcMexicio + pplPerMile);
        return calcMexicio2;
    }

    public int popDensity() {
        int calcAve = USA()  + Canada() + Mexico() / 3;
        System.out.println("The average is " + calcAve);
        return calcAve;
    }

} 

答案

你有几个错误，我认为你没有完全理解OO编程（或只是Java结构）。

1. 首先，当你在循环中返回时，它将结束。就像休息一样。您应该删除循环内的返回参数。
2. 当你返回一个字符串时，你必须对它做一些事情。该方法返回平均值，但不会将其弹出给用户。你在popDensity（）中的位置。
3. PopDensity（）方法又调用了这些国家/地区的方法，因此您必须再次使用params。您应该存储coutries方法返回的结果，以避免再次调用它们。
4. 你再次调用CountryCaller（），递归，所以你再次启动循环...我想你想停止循环不再启动它。
5. 在墨西哥和加拿大的方法中，你没有返回真正的结果。
6. 你正在为用户做一些重复，关心这一点。

这是正确的代码，请与您的代码进行比较，请与您的代码进行比较并尝试理解所有代码，如果有什么是您没有得到的，请在评论中询问:)。

码：

package massilia.export.promotion;

import java.util.Scanner;

public class Q8_PopAndLandArea2 {
    static String pplPerMile = " person/s per square mile.
";
    static Scanner x = new Scanner(System.in);

    public static void main(String[] args) {
        Q8_PopAndLandArea2 obj = new Q8_PopAndLandArea2();
        obj.countryCaller();
    }

    String userSelected;

    void countryCaller() {
        int i;
        int totalDensity = 0;
        for (i = 0; i < 3; i++) {
            int actualDensity = 0;
            System.out.println("Enter one of these 3 countries: 
1.USA 
2.Canada 
3.Mexico");
            userSelected = x.next();
            if (userSelected.equalsIgnoreCase("USA")) {
                actualDensity = USA();
            } else if (userSelected.equalsIgnoreCase("Canada")) {
                actualDensity = Canada();
            } else if (userSelected.equalsIgnoreCase("Mexico")) {
                actualDensity = Mexico();
            } else {
                System.out.println("I don't understand. Care to try again?");
                String tryAgain2 = x.next();
                if (tryAgain2.equalsIgnoreCase("N")) {
                    return;
                }
            }
            totalDensity += actualDensity;
        }
        System.out.println("The average is" + totalDensity / 3);
    }

    public int USA() {
        System.out.println("Enter the approximate population: ");
        int popUSA = x.nextInt();
        System.out.println("Enter the land area of the country (in sq miles): ");
        int areaUSA = x.nextInt();
        int calcUSA = popUSA / areaUSA;
        System.out.println("The population density of the USA is " + calcUSA + pplPerMile + calcUSA);
        return calcUSA;
    }

    public int Canada() {
        System.out.println("Enter the approximate population: ");
        int popCanada = x.nextInt();
        System.out.println("Enter the land area of the country (in sq miles): ");
        int areaCanada = x.nextInt();
        int calcCanada = popCanada / areaCanada;
        System.out.println("The population density of Canada is" + calcCanada + pplPerMile);
        return calcCanada;
    }

    public int Mexico() {
        System.out.println("Enter the approximate population: ");
        int popMexico = x.nextInt();
        System.out.println("Enter the land area of the country (in sq miles): ");
        int areaMexico = x.nextInt();
        int calcMexicio = popMexico / areaMexico;
        System.out.println("The population density of Mexico is " + calcMexicio + pplPerMile);
        return calcMexicio;
    }

}

另一答案

您的问题出在“return”关键字中。当你调用return时，你的程序会退出当前的方法（即使它在循环中）。所以你实际上在做的是输入USA的if条件并通过返回一个字符串退出方法。我的建议是：当你知道已经获得必要的值/条件并且想要退出方法时，只在循环内使用“return”。

另一答案

退货声明正在做你的停止。

您应该更改毛皮婴儿印花内的退货声明。

试试这个：

public String CountryCaller() {
    int i;
    int totalDensity = 0;
    for (i=0; i<3; i++) {
        System.out.println("Enter one of these 3 countries: 
1.USA 
2.Canada 
3.Mexico");
        userSelected = x.next();
        if (userSelected.equalsIgnoreCase("USA")) {
            totalDensity += USA();
            System.out.println("The population density of the USA is " + USA());
        } else if (userSelected.equalsIgnoreCase("Canada")){
            totalDensity += Canada();
            System.out.println("The population density of the Canada is " + Canada());
        } else if (userSelected.equalsIgnoreCase("Mexico")){
            totalDensity += Mexico();
            System.out.println("The population density of the Mexico is " + Mexico());
        } else {
            System.out.println("I don't understand. Care to try again?");
            String tryAgain2 = x.next();
            if (tryAgain2.equalsIgnoreCase("N")) {
                System.out.println("Goodbye!");
            } else {
                CountryCaller();
            }               
        }       
    } return "The average is"+ totalDensity/3;  
}

另一答案

return语句终止函数的执行并将控制返回给调用函数。

看看你的CountryCaller方法 - 你从你的循环中跳出它（returning） - 这将导致循环终止