Java Scanner 未接受有效输入或与整数解析相关的其他问题

Question

请引用以下与数学表格练习相关的代码。对于除 15 * 9 之外的所有数字乘法，它都可以正常工作，但 15 * 9 存在问题。

打包 com.test;

import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.File;
import java.io.FileReader;
import java.io.FileWriter;
import java.io.IOException;
import java.util.ArrayList;
import java.util.List;
import java.util.Random;
import java.util.Scanner;

public class Entry {

    static Scanner scanIn;
    static Random random;
    static List<String> list;
    static List<String> mistakeList;
    static File mistakeFile;

    static {
        scanIn = new Scanner(System.in);
        random = new Random();
        list = new ArrayList<String>();

    }

    /**
     * Main function to call the private methods.
     */
    public static void main(String[] args) throws IOException {
        mistakeFile = new File(args[0]);
        if(!mistakeFile.exists())
        {
            try {
                mistakeFile.createNewFile() ;
            } catch (IOException e) {           
                e.printStackTrace();
            }
        }
        mistakeList = getFileContent(mistakeFile);
        System.out.println("Please enter max limit .....");
        Integer maxLimit = Integer.parseInt(scanIn.nextLine());

        System.out.println("Please enter min limit .....");
        Integer minLimit = Integer.parseInt(scanIn.nextLine());

        System.out.println("Enter the number if iterations ..... ");
        Integer iterations = Integer.parseInt(scanIn.nextLine());

        executeTable(maxLimit, minLimit, iterations);
        scanIn.close();
        writeIntoFile();
    }
    /**
     * 
     * Multiplication logic which outputs two numbers, take input from user and 
     * matches it with result of multiplicaion.
     */
    private static void executeTable(Integer maxLimit, Integer minLimit,
            Integer iterations) {
        int skipCount = 0;
        for (int iTemp = 0; iTemp < iterations; iTemp++) {
            try
            {
            Integer num1 = getNextRandomInt(minLimit, maxLimit);
            Integer num2 = getNextRandomInt(2, 9);
            if (list.contains(num1 + "_X_" + num2) && skipCount < 10) {
                iTemp = iTemp - 1;
                skipCount++;
                continue;
            }
            skipCount = 0;
            System.out.println(num1 + " X " + num2 + " = ");
            Integer usrAnswer = Integer.parseInt(scanIn.nextLine());
            Integer answer = num1 * num2;
            if (usrAnswer != answer) {
                System.out.println("WRONG!! The correct answer is  " + num1
                        + " X " + num2 + " = " + answer);
                if(!mistakeList.contains(num1 + "_X_" + num2))
                {
                    mistakeList.add(num1 + "_X_" + num2);
                }
            } else {
                if(!list.contains(num1 + "_X_" + num2))
                {
                    list.add(num1 + "_X_" + num2);
                }
            }
            }
            catch(Exception e)
            {
                e.printStackTrace();
            }

        }

    }

    /**
     * Gets next random number
     * 
     */
    private static Integer getNextRandomInt(Integer minLimit, Integer maxLimit) {
        Integer number = random.nextInt();
        if(maxLimit < minLimit)
        {
            maxLimit = maxLimit + minLimit ;
            minLimit = maxLimit - minLimit ;
            maxLimit = maxLimit - minLimit ;
        }

        number = (number % (maxLimit - minLimit + 1));
        if (number < 0) {
            number = -number;
        }
        number += minLimit;
        return number;
    }

    /**
     * Records commom mistake in file
     */
    private static void writeIntoFile() throws IOException 
    {
        String fileContent = "" ;
        for(String data : mistakeList)
        {
            fileContent = fileContent +data+ "\r\n" ;
        }
        try (FileWriter fw = new FileWriter(mistakeFile.getAbsoluteFile(), false);
                BufferedWriter bw = new BufferedWriter(fw)) {
            bw.write(fileContent);
            bw.close();
        }

    }

    /**
     *Returns file content.
     *
     */
    private static List<String> getFileContent(File file) {
        List<String> lst = new ArrayList<String>();
        try (BufferedReader br = new BufferedReader(new FileReader(file))) {

            String sCurrentLine;

            while ((sCurrentLine = br.readLine()) != null) {
                lst.add(sCurrentLine);
            }

        } catch (IOException e) {
            e.printStackTrace();
        }
        return lst;
    }
}

The console output is as follows .

Please enter max limit .....
15

Please enter min limit .....
15

Enter the number if iterations ..... 
100

15 X 2 = 
30

15 X 8 = 
120

15 X 9 = 
135

WRONG!! The correct answer is  15 X 9 = 135

虽然提供了 135 正确的输入，但上面的程序显示了错误。不知道 15 * 9 有什么问题

添加更多描述。

上面的程序在一定范围内生成两个随机数，用户必须输入正确的答案。示例。上面的程序显示两个数字，如“15 X 8 =”，用户必须输入 120。上面的程序有效适用于除“15 X 9 =”之外的所有其他组合。用户输入 135，这是正确的答案，但程序显示它是错误的答案。

Answer 1

问题出在这里:

if (usrAnswer != answer) {

您正在比较Integer对象，必须像这样完成:

if (usrAnswer.equals(answer)) {

与!=比较检查对象实例是否相同。它适用于较小数量的原因是实例缓存达到一定限制。

在这种情况下，更好的解决方案是使用原始 int 类型:

int usrAnswer = Integer.parseInt(scanIn.nextLine());
int answer = num1 * num2;
if (usrAnswer != answer) {