java - 如何修复 Java 中的不匹配异常？

Question

我不断收到名为“java.util.Mismatchexpection: null”的错误。我不知道如何解决这个问题。它指向 grade = fileInput.nextInt(); 行。

这是我正在使用的类:

import java.util.Scanner;

import java.io.*;

public class Students
{
    public static void printAllStudents(Student[] students){
        for(int i=0;i<students.length;i++)
        {
            System.out.println(students[i]);
        }
    }

    public static void printAllStudentsByFirstName(Student[] students,String firstName){
        String name=firstName.split(" ")[1];
        int len=name.length();
        for (int i=0;i<students.length;i++){
            try{
                if(students[i].getFname().substring(0, len).equalsIgnoreCase(name)){
                    System.out.println(students[i]);
                }
            }
            catch (Exception e) {
                // TODO: handle exception
            }
        }
    }

    public static void printAllStudentsByLastName(Student[] students,String lastName){
        String name=lastName.split(" ")[1];

        int len = name.length();

        for(int i=0;i<students.length;i++){
            try{
                if(students[i].getLname().substring(0, len).equalsIgnoreCase(name)){
                    System.out.println(students[i]);
                }
            }
            catch (Exception e) {
                // TODO: handle exception
            }
        }
    }

    public static void printAllStudentsByGrades(Student[] students,String interval){
        int start=Integer.parseInt(interval.split(" ")[1]);
        int end=Integer.parseInt(interval.split(" ")[2]);
        for(int i=0;i<students.length;i++){
            if(students[i].getGrade()>=start && students[i].getGrade()<=end){
                System.out.println(students[i]);
            }
        }
    }

    public static void sort(Student[] students)
    {
        int n = students.length;
        int k;
        for (int m = n; m >= 0; m--) {
            for (int i = 0; i < n - 1; i++) {
                k = i + 1;
                if (students[i].getGrade() > students[k].getGrade()) 
                {
                    Student temp=new Student();
                    temp = students[k];
                    students[k]=students[i];
                    students[i]=temp;
                }
            }
        }
        printAllStudents(students);
    }

    public static void main (String[] args) throws IOException
    {
        String first_name, last_name, line = "";
        int grade, lines = 0, i;
        char ch;
        Scanner sc = new Scanner(System.in);
        Scanner fileInput = new Scanner(new File("students.txt"));
        //Counting the number of lines in the file.
        while (fileInput.hasNext())
        {
            line = fileInput.next();
            lines++;}

        fileInput = new Scanner(new File("students.txt"));
        //Declaring Student array of size lines.
        Student[] students=new Student[lines];
        i = 0;

        while (fileInput.hasNext())
        {
            first_name = fileInput.nextLine();
            last_name = fileInput.nextLine();
            grade = fileInput.nextInt();

            Student st = new Student(first_name, last_name, grade);
            students[i++] = st;
        }

        System.out.println("Please choose from below operations: ");
        System.out.println("1. printall");
        System.out.println("2. firstname <name>");
        System.out.println("3. lastname <name>");
        System.out.println("4. interval m n");
        System.out.println("5. sort");
        System.out.println("6. end");

        while (true)
        {
            System.out.println("Enter choice:");
            String choice = sc.nextLine();
            if(choice.equals("printall"))
                printAllStudents(students);
            else if(choice.indexOf("firstname")==0)
                printAllStudentsByFirstName(students, choice);
            else if(choice.indexOf("lastname") == 0)
                printAllStudentsByLastName(students, choice);
            else if(choice.indexOf("interval") == 0)
                printAllStudentsByGrades(students, choice);
            else if(choice.equals("sort"))
                sort(students);
            else if(choice.equals("end"))
                break;
        }
    }

}

    public class Student
{
    private String fname, lname;
    private int grade;

    public Student()
    {
        super(); //gives us the ability to override


    }

    public Student (String fname, String lname, int grade)
    {
        this.fname = fname;
        this.lname = lname;
        this.grade = grade;
    }

    public String getFname() {
        return fname;
    }

    public void setFname (String fname) {
        this.fname = fname;
    }

    public String getLname() {
        return lname;
    }

    public void setLname(String lname) {
        this.lname = lname;
    }

    public int getGrade() {
        return grade;
    }

    public void setGrade(int grade) {
        this.grade = grade;
    }

    public String toString()
    {
        return fname + " " + lname + "\t" + grade;
    }
}

任何帮助将不胜感激。

Answer 1

当扫描器遇到意外输入时，会触发MismatchException。在您的情况下，扫描程序在期望 fileInput.nextInt() 输入 int 时拾取 null。

这与 student.txt 的格式以及解析它的方式有关；尝试在 IDE(如果您使用的是 IDE)中逐步调试运行代码，以查明如何修复文本文件解析。