java - 使用 While 与 If 防止数组中出现重复的整数 ID 号

Question

我创建了一个程序，可以添加、查看、更新和删除学生数组。为了防止用户输入重复的ID号，我使用了以下代码:

public static void addStud() {
    int numID, year;
    String userName, course;

    int addMore;

    do {

        System.out.println("1. Enter Student ID: ");
        numID = sc.nextInt();
        sc.nextLine();
        for(int x = 0; x < count; x++){
             if(numID == stud[x].getNumID()) {
                System.out.println("The Student ID: " +numID+ " already exist.\nEnter New Student ID: ");
                numID = sc.nextInt();
                sc.nextLine();
                x = 0;
            }
        }
        System.out.println("2. Enter Student Name");
        userName = sc.nextLine();
        System.out.println("3. Enter Student Course");
        course = sc.nextLine();
        System.out.println("4. Enter Student Year");
        year = sc.nextInt();

        stud[count] = new Student(numID, year, userName, course);
        ++count;

        System.out.println("To add another Student Record Press 1 [any] number to stop");
        addMore = sc.nextInt();
        sc.nextLine();
    } while (addMore == 1);

}

但是，由于某种原因(一次又一次)输入 ID 号 1-5，程序会接受重复的 ID [1]。我所做的是将 if(numID == Stud[x].getNumID()) 更改为 while(numID == Stud[x].getNumID()) 它解决了我的问题。

我不擅长调试，我只是想知道出了什么问题？为什么它与 while 一起工作而不与 if 一起工作？

贝娄是我的完整程序。尝试输入 ID 号 1 - 5 并重复，如果使用 IF 语句，将接受重复的 ID 号。

import java.util.Scanner;

公共(public)类StudentArray {

static Scanner sc = new Scanner(System.in);
static Student[] stud = new Student[100];
static int count = 0;

public static void main(String[] args) {

    while (true) {
        int select;
        System.out.println("1. Add Student Record");
        System.out.println("2. View Student Record");
        System.out.println("3. Update Student Record");
        System.out.println("4. Delete Student Record");
        System.out.println("0. Exit");
        select = sc.nextInt();

        switch (select) {
            case 1:
                addStud();
                break;
            case 2:
                viewStud();
                break;
            case 3:
                updateStud();
                break;
            case 4:
                deleteStud();
                break;
            case 0:
                return;
            default:
                System.out.println("Invalid Option");
        }
    }

}

public static void addStud() {
    int numID, year;
    String userName, course;

    int addMore;

    do {

        System.out.println("1. Enter Student ID: ");
        numID = sc.nextInt();
        sc.nextLine();
        for(int x = 0; x < count; x++){
             if(numID == stud[x].getNumID()) { // change if to while prevents duplicate
                System.out.println("The Student ID: " +numID+ " already exist.\nEnter New Student ID: ");
                numID = sc.nextInt();
                sc.nextLine();
                x = 0;
            }
        }
        System.out.println("2. Enter Student Name");
        userName = sc.nextLine();
        System.out.println("3. Enter Student Course");
        course = sc.nextLine();
        System.out.println("4. Enter Student Year");
        year = sc.nextInt();

        stud[count] = new Student(numID, year, userName, course);
        ++count;

        System.out.println("To add another Student Record Press 1 [any] number to stop");
        addMore = sc.nextInt();
        sc.nextLine();
    } while (addMore == 1);

}

public static void viewStud() {

    while(true) {
        int select;

        System.out.println("1. View Record by ID number ");
        System.out.println("2. View Record by Course ");
        System.out.println("3. View Record by Course and Year ");
        System.out.println("4. View All ");
        System.out.println("0. Return Main Menu ");
        select = sc.nextInt();
        sc.nextLine();

        switch (select) {

            case 1:
                int view1;
                System.out.println("Please enter Student ID Number: ");
                view1 = sc.nextInt();
                viewArray(view1);
                break;
            case 2:
                String view2;
                System.out.println("Please enter Student Course: ");
                view2 = sc.nextLine();
                viewArray(view2);
                break;
            case 3:
                String view3;
                int view4;
                System.out.println("Please enter Student Course: ");
                view3 = sc.nextLine();
                System.out.println("Please enter Student Year: ");
                view4 = sc.nextInt();
                viewArray(view3, view4);
                break;
            case 4:
                viewArray();
                break;
            case 0:
                return;
            default:
                System.out.println("Invalid Option");

        }

    }
}
public static void viewArray(){
    System.out.println("Student ID\tStudent Name\tStudent Course\tStudent Year");
    for (Student student : stud) {
        if (student != null) {
            System.out.println(student.getNumID()+"\t\t\t\t"+student.getUserName()+ "\t\t\t\t"+student.getCourse()+"\t\t\t\t"+ student.getYear());
        }
    }
}
public static void viewArray(int key){
    boolean isExist = false;
    int temp = 0;

    for(int x = 0; x < count; ++x){
        if(key == stud[x].getNumID()){
            temp = x;
            isExist = true;
            break;
        }

    }

    if(isExist){
        System.out.println("1. Student ID: " + stud[temp].getNumID());
        System.out.println("2. Student Name: " + stud[temp].getUserName());
        System.out.println("3. Student Course: " + stud[temp].getCourse());
        System.out.println("4. Student Year: " + stud[temp].getYear() +"\n");
    }
    else
        System.out.println("The Student ID: " +key+ " is invalid");

}
public static void viewArray(String key){
    boolean isExist = false;

    for(int x = 0; x < count; ++x){
        if(key.equalsIgnoreCase(stud[x].getCourse())){
            System.out.println("1. Student ID: " + stud[x].getNumID());
            System.out.println("2. Student Name: " + stud[x].getUserName());
            System.out.println("3. Student Course: " + stud[x].getCourse());
            System.out.println("4. Student Year: " + stud[x].getYear() +"\n");
            isExist = true;
        }

    }

    if(isExist == false){
        System.out.println("The Student Course: " +key+ " is invalid");
    }

}
public static void viewArray(String course, int year){
    boolean isExist = false;

    for(int x = 0; x < count; ++x){
        if(course.equalsIgnoreCase(stud[x].getCourse()) && year == stud[x].getYear()){
            System.out.println("1. Student ID: " + stud[x].getNumID());
            System.out.println("2. Student Name: " + stud[x].getUserName());
            System.out.println("3. Student Course: " + stud[x].getCourse());
            System.out.println("4. Student Year: " + stud[x].getYear() +"\n");
            isExist = true;
        }

    }

    if(isExist == false){
        System.out.println("The Student Course: " +course+ " and Year: "+year+" is invalid");
    }

}
public static void updateStud(){
    int numID, temp = 0;
    boolean flag = false;
    System.out.println("Student ID\tStudent Name\tStudent Course\tStudent Year");
    for (Student student : stud) {
        if (student != null) {
            System.out.println(student.getNumID()+"\t\t\t\t"+student.getUserName()+ "\t\t\t\t"+student.getCourse()+"\t\t\t\t"+ student.getYear());
        }
    }

    System.out.println("Enter Student ID to update: ");
    numID = sc.nextInt();
    sc.nextLine();

    for(int x = 0; x < count && flag == false; x++){
        if (numID == stud[x].getNumID()){
            temp = x;
            flag = true;
        }
    }
    if(flag) {
        System.out.println("Enter Student Name: ");
        stud[temp].setUserName(sc.nextLine());
        System.out.println("Enter Student Course");
        stud[temp].setCourse(sc.nextLine());
        System.out.println("Enter Student Year");
        stud[temp].setYear(sc.nextInt());
        System.out.println("The Student ID: " + numID + " record has been updated");
    }
    else
        System.out.println("The Student ID: " +numID+ " is invalid");
}
public static void deleteStud() {
    int numID, temp = 0;
    boolean flag = false;
    if (count > 0){ // check if array is empty
        System.out.println("Student ID\tStudent Name\tStudent Course\tStudent Year");
        for (Student student : stud) {
            if (student != null) {
                System.out.println(student.getNumID()+"\t\t\t\t"+student.getUserName()+ "\t\t\t\t"+student.getCourse()+"\t\t\t\t"+ student.getYear());
            }
        }

        System.out.println("Enter Student ID to delete: ");
        numID = sc.nextInt();
        sc.nextLine();

        for(int x = 0; x < count && flag == false; x++){
            if (numID == stud[x].getNumID()){
                temp = x; // get the index
                flag = true; // stops the loop if id is found
            }
        }
        for( ; temp < count -1; temp++){
            stud[temp]=stud[temp+1];
        }
            stud[count-1] = null;
            --count;
    }
    else {
        System.out.println("Cannot delete [Array is Empty]");
    }

}

}

我的学生类(class):

public class Student {

private int numID, year;
private String userName, course;

public Student(int numID, int year, String userName, String course) {

    this.numID = numID;
    this.year = year;
    this.userName = userName;
    this.course = course;

}


public int getNumID() {
    return numID;
}

public void setNumID(int numID) {
    this.numID = numID;
}

public int getYear() {
    return year;
}

public void setYear(int year) {
    this.year = year;
}

public String getUserName() {
    return userName;
}

public void setUserName(String userName) {
    this.userName = userName;
}

public String getCourse() {
    return course;
}

public void setCourse(String course) {
    this.course = course;
}

}

Answer 1

如果您使用 for 解决问题，则不需要 while 循环环形。假设您输入用户 ID 1 至 5，然后再次输入 1。 for 的第一次迭代循环会发现1 == stud[0].getNumID() ，所以你会要求提供新的学生证。然后你会得到一个新的输入并重置 x至0 。

现在，x将增加到1通过x++循环的一部分，因此如果您输入 1同样，循环永远不会比较 1至stud[0].getNumID() ，自 x==1 。因此重复的输入将被接受。

可以通过将 x 重置为 -1 来修复此问题:

for(int x = 0; x < count; x++){
     if(numID == stud[x].getNumID()) {
        System.out.println("The Student ID: " +numID+ " already exist.\nEnter New Student ID: ");
        numID = sc.nextInt();
        sc.nextLine();
        x = -1;
    }
}

当然，有更好的方法来检查重复的 ID(这会更具可读性和更高效)。使用HashSet<Integer>存储所有使用过的 ID，并使用 Set检查新的 ID 是否已被使用。

例如:

System.out.println("1. Enter Student ID: ");
numID = sc.nextInt();
sc.nextLine();
Set<Integer> ids = new HashSet<>();
while (!ids.add(numID)) { // add returns false if numID is already in the Set
    System.out.println("The Student ID: " +numID+ " already exist.\nEnter New Student ID: ");
    numID = sc.nextInt();
    sc.nextLine();
}
...