Java NullPointerException 为什么数组为空？

Question

我可以输出学生的详细信息，但总是在我这样做时显示

Exception in thread "main" java.lang.NullPointerException
    at client.Client.main(Client.java:126)

程序崩溃了。

数组为 null ，我不知道为什么，也不知道如何解决这个问题。请帮我理解，问题应该出在这里..

   if (choice == 3) {
        for (int a = 0; a < list.length; a++) { //To Display all current Student Information.
            //   list[i] = new student();
            list[a].DisplayOutput();

    }

无论如何，我的代码来了。

 package client;

 import java.util.Scanner;

 public class Client {
//my infos
public static void AuthorInformation() {

    System.out.print("__________________________________________\n"
            + "Student Name: xxxxxx xxxxxx\n"
            + "Student ID-Number: xxxxxx\n"
            + "Tutorial Timing: Wednesday 9:30 - 12:30 \n"
            + "Tutor Name: Mr xxxxxx\n\n"
            + "__________________________________________\n");
}

这是我的客户类

public static void main(String[] args) {

    AuthorInformation(); //calls the method for my information

    student[] list = new student[20]; //my array
    student student = new student();

    int choice = 0; //variable for the choise of the menu

    Scanner keyboard = new Scanner(System.in); //Scanner class



    do {        //MY menu in the do-while, so that it runs at least once while user enters 7(quit)

        student.DisplayQuestions();

        choice = keyboard.nextInt();   //takes the entered value from the user

        if (choice == 1) {

            System.out.println("Enter the Number of Students you want to store: ");//Enter amount of students to be stored
             int studentNumber = keyboard.nextInt();

            // student.addStudent();
            for (int i = 0; i < studentNumber; i++) { //for loop is till the student number is achieved

                list[i] = new student();

                System.out.println("\nTitle of the student (eg, Mr, Miss, Ms, Mrs etc): ");
                list[i].SetTitle(keyboard.next());

                System.out.println("First name (given name)");
                list[i].SetFirstName(keyboard.next());

                System.out.println("A last name (family name/surname)");
                list[i].SetFamilyName(keyboard.next());

                System.out.println("Student number (ID):");
                list[i].SetStudentID(keyboard.nextInt());

                System.out.println("Enter the Day of birth(1-31): ");
                list[i].SetDay(keyboard.nextInt());

                System.out.println("Enter the Month of birth (1-12): ");
                list[i].SetMonth(keyboard.nextInt());

                System.out.println("Enter The Year of birth: ");
                list[i].SetYear(keyboard.nextInt());

                System.out.println("Students First Assignment Mark (0 - 100): ");
                list[i].SetFirstMark(keyboard.nextInt());

                System.out.println("Students Second Assignment Mark (0 - 100): ");
                list[i].SetSecondMark(keyboard.nextInt());

                System.out.println("Enter the mark of Student weekly practical work (0-10) ");
                list[i].SetWeeklyMarks(keyboard.nextInt());

                System.out.println("Please Enter the Marks for the final Exam(0 - 100): ");
                list[i].SetFinalMark(keyboard.nextInt());
                /* System.out.println("- - - - - - - - - - - -  -");
                System.out.println("Do you want to add another Student? (Yes/No)");

                String a = keyboard.next();

                if (a.equalsIgnoreCase("yes")) {

                } else if (a.equalsIgnoreCase("no")) {
                    i = list.length + 1;
                }*/

            }

        }

        if (choice == 2) {

            int x = 2;
            double AverageNum = 0;
            for (int p = 0; p < x; p++) {   //This is to take the Average OverallMarks of all students

                AverageNum += list[p].OverallMarking();

            }
            System.out.println("Total Average Of Overall Marks is :" + AverageNum / 2);

        }

        if (choice == 3) {

            for (int a = 0; a < list.length; a++) { //To Display all current Student Information.
                //   list[i] = new student();
                list[a].DisplayOutput();

        }
        }
        if (choice == 4) {    //To Display the distribution of grades awarded.

            System.out.println("\nGrade Distribution: \n" + student.GetCounterHD() + " HDs\n" + student.GetCounterD() + " Ds\n" + student.GetCounterC() + " Cs\n" + student.GetCounterP() + " Ps\n" + student.GetCounterN() + " Ns\n");

        }

        if (choice == 5) {

            System.out.println("Enter Student's ID Number to search for: "); // to take the id number from the user
            int FindID = keyboard.nextInt();

            boolean Found = false;
            // to find with the student ID Number details of the student.
            for (int i = 0; i < 10; i++) {
                if (FindID == list[i].GetStudentID()) {

                    list[i].DisplayOutput();
                    Found = true;
                    break;
                }
            }

        }

        if (choice == 6) {  //

            System.out.println("Enter Student's Name to search for: ");
            String SearchStudentName = keyboard.next(); //take the name of the student 
            boolean Found = false;
            //To find the name of the student it loops till it has it or the limit of studentnumbers are achieved.
            for (int i = 0; i < list.length; i++) {
                if (SearchStudentName.equalsIgnoreCase(list[i].GetFirstName())) {
                    list[i].DisplayOutput();
                    Found = true;
                    break;
                }
            }
        }

    } while (choice != 7);
    { //while statement quit the program

        System.out.println("\nYou Quit.");

    }

}

}

这是我的学生类(class)

 package client;

 import java.util.Scanner;


 public class student {
 //The instance vriables for students (Title, first name, family name,
 Student ID, date of birth in day month and year, first and second 
 assignment mark, mark of weekly practical work and final exam

private String Title;
private String FirstName;
private String FamilyName;
private long StudentID;
private int Day;
private int Month;
private int Year;
private float FirstMark;
private float SecondMark;
private float WeeklyMarks;
private float FinalMark;

 //those private instance variables are for the calculation of (first and
 second assignment mark, mark of weekly practical work and exam mark, final 
mark and overall mark)
private float FirstMarkPercentage;
private float SecondMarkPercentage;
private float WeeklyMarksPercentage;
private float ExamPercentage;
private String FinalGrade;
private float OverallMarks = 0;

//those private instance variables are to count the the marks(
private int CounterN = 0;
private int CounterP = 0;
private int CounterC = 0;
private int CounterD = 0;
private int CounterHD = 0;



public student(String Title, String FirstName, String FamilyName, long StudentID, int Day, int Month, int Year, float FirstMark, float SecondMark, float WeeklyMarks, float FinalMark) {

    this.Title = Title;
    this.FirstName = FirstName;
    this.FamilyName = FamilyName;
    this.StudentID = StudentID;
    this.Day = Day;
    this.Month = Month;
    this.Year = Year;
    this.FirstMark = FirstMark;
    this.SecondMark = SecondMark;
    this.WeeklyMarks = WeeklyMarks;
    this.FinalMark = FinalMark;
    this.FinalGrade = FinalGrade;
}

//This Method is to display  (Title, first name, family name, Student ID, date of birth in day month and year, first and second assignment mark, mark of weekly practical work and final exam)
public student() {

    Title = "";
    FirstName = "";
    FamilyName = "";
    StudentID = 0;
    Day = 0;
    Month = 0;
    Year = 0;
    FirstMark = 0;
    SecondMark = 0;
    WeeklyMarks = 0;
    FinalMark = 0;
}

//The methods starting with Get...(), are to return the (Title, first name, family name, Student ID, date of birth in day month and year, first and second assignment mark, mark of weekly practical work and final exam and the marks N, P, C, D & HD)
public String GetTitle() {
    return Title;
}

public String GetFirstName() {
    return FirstName;
}

public String GetFamilyName() {
    return FamilyName;
}

public long GetStudentID() {
    return StudentID;
}

public int GetDay() {
    return Day;
}

public int GetMonth() {
    return Month;
}

public int GetYear() {
    return Year;
}

public float GetFirstMark() {
    return FirstMark;
}

public float GetSecondMark() {
    return SecondMark;
}

public float GetWeeklyMarks() {
    return WeeklyMarks;
}

public float GetFinalMark() {
    return FinalMark;
}

public String GetFinalGrade() {
    return FinalGrade;
}

public int GetCounterHD() {
    return CounterHD;
}

public int GetCounterD() {
    return CounterD;
}

public int GetCounterC() {
    return CounterC;
}

public int GetCounterP() {
    return CounterP;
}

public int GetCounterN() {
    return CounterN;
}

public float GetOverallMarks() {
    return OverallMarks;
}

//The methods starting with Set...(), are to set the (Title, first name, family name, Student ID, date of birth in day month and year, first and second assignment mark, mark of weekly practical work and final exam and the marks N, P, C, D & HD)
public void SetTitle(String Title) {
    this.Title = Title;
}

public void SetFirstName(String FirstName) {
    this.FirstName = FirstName;
}

public void SetFamilyName(String FamilyName) {
    this.FamilyName = FamilyName;
}

public void SetStudentID(int StudentID) {
    this.StudentID = StudentID;
}

public void SetDay(int Day) {
    this.Day = Day;
}

public void SetMonth(int Month) {
    this.Month = Month;
}

public void SetYear(int Year) {
    this.Year = Year;
}

public void SetFirstMark(float FirstMark) {
    this.FirstMark = FirstMark;
}

public void SetSecondMark(float SecondMark) {
    this.SecondMark = SecondMark;
}

public void SetWeeklyMarks(float WeeklyMarks) {
    this.WeeklyMarks = WeeklyMarks;
}

public void SetFinalMark(float FinalMark) {
    this.FinalMark = FinalMark;
}

public void SetFinalGrade(String FinalGrade) {
    this.FinalGrade = FinalGrade;
}

public void SetOverallMarks(float OverallMarks) {
    this.OverallMarks = OverallMarks;
}

public boolean equals(student OtherStudent) {
    return (this.FirstName.equalsIgnoreCase(OtherStudent.FirstName)) && (this.FamilyName.equalsIgnoreCase(OtherStudent.FamilyName));
}

//this method is for the calculation of (first and second assignment mark, mark of weekly practical work and exam mark, final mark and overall mark)
public float OverallMarking() {
    FirstMarkPercentage = ((FirstMark / 100) * 20);

    SecondMarkPercentage = ((SecondMark / 100) * 20);

    WeeklyMarksPercentage = ((WeeklyMarks / 10) * 10);

    ExamPercentage = ((FinalMark / 100) * 50);

    OverallMarks = FirstMarkPercentage + SecondMarkPercentage + WeeklyMarksPercentage + ExamPercentage; //for the overall mark returns 
    return OverallMarks;
}

//this function arranges the grade calculations and returns the final grade
public String GradeCalculations() {

    if (OverallMarks >= 80 && OverallMarks <= 100) { // if grade lies within this range print HD
        FinalGrade = "HD";
        CounterHD++;
    } else if (OverallMarks >= 70 && OverallMarks < 80) { // if grade lies within this range print D
        FinalGrade = "D";
        CounterD++;
    } else if (OverallMarks >= 60 && OverallMarks < 70) { // if grade lies within this range print C
        FinalGrade = "C";
        CounterC++;
    } else if (OverallMarks >= 50 && OverallMarks < 60) { // if grade lies within this range print P
        FinalGrade = "P";
        CounterP++;
    } else if (OverallMarks < 50 && OverallMarks >= 0) { // if grade lies within this range print N
        FinalGrade = "N";
        CounterN++;
    }
    return FinalGrade;
}

public void DisplayQuestions() {

    System.out.println("\n Welcome to the Menu to perform one of the following operations (You must enter a number between 1-7):");
    System.out.println("(1) To add the Student Information.");
    System.out.println("(2) To Display the Output from the Average Overall Mark for students.");
    System.out.println("(3) To Display all current Student Information.");
    System.out.println("(4) To Display the distribution of grades awarded.");
    System.out.println("(5) for entering a student ID Number To view all details of the student.");
    System.out.println("(6) for entering a student name To view all details of the student.");
    System.out.println("(7) Quit");
    System.out.println("\n__________________________________________");
}

//This function displays the details of the student with before calculated marks.
public void DisplayOutput() {

    System.out.println("\nName: " + GetTitle() + " " + GetFirstName() + " " + GetFamilyName());
    System.out.println("Student ID: " + GetStudentID());
    System.out.println("Date of Birth: " + GetDay() + "/" + GetMonth() + "/" + GetYear());
    System.out.println("Assignment 1 Marks: " + GetFirstMark());
    System.out.println("Assignment 2 Marks: " + GetSecondMark());
    System.out.println("Weekly Practical Marks: " + GetWeeklyMarks());
    System.out.println("Final Exam Marks: " + GetFinalMark());
    System.out.println("Final Marks & Grade: " + OverallMarking() + "/" + GradeCalculations());

}

public void addStudent() {
   /*Scanner keyboard = new Scanner(System.in);
    for (int i = 0; i < list.length; i++) { //for loop is till the student number is achieved

        list[i] = new student();

        System.out.println("\nTitle of the student (eg, Mr, Miss, Ms, Mrs etc): ");
        list[i].SetTitle(keyboard.next());

        System.out.println("First name (given name)");
        list[i].SetFirstName(keyboard.next());

        System.out.println("A last name (family name/surname)");
        list[i].SetFamilyName(keyboard.next());

        System.out.println("Student number (ID):");
        list[i].SetStudentID(keyboard.nextInt());

        System.out.println("Enter the Day of birth(1-31): ");
        list[i].SetDay(keyboard.nextInt());

        System.out.println("Enter the Month of birth (1-12): ");
        list[i].SetMonth(keyboard.nextInt());

        System.out.println("Enter The Year of birth: ");
        list[i].SetYear(keyboard.nextInt());

        System.out.println("Students First Assignment Mark (0 - 100): ");
        list[i].SetFirstMark(keyboard.nextInt());

        System.out.println("Students Second Assignment Mark (0 - 100): ");
        list[i].SetSecondMark(keyboard.nextInt());

        System.out.println("Enter the mark of Student weekly practical work (0-10) ");
        list[i].SetWeeklyMarks(keyboard.nextInt());

        System.out.println("Please Enter the Marks for the final Exam(0 - 100): ");
        list[i].SetFinalMark(keyboard.nextInt());
        System.out.println("- - - - - - - - - - - -  -");
        System.out.println("Do you want to add another Student? (Yes/No)");

        String a = keyboard.next();

        if (a.equalsIgnoreCase("yes")) {
            addStudent();
        } else if (a.equalsIgnoreCase("no")) {
           i=list.length+1;
        }

    }*/
   }
 }

Answer 1

数组不为空。

当您尝试对数组的空成员调用方法时，会引发异常。

您迭代整个数组，但不一定填满整个数组。您尚未指定引用对象的成员为 null。

for (int a = 0; a < list.length; a++) { //To Display all current Student Information.
    //   list[i] = new student();
    list[a].DisplayOutput();
}

一个修复方法是在遇到第一个 null 后停止迭代。

for (int a = 0; a < list.length; a++) { //To Display all current Student Information.
    //   list[i] = new student();
    student student = list[a];
    if ( null == student ) {
       break;
    }
    else {
       list[a].DisplayOutput();
    }
}

另一个修复方法是记住情况 1 中存储了多少学生，并更改循环条件以反射(reflect)这一点。

for (int a = 0; a < cntStudents; a++) { //To Display all current Student Information.

顺便说一下，在 Java 代码中，几乎普遍认为:

• 类名称以大写字符开头。
• 方法名称以小写字符开头。