Java NullPointerException错误数组

尝试创建一个基本程序，其中创建一个对象游泳者及其数据并将其存储到一个文件（游泳者的简单数据库）。但是，我遇到了许多运行时错误，其中许多被归类为NullPointerException。我有一个游泳者类，没有一个构造函数，它的方法可以获取姓名、年龄、性别等信息。这个类还有一些方法可以返回所有这些信息。在我的Tester类中，我创建了一个开关，提示用户显示游泳者、添加游泳者、编辑游泳者、删除游泳者和退出程序。我当前的错误出现在添加游泳者时，特别是在设置名称时。myList[maxIndex].setName（a）处存在空指针异常

这是我的游泳课代码： 导入java.io。； 导入java.util

这是我的主要课程的代码： 导入java.io。； 导入java.util。； 导入java.util.array

public class Tester
{
public static Swimmer[] myList = new Swimmer[1000];
public static int maxIndex = 0; 

public static void main(String args[]) throws IOException
{
   Scanner kbReader = new Scanner(System.in);

   int choice;
   String swimmerName = "Default";
   boolean condition = true;

   while(condition)
    {
       System.out.println("Which of the following would you like to do?");
       System.out.println("1. Show Swimmers" + "\n" + "2. Add Swimmer" + "\n"    + "3. Edit Swimmer" + "\n" + "4. Delete Swimmer" + "\n" + "5. Exit");
       choice = kbReader.nextInt();

       switch (choice)
       {
           case 1:
           showSwimmer();
           break;
           case 2:
           addSwimmer();
           break;
           case 3:
           editSwimmer();
           break;
           case 4:
           deleteSwimmer();
           break;
           case 5:
           condition = false;
           break; 
        }
    }
    FileWriter();
}

public static void FileWriter() throws IOException
{
    FileWriter fw = new FileWriter("database.txt");
    PrintWriter output = new PrintWriter(fw);

    for(int j = 0; j<=maxIndex; j++)
    {
        output.println(myList[j].getName() + " ~ " + myList[j].getAge() + " ~      " + myList[j].getGender() + " ~ " + myList[j].getGrade() + " ~ " +      myList[j].getFiftyFree() + " ~ " + myList[j].getHundredFree() + " ~ " +      myList[j].getTwoFree() + " ~ " + myList[j].getFiveFree() + " ~ " +      myList[j].getHundredBack() + " ~ " + myList[j].getHundredBreast() + " ~ " +     myList[j].getHundredFly() + myList[j].getTwoIM());
    }

    output.close();
    fw.close();
}

public static void FileReader() throws IOException
{
    Scanner sf = new Scanner(new File("database.txt"));
    String s, sp[];
    while(sf.hasNext())
    {
        maxIndex++;
        s = sf.nextLine();
        sp = s.split("~");
        myList[maxIndex] = new Swimmer();
    }
    sf.close();

}

public static void swap(int a, int b)
{
    Swimmer temp = myList[a];
    myList[a] = myList[maxIndex];
    myList[maxIndex] = temp;



}

public static void showSwimmer()

{
    for(int j = 1; j < maxIndex; j++)
    {
        System.out.println( (j) + ". " + myList[j].getName());
    }
}

public static void addSwimmer()
{
    Scanner kbReader = new Scanner (System.in);

    System.out.println("What is the swimmer's name? ");
    String a = kbReader.nextLine();
    myList[maxIndex].setName(a);

    System.out.println("Is the swimmer male or female? Please enter \"m\" for     male and \"f\" for female. ");
    String b = kbReader.nextLine();

    System.out.println("How old is he/she? ");
    int c = kbReader.nextInt();
    myList[maxIndex].setAge(c);

    System.out.println("What is the numerical value of the swimmer's grade?");
    int d = kbReader.nextInt();
    myList[maxIndex].setGrade(d);

    System.out.println("How many minutes does it take for this swimmer to     complete the 50 Freestyle?");
    double e = kbReader.nextDouble();
    System.out.println("How many additional seconds does it take for this     swimmer to complete the 50 freestyle?");
    double f = kbReader.nextDouble();
    double g = (e*60.0) + f;
    myList[maxIndex].setFiftyFree(g);

    System.out.println("How many minutes does it take for this swimmer to     complete the 100 Freestyle?");
    double h = kbReader.nextDouble();
    System.out.println("How many additional seconds does it take for this     swimmer to complete the 100 Freestyle?");
    double i = kbReader.nextDouble();
    double j = (h*60.0) + i;
    myList[maxIndex].setHundredFree(j);

    System.out.println("How many minutes does it take for this swimmer to     complete the 200 Freestyle?");
    double k = kbReader.nextDouble();
    System.out.println("How many additional seconds does it take for this     swimmer to complete the 200 Freestyle?");
    double l = kbReader.nextDouble();
    double m = (k*60.0) + l;
    myList[maxIndex].setTwoFree(k);

    System.out.println("How many minutes does it take for this swimmer to     complete the 500 Freestyle?");
    double n = kbReader.nextDouble();
    System.out.println("How many additional seconds does it take for this     swimmer to complete the 200 Freestyle?");
    double o = kbReader.nextDouble();
    double p = (n*60.0) + o;
    myList[maxIndex].setFiveFree(p);

    System.out.println("How many minutes does it take for this swimmer to     complete the 100 Backstroke?");
    double q = kbReader.nextDouble();
    System.out.println("How many additional seconds does it take for this      swimmer to complete the 100 Backstroke?");
    double r = kbReader.nextDouble();
    double s = (q*60.0) + r;
    myList[maxIndex].setHundredBack(s);

    System.out.println("How many minutes does it take for this swimmer to     complete the 100 Breastroke?");
    double t = kbReader.nextDouble();
    System.out.println("How many additional seconds does it take for this      swimmer to complete the 100 Breastroke?");
    double u = kbReader.nextDouble();
    double v = (t*60.0) + u;
    myList[maxIndex].setHundredBreast(v);

    System.out.println("How many minutes does it take for this swimmer to      complete the 100 Butterfly?");
    double w = kbReader.nextDouble();
    System.out.println("How many additional seconds does it take for this      swimmer to complete the 100 Butterfly?");
    double x = kbReader.nextDouble();
    double y = (w*60.0) + x;
    myList[maxIndex].setHundredFly(y);

    System.out.println("How many minutes does it take for this swimmer to      complete the 200 IM?");
    double z = kbReader.nextDouble();
    System.out.println("How many additional seconds does it take for this      swimmer to complete the 200 IM?");
    double aa = kbReader.nextDouble();
    double bb = (z*60.0) + aa;
    myList[maxIndex].setTwoIM(bb);
    maxIndex++;
}

public static void editSwimmer()
{
    Scanner kbReader = new Scanner(System.in);
    System.out.println("Which swimmer would you like to edit? Please enter the      corresponding number.");
    System.out.println(Arrays.toString(myList));
    int choice = kbReader.nextInt();
}

public static void deleteSwimmer()
{
    int a;
    Scanner kbReader = new Scanner(System.in);
    System.out.println("Which swimmer would you like to delete? Please enter      the corresponding number.");
    System.out.println(Arrays.toString(myList));
    a = kbReader.nextInt();

    swap(a-1, maxIndex);
    maxIndex--;
}
}

公共类测试器
{
公众静态游泳者[]myList=新游泳者[1000]；
公共静态int maxIndex=0；
公共静态void main（字符串args[]）引发IOException
{
扫描仪kbReader=新扫描仪（System.in）；
智力选择；
字符串name=“Default”；
布尔条件=真；
while（条件）
{
System.out.println（“您希望执行以下哪项？”；
System.out.println（“1.显示游泳者”+“\n”+”2.添加游泳者”+“\n”+”3.编辑游泳者”+“\n”+”4.删除游泳者”+“\n”+”5.退出”）；
choice=kbReader.nextInt（）；
开关（选择）
{
案例1：
showswiller（）；
打破
案例2：
游泳运动员（）；
打破
案例3：
editswiller（）；
打破
案例4：
删除游泳者（）；
打破
案例5：
条件=假；
打破
}
}
FileWriter（）；
}
公共静态void FileWriter（）引发IOException
{
FileWriter fw=新的FileWriter（“database.txt”）；
PrintWriter输出=新的PrintWriter（fw）；
对于（int j=0；j这是因为您从未创建过任何新的游泳者。您的数组myList被声明为1000个游泳者的数组。但是，您的问题是，您没有创建游泳者对象来占据这些位置，它们都是空的。因此，当您尝试设置游泳者的名称时，您会得到一个NullPointerException

您应该创建如下游泳运动员：

for (int i = 0; i < 1000; i++) {
    myList[i] = new Swimmer();
}

for（int i=0；i<1000；i++）{
myList[i]=新游泳运动员（）；
}
发布错误的完整堆栈跟踪将很有帮助。
for (int i = 0; i < 1000; i++) {
    myList[i] = new Swimmer();
}