尝试创建一个基本程序，其中创建对象Swimmer及其数据并将其存储到文件（游泳者的简单数据库）。但是，我遇到了许多运行时错误，其中许多错误被归类为NullPointerExceptions。我有一个没有构造函数的Swimmer类，其方法可以获取名称，年龄，性别等。此类还有返回所有这些信息的方法。在我的Tester类中，我创建了一个开关，提示用户显示Swimmers，Add Swimmer，Edit Swimmer，Delete Swimmer和退出程序。添加游泳者时会出现我当前的错误，特别是在设置名称时。 myList [maxIndex] .setName（a）中存在空指针异常。
这是我的Swimmer课程的代码： import java.io. ; import java.util。;
public class Swimmer
{
private String name;
private int age;
private int gradeLevel;
private String gender;
private int grade;
private double fiftyFree;
private double hundredFree;
private double twoFree;
private double fiveFree;
private double hundredBack;
private double hundredBreast;
private double hundredFly;
private double twoIM;
Scanner kbReader = new Scanner(System.in);
public void setName(String a)
{
name = a;
}
public String getName()
{
return name;
}
public void setAge(int a)
{
age = a;
}
public int getAge()
{
return age;
}
public void setGender(String a)
{
gender = a;
}
public String getGender()
{
return gender;
}
public void setGrade(int a)
{
grade = a;
}
public int getGrade()
{
return grade;
}
public void setFiftyFree(double a)
{
fiftyFree = a;
}
public double getFiftyFree()
{
return fiftyFree;
}
public void setHundredFree(double a)
{
hundredFree = a;
}
public double getHundredFree()
{
return hundredFree;
}
public void setTwoFree(double a)
{
twoFree = a;
}
public double getTwoFree()
{
return twoFree;
}
public void setFiveFree(double a)
{
fiveFree = a;
}
public double getFiveFree()
{
return fiveFree;
}
public void setHundredBack(double a)
{
hundredBack = a;
}
public double getHundredBack()
{
return hundredBack;
}
public void setHundredBreast(double a)
{
hundredBreast = a;
}
public double getHundredBreast()
{
return hundredBreast;
}
public void setTwoIM(double a)
{
twoIM = a;
}
public double getTwoIM()
{
return twoIM;
}
public void setHundredFly(double a)
{
hundredFly = a;
}
public double getHundredFly()
{
return hundredFly;
}
public void getInfo()
{
System.out.println("Name: " + name + "\n" + "Age: " + age + "/n" + "Grade enter code here`Level: " + gradeLevel + "\n" + "Gender: " + gender);
}
}
这是我的Main类的代码： import java.io. ; import java.util。; import java.util.Arrays;
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--;
}
}
我只需要帮助Null Pointer Exception。备份到文件的任何帮助也会很棒。我有一台Mac，我不认为这会过多地改变代码。 （也许只是文件扩展名）
答案 0 :(得分：1)
这是因为你从未创造过任何新的游泳运动员。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字时，您会收到NullPointerException。
你应该像这样创造游泳运动员：
for (int i = 0; i < 1000; i++) {
myList[i] = new Swimmer();
}