我编写了这段代码，除了一件事，一切正常。
在方法whatIsThebestEmployeeName我有问题将字符串从一个对象复制到另一个对象。它告诉我nullpointerexception。
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enter code here
import java.util.Scanner;
public class HW3 {
public static class Employee {
private int id;
private String name;
private int salary;
public Employee(int id_num, String n, int s) {
id = id_num;
name = n;
salary = s;
}
public String toString() {
return id + " " + name + " " + salary;
}
public String getName() {
return name;
}
public void setName(String n) {
name = n;
}
public int getId() {
return id;
}
public void setName(int id_num) {
id = id_num;
}
public int getSalary() {
return salary;
}
public void setSalary(int s) {
salary = s;
}
}
public static class bestName {
private String name;
private double avg;
public bestName(String s, double n) {
name = s;
avg = n;
}
public String getName() {
return name;
}
public void setName(String n) {
name = n;
}
public String toString() {
return "Best name: " + name + "\n" + "avg = " + avg;
}
}
public static Employee getOneEmployee(int i) {
Scanner s = new Scanner(System.in);
System.out.println("Please Enter Employee's #" + i + " id:");
int id = s.nextInt();
s.nextLine();
System.out.println("Please Enter Employee's #" + i + " name:");
String name = s.nextLine();
System.out.println("Please Enter Employee's #" + i + " salary:");
int salary = s.nextInt();
Employee temp = new Employee(id, name, salary);
return temp;
}
public static Employee[] getEmployeesArray(int n) {
Employee[] arr = new Employee[n];
for (int i = 0; i < arr.length; i++) {
arr[i] = getOneEmployee(i + 1);
}
return arr;
}
public static void printEmployeesArray(Employee[] arr) {
for (int i = 0; i < arr.length; i++) {
System.out.println(arr[i]);
}
}
public static void swap(Employee[] arr, int from, int to) {
Employee temp = arr[from];
arr[from] = arr[to];
arr[to] = temp;
}
public static void bubbleSort(Employee[] arr) {
boolean bChanged = true;
for (int i = arr.length - 1; i > 0 && bChanged; i--) {
bChanged = false;
for (int j = 0; j < i; j++) {
if (arr[j].name.compareTo(arr[j + 1].name) > 0) {
swap(arr, j, j + 1);
bChanged = true;
} else if ((arr[j].name.compareTo(arr[j + 1].name) == 0)
&& (arr[j].salary > arr[j + 1].salary)) {
swap(arr, j, j + 1);
bChanged = true;
}
}
}
}
public static int howManyRows(Employee[] arr) {
if (arr.length == 1)
return 1;
int count = 0;
int i = 0;
Employee temp = arr[i];
for (i = i + 1; i < arr.length; i++) {
if (!(temp.name.equals(arr[i].name)))
count++;
}
if ((count == 0) || (count == 1))
return ++count;
else
return count;
}
public static int howManyColumns(Employee[] arr, int k) {
if (arr.length == 1)
return 1;
int count = 1;
Employee temp = arr[k];
for (int i = k + 1; i < arr.length; i++) {
if (temp.name.equals(arr[i].name))
count++;
else
break;
}
return count;
}
public static Employee[][] setEmployeeMatrix(Employee[] arr) {
int rows = howManyRows(arr);
Employee[][] employeeMat = new Employee[rows][];
int k = 0;
for (int i = 0; i < employeeMat.length; i++) {
int columns = howManyColumns(arr, k);
employeeMat[i] = new Employee[columns];
for (int j = 0; j < employeeMat[i].length; j++) {
employeeMat[i][j] = arr[k];
k++;
}
}
return employeeMat;
}
public static void printEmployeeIdMatrix(Employee[][] mat) {
for (int i = 0; i < mat.length; i++) {
for (int j = 0; j < mat[i].length; j++) {
System.out.print(" " + mat[i][j].id);
}
System.out.println();
}
}
public static double getAvg(Employee[][] arr, int i) {
double avg = 0;
for (int j = 0; j < arr[i].length; j++)
avg += arr[i][j].salary;
return avg / arr.length;
}
public static bestName bestAvg(bestName[] arr) {
int k = 0;
double max = arr[k].avg;
for (int i = ++k; i < arr.length; i++) {
if (arr[i].avg > max) {
max = arr[i].avg;
k = i;
}
}
return arr[k];
}
public static bestName whatIsThebestEmployeeName(
Employee[][] employeeMatrix, Employee[] employeeArr) {
bestName[] arr = new bestName[employeeMatrix.length];
for (int i = 0; i < arr.length; i++) {
arr[i].name = employeeArr[i].name;
arr[i].avg = getAvg(employeeMatrix, i);
}
return bestAvg(arr);
}
public static void main(String[] args) {
// TODO Auto-generated method stub
Scanner s = new Scanner(System.in);
System.out.println("Please Enter how many Employees:");
int n = s.nextInt();
Employee[] employeeArr = getEmployeesArray(n);
System.out.println("Original array from user:\n");
printEmployeesArray(employeeArr);
System.out.println("\nArray after sorting:\n");
bubbleSort(employeeArr);
printEmployeesArray(employeeArr);
System.out.println("\nMatrix:");
Employee[][] employeeMatrix = setEmployeeMatrix(employeeArr);
printEmployeeIdMatrix(employeeMatrix);
bestName bestEmployeeName = whatIsThebestEmployeeName(employeeMatrix,
employeeArr);
System.out.println(bestEmployeeName);
s.close();
}
}
答案 0 :(得分：0)
您需要先向数组添加new bestNames，然后才能尝试更改数据。
bestName[] arr = new bestName[employeeMatrix.length]; // all null
arr[i].name = employeeArr[i].name;
arr[i].avg = getAvg(employeeMatrix, i);
第一行代码创建一个nestName类型的数组，其中包含所有null个值。您需要先使用new bestName初始化所有索引，然后才能更改arr[i].name和arry[i].avg。
也许你想要像
这样的东西public static bestName whatIsThebestEmployeeName(
Employee[][] employeeMatrix, Employee[] employeeArr) {
bestName[] arr = new bestName[employeeMatrix.length];
for (int i = 0; i < arr.length; i++) {
array[i] = new bestName(); <=-------- Change --------<<<<
arr[i].name = employeeArr[i].name;
arr[i].avg = getAvg(employeeMatrix, i);
}
return bestAvg(arr);
}
另外，我刚注意到bestName的字段是私有的。您可能想要使用它的构造函数。
for (int i = 0; i < arr.length; i++) {
String name = employeeArr[i].getName(); // notice getName() change
double avg = getAvg(employeeMatrix, 1);
array[i] = new bestName(name, avg);
}
请注意getName()更改。因为你所有的领域都是私人的。您希望确保尝试使用getMethods()访问数据并使用setMethods()或通过构造函数设置其值。您应该查看所有代码以确保正确无误。
答案 1 :(得分：0)
在尝试设置其属性之前必须实例化arr[i]
arr[i] = new bestName();
arr[i].name = employeeArr[i].name;
arr[i].avg = getAvg(employeeMatrix, i);