为NullPointerException异常
问题描述：
仍然有错误我试图调整here这里，但仍然出来的错误。 错误是当我想要做一些与数组内的元素或者要显示它，存储元素或使用索引搜索 。这里是我的代码：为NullPointerException异常
import java.util.*;
public class RegisterMenu {
private Driver[] newOwner; //array
private final int MAX_ITEMS = 30;
private int size = 0;
public RegisterMenu(){
newOwner = new Driver[MAX_ITEMS];
Scanner scan = new Scanner(System.in);
System.out.println("1. Register New Car");
System.out.println("2. Edit Car Information");
System.out.println("3. Search Car Information");
System.out.println("4. Display Car List");
System.out.println("5. Exit");
System.out.print("Enter Selection: ");
int s = scan.nextInt();
switch(s){
case 1:
System.out.println("--Register New Car--");
Driver owner = newReg();
newOwner[size++] = owner;
break;
case 2:
System.out.println("--Edit Car Infomation--");
System.out.print("Enter RegNo to be edit: ");
int input = scan.nextInt(); //getting user input of index number which will be edited
if(newOwner[input].getName() == ""){
System.out.println("No data in RegNo "+input);
}
else{
Driver editOwner = newReg();
newOwner[input] = editOwner;
}
break;
case 3:
System.out.println("--Search Car Infomation--");
int index = searchReg();
newOwner[index].toString();
break;
case 4:
System.out.println("--Display Car Infomation--");
displayReg();
break;
case 5:
System.exit(0);
default:
System.out.println("Error selection");
}
}
public Driver newReg(){ //newReg class for new registration
Driver owner = new Driver();
Scanner scan = new Scanner(System.in);
owner.setRegNo(size+1); //registration number will auto update on each array
System.out.print("Enter Name: "); //getting info from user and put in setter
owner.setName(scan.nextLine());
System.out.print("Enter IC: ");
owner.setIc(scan.nextLine());
System.out.print("Enter PlateNo: ");
owner.carInfo.setPlateNum(scan.nextLine());
System.out.print("Enter Color: ");
owner.carInfo.setColor(scan.nextLine());
System.out.print("Enter Year: ");
owner.carInfo.setYear(scan.nextLine());
System.out.print("Enter Make: ");
owner.carInfo.setMake(scan.nextLine());
System.out.print("Enter Model: ");
owner.carInfo.setModel(scan.nextLine());
System.out.print("Enter Capacity: ");
owner.carInfo.setCapacity(scan.nextLine());
return owner; //return all back
}
public int searchReg(){ //searchReg will return index number of array
Scanner scan = new Scanner(System.in);
System.out.print("Enter Name or Ic to search: ");
String search = scan.nextLine();
int dataIndex = 0;
for(int i = 0; i < newOwner.length; i++){
if(search == newOwner[i].getIc() || search == newOwner[i].getName()){
dataIndex = i;
}
}
return dataIndex;
}
public void displayReg(){ //display all array
for(int i = 0; i < newOwner.length; i++){
System.out.println(newOwner[i].toString());
}
}
public static void main (String args[]){
while(true){
RegisterMenu reg = new RegisterMenu();
}
}
}
Car类
public class Car {
public String plateNum; //variable
public String make;
public String model;
public String color;
public String year;
public String capacity;
public Car(){ //empty constructor
}
public Car(String plateNum, String color, String year, String make, String model, String capacity){
this.plateNum = plateNum;
this.color = color;
this.year = year;
this.make = make;
this.model = model;
this.capacity = capacity;
}
public String getPlateNum(){ //get
return plateNum;
}
public String getMake(){
return make;
}
public String getModel(){
return model;
}
public String getColor(){
return color;
}
public String getYear(){
return year;
}
public String getCapacity(){
return capacity;
}
public void setPlateNum(String plateNum){ //setter
this.plateNum = plateNum;
}
public void setMake(String make){
this.make = make;
}
public void setModel(String model){
this.model = model;
}
public void setColor(String color){
this.color = color;
}
public void setYear(String year){
this.year = year;
}
public void setCapacity(String capacity){
this.capacity = capacity;
}
}
Driver类
public class Driver {
private int regNo; //variable
private String name;
private String ic;
Car carInfo = new Car(); //calling car class
public Driver(){ //empty constructor
}
public Driver(int regNo, String name, String ic, Car carInfo){ //constructor with argument
this.regNo = regNo;
this.name = name;
this.ic = ic;
this.carInfo = carInfo;
}
public int getRegNo(){ //get info
return regNo;
}
public String getName(){
return name;
}
public String getIc(){
return ic;
}
public void setRegNo(int regNo){ //set from user
this.regNo = regNo;
}
public void setName(String name){
this.name = name;
}
public void setIc(String ic){
this.ic = ic;
}
public String toString(){ //to string for display
return "RegNo: "+getRegNo()+"\t\tName: "+getName()+"\t\tIc: "+getIc()+
"\t\tPlateNo: "+carInfo.getPlateNum()+"\t\tColor: "+carInfo.getColor()+"\t\tYear: "+carInfo.getYear()+
"\t\tMake: "+carInfo.getMake()+"\t\tModel: "+carInfo.getModel()+"\t\tCapacity: "+carInfo.getCapacity()+"cc";
}
}
答
您反复在main方法
while (true) {
RegisterMenu reg = new RegisterMenu();
}
RegisterMenu对象
这有效地“湿巾”阵列newOwner这里以前的任何内容：
newOwner = new Driver[MAX_ITEMS];
因此在displayReg数组为空导致NPE。即使数组包含某些元素，也没有检查以确保当前元素不是null。
对于立即修复，你需要使用的RegisterMenu一个实例，并
for (int i = 0; i < newOwner.length; i++) {
if (newOwner[i] != null) {
System.out.println(newOwner[i].toString());
}
}
但这守卫检查可以通过使用ArrayList，而不是被淘汰。
注：在当前的IDE调试器是伟大的采购问题，如这些
+0
谢谢！这真的帮助我很多！我只是把RegisterMenu（）中的（true）循环放在main方法中，而不是像冠军一样工作:) –
答
我看不到在你的索引变量，每次访问newOwner阵列时间一个单一的控制。以此示例为例：
int input = scan.nextInt(); //getting user input of index number which will be edited
if(newOwner[input].getName() == ""){
您是否确定input索引与现有用户有关？如果不是，newOwner[index]返回null，当你调用getName时，你会得到一个NPE。
你正确定义的数组，但你只能创建和按需添加新的元素：
case 1:
System.out.println("--Register New Car--");
Driver owner = newReg();
newOwner[size++] = owner;
break;
你的第一个初始化做到这一点：
newOwner = new Driver[MAX_ITEMS];
分配适当长度的数组但充满了null元素。您需要确保您没有访问空所有者，例如，在获取元素之前进行空值检查（加上范围检查以避免出现越界异常）。
在这种情况下，堆栈跟踪确实很有帮助。请将其添加到您的问题。 – Medo42
'if（newOwner [input] .getName（）==“”）'。不要使用==比较String的内容。 –
作为您的问题的附注，您最好重复使用'RegisterMenu'对象，方法如下：'RegisterMenu reg = new RegisterMenu（）; while（true）{reg。interactiveRecordImport（）; }'这样你可以避免在每次迭代中重新生成这个对象，并且为了后面的改进，你可以使它以某种方式存储结果。 – zmo