我试图调整here并在这里但仍然出错。 错误是当我想对数组中的元素做某事要么想显示它,存储元素或 使用索引搜索。这是我的代码:
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
上课
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
上课
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";
}
}
答案 0 :(得分:2)
您正在RegisterMenu
方法
main
个对象
while (true) {
RegisterMenu reg = new RegisterMenu();
}
在这里有效地“擦除”数组newOwner
的任何先前内容:
newOwner = new Driver[MAX_ITEMS];
因此,在displayReg
数组中,数组为空,从而生成NPE
。即使数组包含某些元素,也不会检查以确保当前元素不是null
。
要立即修复,您需要使用RegisterMenu
和
for (int i = 0; i < newOwner.length; i++) {
if (newOwner[i] != null) {
System.out.println(newOwner[i].toString());
}
}
但是,可以使用ArrayList
来消除此保护检查。
注意:当前IDE中的调试器非常适合采购这些
等问题答案 1 :(得分:1)
每次访问newOwner
数组时,我都看不到对索引变量的单一控制。举个例子:
int input = scan.nextInt(); //getting user input of index number which will be edited
if(newOwner[input].getName() == ""){
您是否完全确定input
索引与现有用户相关?如果不是,newOwner[index]
会返回null
,当您调用getName
时,您将获得NPE。
您正确定义了数组,但您只需按需创建和添加新元素:
case 1:
System.out.println("--Register New Car--");
Driver owner = newReg();
newOwner[size++] = owner;
break;
您的第一次初始化是这样做的:
newOwner = new Driver[MAX_ITEMS];
分配适当长度的数组但填充null
个元素。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检查(加上范围检查以避免超出范围的异常)。