我遇到了向ObservableList添加数组的问题，使用它将该数组添加到ListView中。编译时，我得到了NullPointerException。当我调用listAllItems（）方法时会发生此异常。如果我不使用GUI控件，而不是使用switch-statement并让用户选择该选项，那么在运行时不会发生异常。有人可以帮我弄清楚会发生什么吗？感谢
要求
创建LibraryGUI类 ◦此类应具有Library对象作为其字段之一。在LibraryGUI构造函数中 您将需要调用库构造函数方法来初始化此字段，然后调用它 open（）方法从数据文件中读取项目。 ◦使用JList显示库的内容。使用JList的setListData方法 class与Library类的listAllItems方法一起使列表保持当前状态 图书馆内容。
import javax.swing.JOptionPane;
public class MediaItem {
/*Fields*/
private String title;
private String format;
private boolean onLoan;
private String loanedTo;
private String dateLoaned;
/*Default Constructor - Initialize string variables for null and boolean variable for false*/
public MediaItem()
{
title = null;
format = null;
onLoan = false;
loanedTo = null;
dateLoaned = null;
}
/*Parameterized Constructor*/
public MediaItem(String title, String format)
{
this.title = title;
this.format = format;
this.onLoan = false;
}
public String getTitle() {
return title;
}
public void setTitle(String title) {
this.title = title;
}
public String getFormat() {
return format;
}
public void setFormat(String format) {
this.format = format;
}
public boolean isOnLoan() {
return onLoan;
}
public void setOnLoan(boolean onLoan) {
this.onLoan = onLoan;
}
public String getLoanedTo() {
return loanedTo;
}
public void setLoanedTo(String loanedTo) {
this.loanedTo = loanedTo;
}
public String getDateLoaned() {
return dateLoaned;
}
public void setDateLoaned(String dateLoaned) {
this.dateLoaned = dateLoaned;
}
/*Mark item on loan*/
void markOnLoan(String name, String date){
if(onLoan == true)
JOptionPane.showMessageDialog(null,this.title + " is already loaned out to " + this.loanedTo + " on " + this.dateLoaned);
else {
onLoan = true;
loanedTo = name;
dateLoaned = date;
}
}
/*Mark item return*/
void markReturned(){
if(onLoan == false)
JOptionPane.showMessageDialog(null, this.title + " is not currently loaned out");
onLoan = false;
}
}
import java.io.File;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.PrintWriter;
import java.util.ArrayList;
import java.util.Scanner;
import javax.swing.JOptionPane;
import javafx.scene.control.TextInputDialog;
public class Library extends MediaItem {
/*Fields*/
private ArrayList<MediaItem> itemsList = new ArrayList<>();
public ArrayList<MediaItem> getItemsList() {
return itemsList;
}
public void setItemsList(ArrayList<MediaItem> itemsList) {
this.itemsList = itemsList;
}
public int displayMenu()
{
Scanner input = new Scanner(System.in);
System.out.println("\nMenu Options \n"
+"\n1. Add new item\n"
+"\n2. Mark an item as on loan\n"
+"\n3. List all item\n"
+"\n4. Mark an item as returned\n"
+"\n5. Quit\n"
+"\n\nWhat would you like to do?\n");
int choices = input.nextInt();
while (choices < 1 || choices >5)
{
System.out.println("I'm sorry, " + choices + " is invalid option");
System.out.println("Please choose another option");
choices = input.nextInt();
}
return (choices);
}
public void addNewItem(String title, String format)
{
MediaItem item = new MediaItem(); // the object of item
/*Use the JOptionPane.showInputDialog(null,prompt) to let the user input the title and the format
* store the title and the format to the strings and pass it to the parameter of addNewItem(String, String) method
* title and format will go through the loop if title already exited let the user input another title and format
* Using the same method JOptionPan.ShowInputDialog and JOptionPane.showMessage()
*/
boolean matches = false;
for(int i = 0; i< itemsList.size(); i++){
if(title.equals(itemsList.get(i).getTitle())){
String newTitle = JOptionPane.showInputDialog(null, "this " + title + " already exited. Please enter the title again");
String newFormat = JOptionPane.showInputDialog(null, "Enter the format again");
item = new MediaItem(newTitle,newFormat);
itemsList.add(item);
matches = true;
}
}if(!matches){
item = new MediaItem(title,format);
itemsList.add(item);
}
}
public void markItemOnLoan(String title, String name, String date)
{
boolean matches = false;
for(int i = 0; i < itemsList.size(); i++)
{ if (title.equals(itemsList.get(i).getTitle()))
{
itemsList.get(i).markOnLoan(name,date);
matches = true;
}
}
if (!matches)
{
JOptionPane.showMessageDialog(null,"I am sorry. I couldn't find " + title + " in the library");
}
}
public String[] listAllItems()
{
String[] list = new String[itemsList.size()];
for (int n = 0; n < itemsList.size(); n++)
{
if(itemsList.get(n).isOnLoan() == true)
list[n] = itemsList.get(n).getTitle() + " (" + itemsList.get(n).getFormat() + " ) loaned to " + itemsList.get(n).getLoanedTo() + " on " + itemsList.get(n).getDateLoaned();
else
list[n] = itemsList.get(n).getTitle() + " (" + itemsList.get(n).getFormat() + ")";
if(list[n] == null){
String title = "title ";
String format = "format ";
String loanTo = "name ";
String date = "date ";
list[n] = title + format + loanTo + date;
}
}
return list;
}
public void markItemReturned(String title)
{
boolean matches = false;
for( int i = 0; i < itemsList.size(); i++)
{
if(title.equals(itemsList.get(i).getTitle()))
{
matches = true;
itemsList.get(i).markReturned();
}
}
if(!matches)
{
JOptionPane.showMessageDialog(null,"I am sorry. I couldn't find " + title + " in the library");
}
}
public void deleteItem(String title){
boolean matches = false;
for(int i = 0; i < itemsList.size(); i++){
if(title.equals(itemsList.get(i).getTitle())){
itemsList.get(i).setTitle(null);
itemsList.get(i).setFormat(null);
itemsList.get(i).setDateLoaned(null);
itemsList.get(i).setLoanedTo(null);
itemsList.get(i).setOnLoan(false);
}
}
if(!matches)
JOptionPane.showMessageDialog(null,"I am sorry. I couldn't find " + title + " in the library");
}
public void openFile(){
Scanner input = null;
try{
input = new Scanner(new File("library.txt"));
Scanner dataInput = null;
MediaItem item = new MediaItem();
int index = 0;
String title = " ";
String format = " ";
String date = " ";
String onLoanTo = " ";
Boolean onLoan = false;
Boolean checkString= false;
Boolean checkBoolean = false;
itemsList = new ArrayList<>();
while(input.hasNextLine()){
dataInput = new Scanner(input.nextLine());
dataInput.useDelimiter("-");
while(input.hasNext()){
Boolean dataBoolean = dataInput.nextBoolean();
String dataString = dataInput.next();
if(index == 0){
item.setTitle(dataString);
title = item.getTitle();
checkString = true;
}else if(index == 1){
item.setFormat(dataString);
format = item.getFormat();
checkString= true;
}else if(index == 2){
item.setOnLoan(dataBoolean);
checkBoolean = true;
}else if(index == 3){
item.setLoanedTo(dataString);
checkString = true;
}else if(index == 4){
item.setDateLoaned(dataString);
checkString = true;
}else {
if(checkString == false)
throw new Exception("Invalid data " + dataString);
if(checkBoolean == false)
throw new Exception("Invalid data " + dataBoolean);
}
index++;
itemsList.add(new MediaItem(title,format));
}
index = 0;
title = " ";
format = " ";
date = " ";
onLoan = false;
onLoanTo = " ";
checkString= false;
checkBoolean = false;
}
}catch(Exception ex){
JOptionPane.showMessageDialog(null, ex.getMessage());
}finally{
if(input != null){
try{
input.close();
}catch(NullPointerException ex){
JOptionPane.showMessageDialog(null,"Error: File can not be closed");
}
}
}
}
public void saveFile(){
PrintWriter output = null;
try{
output = new PrintWriter(new File("library.txt"));
for(int i = 0; i < itemsList.size(); i++){
output.println(itemsList.get(i).getTitle() + "-" + itemsList.get(i).getFormat()+ "-" + itemsList.get(i).isOnLoan() + "-"
+ itemsList.get(i).getLoanedTo() + "-" + itemsList.get(i).getDateLoaned());
}
}catch(FileNotFoundException e1){
JOptionPane.showMessageDialog(null,"Could not find the data file. Program is exiting.");
}catch(IOException e2){
JOptionPane.showMessageDialog(null, "Something went wrong with writing to the data file.");
}catch(Exception e3){
JOptionPane.showMessageDialog(null,"Something went wrong.");
}
finally{
if( output != null){
try{
output.close();
System.exit(0);
}catch (NullPointerException e4){
JOptionPane.showMessageDialog(null, "Error: File can not be closed");
}
}
}
}
}
enter code here
import java.util.ArrayList;
import javafx.application.Application;
import javafx.collections.FXCollections;
import javafx.collections.ObservableList;
import javafx.geometry.Pos;
import javafx.stage.Stage;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.layout.BorderPane;
import javafx.scene.layout.FlowPane;
import javafx.scene.control.ListView;
import javafx.scene.control.SelectionMode;
public class LibraryGUI extends Application{
private ListView<String> listView;
private Library library;
private Button btAdd, btCheckIn, btCheckOut, btDelete;
public void LibraryGUI(){
library = new Library();
library.openFile();
}
public void start(Stage primaryStage) throws Exception {
listView = new ListView<>();
//Add all item in the listAllItem() method to the list view
String[] list = library.listAllItems();
int length = library.getItemsList().size();
for (int i = 0; i < length; i++)
{
ObservableList<String> element = FXCollections.observableArrayList( list[i].toString());
listView.setItems(element);
}
listView.getSelectionModel().setSelectionMode(SelectionMode.SINGLE);
btAdd = new Button(" Add");
btCheckIn = new Button(" CheckIn ");
btCheckOut = new Button(" CheckOut ");
btDelete = new Button(" Delete ");
FlowPane flowP = new FlowPane();
listView.setPrefSize(400, 275);
btAdd.setPrefSize(100, 25);
btAdd.setAlignment(Pos.BOTTOM_LEFT);
btCheckIn.setPrefSize(100, 25);
btCheckIn.setAlignment(Pos.BOTTOM_LEFT);
btCheckOut.setPrefSize(100, 25);
btCheckOut.setAlignment(Pos.BOTTOM_LEFT);
btDelete.setPrefSize(100,25);
btDelete.setAlignment(Pos.BOTTOM_LEFT);
flowP.getChildren().addAll(listView,btAdd,btCheckIn,btCheckOut,btDelete);
Scene scene = new Scene (flowP,400,300);
primaryStage.setTitle("Calculator");
primaryStage.setScene(scene);
primaryStage.show();
}
public static void main(String[] args) {
launch(args);
}
}
Switch-Statement的代码
import java.util.Scanner;
public class Code8 {
public static void main(String[] args) {
MediaItem newItem = new MediaItem();
Library library = new Library();
Scanner in = new Scanner(System.in);
int option = 0;
while (option != 5)
{
option = library.displayMenu();
switch(option)
{
case 1:
System.out.print("What is the title? ");
String newTitle = in.nextLine();
newItem.setTitle(newTitle);
System.out.println("What is the format? ");
String newFormat = in.nextLine();
newItem.setFormat(newFormat);
library.addNewItem(newTitle, newFormat);
break;
case 2:
System.out.println("Which item (enter the title)? ");
newTitle = in.nextLine();
newItem.setTitle(newTitle);
System.out.println("Who are you loaning it to? ");
String name = in.nextLine();
newItem.setLoanedTo(name);
System.out.println("When did you loan it to them? ");
String date = in.nextLine();
library.markItemOnLoan(newTitle, name, date);
break;
case 3:
String[] list = library.listAllItems();
int length = library.getItemsList().size();
for (int i = 0; i < length; i++)
{
System.out.println(list[i].toString());
}
break;
case 4:
System.out.println("Which item (enter the title)? ");
newTitle = in.nextLine();
newItem.setTitle(newTitle);
library.markItemReturned(newTitle);
break;
case 5:
System.out.println("Goodbye");
break;
}// switch statement
}//while statement
} //main method
}