基本上，我正在尝试为学校教师建立图书馆系统。该程序必须能够添加书籍，添加学生，搜索书籍，签出书籍等。但是问题是，当添加一个学生并要求显示该学生时，它在显示中设置的数组甚至是空的尽管正在使用同一对象。
我尝试使用setter和getter来设置数组和对象的值，但似乎没有任何效果。
我了解到，学生类中的showAllstudents（）方法认为该数组为空，但我不明白为什么会这样认为，因为我为调用该方法的对象设置了值。
package Library;
import java.util.Scanner;
public class book {
public int sNo;
public String bookName;
public String authorName;
public int bookQty;
public int bookQtyCopy;
public int getsNo() {
return sNo;
}
public void setsNo(int sNo) {
this.sNo = sNo;
}
public String getBookName() {
return bookName;
}
public void setBookName(String bookName) {
this.bookName = bookName;
}
public String getAuthorName() {
return authorName;
}
public void setAuthorName(String authorName) {
this.authorName = authorName;
}
public int getBookQty() {
return bookQty;
}
public void setBookQty(int bookQty) {
this.bookQty = bookQty;
}
public int getBookQtyCopy() {
return bookQtyCopy;
}
public void setBookQtyCopy(int bookQtyCopy) {
this.bookQtyCopy = bookQtyCopy;
}
Scanner input = new Scanner(System.in);
public book(){
System.out.println("Enter Serial No of Book:");
this.sNo = input.nextInt();
input.nextLine();
System.out.println("Enter Book Name:");
this.bookName = input.nextLine();
System.out.println("Enter Author Name:");
this.authorName = input.nextLine();
System.out.println("Enter Quantity of Books:");
this.bookQty = input.nextInt();
bookQtyCopy = this.bookQty;
}
}
package Library;
import java.util.Scanner;
public class books {
book theBooks[] = new book[50]; // Array that stores 'book' Objects.
public static int count; // Counter for No of book objects Added in Array.
Scanner input = new Scanner(System.in);
public int compareBookObjects(book b1, book b2){
if (b1.bookName.equalsIgnoreCase(b2.bookName)){
System.out.println("Book of this Name Already Exists.");
return 0;
}
if (b1.sNo==b2.sNo){
System.out.println("Book of this Serial No Already Exists.");
return 0;
}
return 1;
}
public void addBook(book b){
for (int i=0; i<count; i++){
if (this.compareBookObjects(b, this.theBooks[i]) == 0)
return;
}
if (count<50){
theBooks[count] = b;
count++;
}
else{
System.out.println("No Space to Add More Books.");
}
}
public void searchBySno(){
System.out.println("\t\t\t\tSEARCH BY SERIAL NUMBER\n");
int sNo;
System.out.println("Enter Serial No of Book:");
sNo = input.nextInt();
int flag = 0;
System.out.println("S.No\t\tName\t\tAuthor\t\tAvailable Qty\t\tTotal Qty");
for (int i=0; i<count; i++){
if (sNo == theBooks[i].sNo){
System.out.println(theBooks[i].sNo + "\t\t" + theBooks[i].bookName + "\t\t" + theBooks[i].authorName + "\t\t" +
theBooks[i].bookQtyCopy + "\t\t" + theBooks[i].bookQty);
flag++;
return;
}
}
if (flag == 0)
System.out.println("No Book for Serial No " + sNo + " Found.");
}
public void searchByAuthorName(){
System.out.println("\t\t\t\tSEARCH BY AUTHOR'S NAME");
input.nextLine();
System.out.println("Enter Author Name:");
String authorName = input.nextLine();
int flag = 0;
System.out.println("S.No\t\tName\t\tAuthor\t\tAvailable Qty\t\tTotal Qty");
for (int i=0; i<count; i++){
if (authorName.equalsIgnoreCase(theBooks[i].authorName)){
System.out.println(theBooks[i].sNo + "\t\t" + theBooks[i].bookName + "\t\t" + theBooks[i].authorName + "\t\t" +
theBooks[i].bookQtyCopy + "\t\t" + theBooks[i].bookQty);
flag++;
}
}
if (flag == 0)
System.out.println("No Books of " + authorName + " Found.");
}
public void showAllBooks(){
System.out.println("\t\t\t\tSHOWING ALL BOOKS\n");
System.out.println("S.No\t\tName\t\tAuthor\t\tAvailable Qty\t\tTotal Qty");
for (int i=0; i<count; i++){
System.out.println(theBooks[i].sNo + "\t\t" + theBooks[i].bookName + "\t\t" + theBooks[i].authorName + "\t\t" +
theBooks[i].bookQtyCopy + "\t\t" + theBooks[i].bookQty);
}
}
public void upgradeBookQty(){
System.out.println("\t\t\t\tUPGRADE QUANTITY OF A BOOK\n");
System.out.println("Enter Serial No of Book");
int sNo = input.nextInt();
for (int i=0; i<count; i++){
if (sNo == theBooks[i].sNo){
System.out.println("Enter No of Books to be Added:");
int addingQty = input.nextInt();
theBooks[i].bookQty += addingQty;
theBooks[i].bookQtyCopy += addingQty;
return;
}
}
}
public int isAvailable(int sNo){
//returns the index number if available
for (int i=0; i<count; i++){
if (sNo == theBooks[i].sNo){
if(theBooks[i].bookQtyCopy > 0){
System.out.println("Book is Available.");
return i;
}
System.out.println("Book is Unavailable");
return -1;
}
}
System.out.println("No Book of Serial Number " + " Available in Library.");
return -1;
}
public book checkOutBook(){
System.out.println("Enter Serial No of Book to be Checked Out.");
int sNo = input.nextInt();
int bookIndex =isAvailable(sNo);
if (bookIndex!=-1){
//int bookIndex = isAvailable(sNo);
theBooks[bookIndex].bookQtyCopy--;
return theBooks[bookIndex];
}
return null;
}
public void checkInBook(book b){
for (int i=0; i<count; i++){
if (b.equals(theBooks[i]) ){
theBooks[i].bookQtyCopy++;
return;
}
}
}
}
package Library;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Font;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.util.Scanner;
import javax.swing.ImageIcon;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.Timer;
public class Library extends JFrame implements ActionListener{
private JPanel controlPanel;
private JButton btnAddBook, btnChangeBookQuantity, btnSearchBook,btnShowAllBooks,btnRegisterStudent,btnShowStudents,btnCheckOut,btnCheckIn,btnExit;
private int width, height;
private books ob;
private students obStudent;
private student s;
private book b;
public void GUI() {
width = 500;
height = 500;
setLayout(null); // layout for my frame
controlPanel = new JPanel(); // panel for buttons
btnAddBook = new JButton("Add Book"); // create button
btnChangeBookQuantity = new JButton("Change Book Quantity");
btnSearchBook = new JButton("Search Book");
btnShowAllBooks = new JButton("Show All Books");
btnRegisterStudent = new JButton("Register Student");
btnShowStudents = new JButton("Show Students");
btnCheckOut = new JButton("Check Out");
btnCheckIn = new JButton("Check In");
btnExit = new JButton("Exit");
// set the size and position of the panels
controlPanel.setBounds(0,0,height,width);
btnAddBook.setPreferredSize(new Dimension(200, 40));
btnChangeBookQuantity.setPreferredSize(new Dimension(200, 40));
btnSearchBook.setPreferredSize(new Dimension(200, 40));
btnShowAllBooks.setPreferredSize(new Dimension(200, 40));
btnRegisterStudent.setPreferredSize(new Dimension(200, 40));
btnShowStudents.setPreferredSize(new Dimension(200, 40));
btnCheckOut.setPreferredSize(new Dimension(200, 40));
btnCheckIn.setPreferredSize(new Dimension(200, 40));
btnExit.setPreferredSize(new Dimension(200, 40));
// add components to panels
controlPanel.add(btnAddBook); //button to control panel
controlPanel.add(btnSearchBook);
controlPanel.add(btnChangeBookQuantity);
controlPanel.add(btnShowAllBooks);
controlPanel.add(btnRegisterStudent);
controlPanel.add(btnShowStudents);
controlPanel.add(btnCheckOut);
controlPanel.add(btnCheckIn);
controlPanel.add(btnExit);
add(controlPanel); // add the button and drawing panels to the frame
// add button as a listener in this frame
btnAddBook.addActionListener(this);
btnChangeBookQuantity.addActionListener(this);
btnSearchBook.addActionListener(this);
btnShowAllBooks.addActionListener(this);
btnRegisterStudent.addActionListener(this);
btnShowStudents.addActionListener(this);
btnCheckOut.addActionListener(this);
btnCheckIn.addActionListener(this);
btnExit.addActionListener(this);
// set size and location of frame
setSize(width,height);
setLocation(100,100);
setVisible(true);
setDefaultCloseOperation(EXIT_ON_CLOSE);
}
public void actionPerformed (ActionEvent e){
//This method recieves an action from either button or the timer and executes the code for each. The start button starts the timer. The timer moves the pictures
// depending on what they rolled. The exit button quits the application. The restart button removes everything from the drawing panel and reinitializes the variables.
ob = new books();
obStudent = new students();
setVisible(false);
if (e.getSource()==btnAddBook){
b = new book();
ob.addBook(b);
}
else if (e.getSource()==btnChangeBookQuantity) {
ob.upgradeBookQty();
}
else if (e.getSource()==btnSearchBook) {
ob.searchByAuthorName();
}
else if(e.getSource()==btnShowAllBooks) {
ob.showAllBooks();
}
else if(e.getSource()==btnRegisterStudent) {
s = new student();
obStudent.addStudent(s);
}
else if (e.getSource()==btnShowStudents) {
obStudent.showAllStudents();
}
else if(e.getSource()==btnCheckOut) {
obStudent.checkOutBook(ob);
}
else if(e.getSource()==btnCheckIn) {
obStudent.checkInBook(ob);
}
else if (e.getSource()==btnExit) {
System.exit(0);
}
setVisible(true);
}
public static void main(String[] args) {
Library c = new Library();
c.GUI();
}
}
package Library;
import java.util.Scanner;
public class student {
String studentName;
String regNum;
book borrowedBooks[] = new book[3];
public String getStudentName() {
return studentName;
}
public void setStudentName(String studentName) {
this.studentName = studentName;
}
public String getRegNum() {
return regNum;
}
public void setRegNum(String regNum) {
this.regNum = regNum;
}
public book[] getBorrowedBooks() {
return borrowedBooks;
}
public void setBorrowedBooks(book[] borrowedBooks) {
this.borrowedBooks = borrowedBooks;
}
public int getBooksCount() {
return booksCount;
}
public void setBooksCount(int booksCount) {
this.booksCount = booksCount;
}
public Scanner getInput() {
return input;
}
public String toString() {
return getStudentName() + " " + getRegNum() + "\n";
}
public void setInput(Scanner input) {
this.input = input;
}
public int booksCount = 0;
Scanner input = new Scanner(System.in);
public student(){
String test = "";
System.out.println("Enter Student Name:");
test = input.nextLine();
this.setStudentName(test);
System.out.println("Enter Reg Number:");
test = input.nextLine();
this.setRegNum(test);
}
}
package Library;
import java.util.Scanner;
import javax.swing.JOptionPane;
public class students {
Scanner input = new Scanner(System.in);
student theStudents[] = new student[50];
public static int count = 0;
public void addStudent(student s){
for (int i=0; i<count; i++){
if(s.getRegNum().equalsIgnoreCase(theStudents[i].getRegNum())){
System.out.println("Student of Reg Num " + s.getRegNum() + " is Already Registered.");
return;
}
}
JOptionPane.showMessageDialog(null,count);
if (count<=50){
setTheStudents(s);
setCount(getCount() + 1);
}
System.out.print(theStudents[0].toString());
}
public void showAllStudents(){
System.out.println("Student Name\t\tReg Number");
for (int i=0; i<count; i++){
System.out.println(theStudents[i].toString());
}
}
public student[] getTheStudents() {
return theStudents;
}
public void setTheStudents(student theStudents) {
this.theStudents[count] = theStudents;
}
public static int getCount() {
return count;
}
public static void setCount(int count) {
students.count = count;
}
public int isStudent(){
//return index number of student if available
//System.out.println("Enter Student Name:");
//String getStudentName() = input.nextLine();
System.out.println("Enter Reg Number:");
String regNum = input.nextLine();
for (int i=0; i<count; i++){
if (theStudents[i].getRegNum().equalsIgnoreCase(regNum)){
return i;
}
}
System.out.println("Student is not Registered.");
System.out.println("Get Registered First.");
return -1;
}
public void checkOutBook(books book){
int studentIndex =this.isStudent();
if (studentIndex!=-1){
System.out.println("checking out");
book.showAllBooks();//jjjjjjjjjjjj
book b = book.checkOutBook();
System.out.println("checking out");
if (b!= null){
if (theStudents[studentIndex].getBooksCount()<=3){
System.out.println("adding book");
theStudents[studentIndex].getBorrowedBooks()[theStudents[studentIndex].getBooksCount()] = b;
theStudents[studentIndex].booksCount++;
return;
}
else {
System.out.println("Student Can not Borrow more than 3 Books.");
return;
}
}
System.out.println("Book is not Available.");
}
}
public void checkInBook(books book){
int studentIndex = this.isStudent();
if (studentIndex != -1){
System.out.println("S.No\t\t\tBook Name\t\t\tAuthor Name");
student s = theStudents[studentIndex];
for (int i=0; i<s.getBooksCount(); i++){
System.out.println(s.getBorrowedBooks()[i].sNo+ "\t\t\t" + s.getBorrowedBooks()[i].getBookName() + "\t\t\t"+
s.getBorrowedBooks()[i].getAuthorName());
}
System.out.println("Enter Serial Number of Book to be Checked In:");
int sNo = input.nextInt();
for (int i=0; i<s.getBooksCount(); i++){
if (sNo == s.getBorrowedBooks()[i].sNo){
book.checkInBook(s.getBorrowedBooks()[i]);
s.getBorrowedBooks()[i]=null;
return;
}
}
System.out.println("Book of Serial No "+sNo+"not Found");
}
}
}
单击showallstudents按钮时，它应该显示控制台中所有已注册的学生，但是显示此错误：
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at Library.students.showAllStudents(students.java:43)
at Library.Library.actionPerformed(Library.java:109)
at javax.swing.AbstractButton.fireActionPerformed(Unknown Source)
at javax.swing.AbstractButton$Handler.actionPerformed(Unknown Source)
at javax.swing.DefaultButtonModel.fireActionPerformed(Unknown Source)
at javax.swing.DefaultButtonModel.setPressed(Unknown Source)
at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(Unknown Source)
at java.awt.Component.processMouseEvent(Unknown Source)
at javax.swing.JComponent.processMouseEvent(Unknown Source)
at java.awt.Component.processEvent(Unknown Source)
at java.awt.Container.processEvent(Unknown Source)
at java.awt.Component.dispatchEventImpl(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.LightweightDispatcher.retargetMouseEvent(Unknown Source)
at java.awt.LightweightDispatcher.processMouseEvent(Unknown Source)
at java.awt.LightweightDispatcher.dispatchEvent(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Window.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
at java.awt.EventQueue.access$500(Unknown Source)
at java.awt.EventQueue$3.run(Unknown Source)
at java.awt.EventQueue$3.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(Unknown Source)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(Unknown Source)
at java.awt.EventQueue$4.run(Unknown Source)
at java.awt.EventQueue$4.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(Unknown Source)
at java.awt.EventQueue.dispatchEvent(Unknown Source)
at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.run(Unknown Source)