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分享NullPointerException的问题
//下面是Book类
public class Book implements Comparable{
private String bookname;
private String author;
private String press;
private String date;
private int price;
public Book (){
}
public Book (String bookname,String author,String press,String date,int price) {
this.bookname = bookname;
this.author = author;
this.press = press;
this.date = date;
this.price = price;
}
public void setBookname (String bookname) {
this.bookname = bookname;
}
public void setAuthor (String author) {
this.author = author;
}
public void setPress (String press) {
this.press = press;
}
public void setDate(String date) {
this.date = date;
}
public void setPrice(int price) {
this.price = price;
}
public String getBookname() {
return bookname;
}
public String getAuthor() {
return author;
}
public String getpress() {
return press;
}
public int getPrice() {
return price;
}
public String toString(){
return bookname+" "+author+" "+press+" "+date+" "+price;
}
public int compareTo(Object o){
Book b = (Book)o;
if((this.bookname.compareTo(b.bookname))>0){
return 1;
}
else if((this.bookname.compareTo(b.bookname))==0){
return 0;
}
else return -1;
}
public static void main(String[] args){
Book myBook= new Book("ff","yy","love you","haha",712);
System.out.println(myBook);
}
}
//下面是SchoolBag类
import java.util.*;
public class SchoolBag {
private TreeSet<Book> booklist = new TreeSet<Book>();
public SchoolBag() {
}
public SchoolBag (Book book) {
booklist.add(book);
}
public ArrayList<Integer> getAllPrice(){
ArrayList<Integer> bookprices = new ArrayList<Integer> ();
for(Book element: booklist) {
bookprices.add(element.getPrice());
}
return bookprices;
}
public boolean add(Book book) {
return booklist.add(book);
}
public <E extends TreeSet<Book>> void setBooklist(E treeBook){
this.booklist = treeBook;
} //泛型方法，将泛型放在函数放回类型之前
public TreeSet<Book> getBooklist(){
return booklist;
}
public boolean remove(Book book) {
if(booklist.isEmpty()) {
System.out.println("Warning ,SchoolBag is Empty!");
return false;
}
else{
booklist.remove(book);
return true;
}
}
public static void main(String[] args){
Book mybook = new Book("xx","ff","geg","fege",12);
Book herbook = new Book("ss","ll","hh","rr",712);
//测试构造方法
SchoolBag sb = new SchoolBag(mybook);
//测试add
sb.add(herbook);
System.out.println(sb.booklist);
TreeSet<Book> bl = sb.getBooklist();
System.out.println(bl);
//测试remove
sb.remove(mybook);
System.out.println(bl);
//测试setBooklist
Book hisbook = new Book("hh","ss","nn","gg",464);
Book itsbook = new Book("ff","hh","KK","tt",375);
TreeSet<Book> b2 = new TreeSet<Book>();
b2.add(hisbook);
b2.add(itsbook);
sb.setBooklist(b2);
System.out.println("-----------------------");
System.out.println(sb.booklist);
//测试printAllPrice
ArrayList al = sb.getAllPrice();
System.out.println(al);
}
}
import java.util.*;
import java.io.*;
public class BookManager {
private SchoolBag schoolBag = new SchoolBag();
public BookManager(){
}
public BookManager(SchoolBag s){
}
public void readData() {
try {
File data = new File("BookList.txt");
FileReader fr = new FileReader(data);
BufferedReader br = new BufferedReader(fr);
String[] content = new String[5];
/*
int rowNum = 0;
String str;
while((str = br.readLine())!=null){
rowNum++;
}
Book[] bookFromData = new Book[rowNum];
int numBook=0;
String s1;
while((s1 = br.readLine())!=null){
System.out.println(s1);
content = s1.trim().split(",|，");
bookFromData[numBook].setBookname(content[0].trim());
bookFromData[numBook].setAuthor(content[1].trim());
bookFromData[numBook].setPress(content[2].trim());
bookFromData[numBook].setDate(content[3].trim());
bookFromData[numBook].setPrice(Integer.parseInt(content[4].trim()));
schoolBag.add(bookFromData[numBook]);
System.out.println(bookFromData[numBook]);
numBook++;
}
*/
Book[] book = new Book[7];
for(int i=0;i<7;i++){
book[i] = new Book();
}
int i=0;
String str;
while((str = br.readLine())!=null){
content = str.trim().split(",|，");
//测试Str
System.out.println(str);
book[i++].setBookname(content[0].trim());
book[i++].setAuthor(content[1].trim());
book[i++].setPress(content[2].trim());
book[i++].setDate(content[3].trim());
book[i++].setPrice(Integer.parseInt(content[4].trim()));
schoolBag.add(book[i++]);
}
br.close();
}catch(IOException e) {
e.printStackTrace();
}
}
public void print() {
System.out.println("-------------------------------------------------------");
System.out.println("书名 作者 出版号 出版日期 价格 ");
System.out.println("-------------------------------------------------------");
TreeSet<Book> treeBook = schoolBag.getBooklist();
for(Book book:treeBook){
System.out.println(book.toString());
}
}
public static void main(String[] args) {
BookManager bm = new BookManager();
bm.readData();
bm.print();
}
}
...全文
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daizhe 2011-12-16
- 打赏
- 举报
Book.java的55行compareTo()方法出的错，加个断点调试一下，看看this.bookname、b、b.bookname这几谁是null，就知道哪里错了
脉动 2011-12-16
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import java.util.*;
import java.io.*;
public class BookManager {
private SchoolBag schoolBag = new SchoolBag();
public BookManager(){
}
public BookManager(SchoolBag s){
}
public void readData() {
try {
File data = new File("BookList.txt");
FileReader fr = new FileReader(data);
BufferedReader br = new BufferedReader(fr);
String[] content = new String[5];
/*
int rowNum = 0;
String str;
while((str = br.readLine())!=null){
rowNum++;
}
Book[] bookFromData = new Book[rowNum];
int numBook=0;
String s1;
while((s1 = br.readLine())!=null){
System.out.println(s1);
content = s1.trim().split(",|，");
bookFromData[numBook].setBookname(content[0].trim());
bookFromData[numBook].setAuthor(content[1].trim());
bookFromData[numBook].setPress(content[2].trim());
bookFromData[numBook].setDate(content[3].trim());
bookFromData[numBook].setPrice(Integer.parseInt(content[4].trim()));
schoolBag.add(bookFromData[numBook]);
System.out.println(bookFromData[numBook]);
numBook++;
}
*/
Book[] book = new Book[7];
for(int i=0;i<7;i++){
book[i] = new Book();
}
int i=0;
String str;
while((str = br.readLine())!=null){
content = str.trim().split(",|，");
//测试Str
System.out.println(str);
book[i].setBookname(content[0].trim());
book[i].setAuthor(content[1].trim());
book[i].setPress(content[2].trim());
book[i].setDate(content[3].trim());
book[i].setPrice(Integer.parseInt(content[4].trim()));
schoolBag.add(book[i]);
i++; //添加该语句，并将上面几句的i++改为i后，测试运行正常;不过代码未改之前，测试是ArrayIndexOutOfBoundsException异常
}
br.close();
}catch(IOException e) {
e.printStackTrace();
}
}
public void print() {
System.out.println("-------------------------------------------------------");
System.out.println("书名 作者 出版号 出版日期 价格 ");
System.out.println("-------------------------------------------------------");
TreeSet<Book> treeBook = schoolBag.getBooklist();
for(Book book:treeBook){
System.out.println(book.toString());
}
}
public static void main(String[] args) {
BookManager bm = new BookManager();
bm.readData();
bm.print();
}
}
Yori Fang 2011-12-16
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- 举报
运行的时候总是抛出：NullPointer异常
Java 2大全，Herbert Schildt,电子工业出版社， 2003/01, 86
Exception in thread "main" java.lang.NullPointerException
at Book.compareTo(Book.java:55)
at java.util.TreeMap.compare(TreeMap.java:1188)
at java.util.TreeMap.put(TreeMap.java:531)
at java.util.TreeSet.add(TreeSet.java:255)
at SchoolBag.add(SchoolBag.java:22)
at BookManager.readData(BookManager.java:61)
at BookManager.main(BookManager.java:83)
Java 2大全，Herbert Schildt,电子工业出版社， 2003/01, 86
Exception in thread "main" java.lang.NullPointerException
at Book.compareTo(Book.java:55)
at java.util.TreeMap.compare(TreeMap.java:1188)
at java.util.TreeMap.put(TreeMap.java:531)
at java.util.TreeSet.add(TreeSet.java:255)
at SchoolBag.add(SchoolBag.java:22)
at BookManager.readData(BookManager.java:61)
at BookManager.main(BookManager.java:83)
Yori Fang 2011-12-16
- 打赏
- 举报
下面是BookList.txt的内容
Java 2大全，Herbert Schildt,电子工业出版社， 2003/01, 86
面向对象软件工程，Stephen R.Schach,机械工业出版社，2002/08，56
软件工程，Roger S. Pressman,清华大学出版社，2001/01，42
Design Patterns，Erich Gamma，机械工业出版社，2002/09，98
软件工程，张海藩，人民邮电出版社，2002/07，37
现代软件工程，孙涌，北京希望电子出版社，2002/08，42
面向对象技术UML教程,王少锋，清华大学出版社，2004/02,54
Java 2大全，Herbert Schildt,电子工业出版社， 2003/01, 86
面向对象软件工程，Stephen R.Schach,机械工业出版社，2002/08，56
软件工程，Roger S. Pressman,清华大学出版社，2001/01，42
Design Patterns，Erich Gamma，机械工业出版社，2002/09，98
软件工程，张海藩，人民邮电出版社，2002/07，37
现代软件工程，孙涌，北京希望电子出版社，2002/08，42
面向对象技术UML教程,王少锋，清华大学出版社，2004/02,54
