Java 更改嵌套循环中静态类变量的值
在嵌套循环中更改类变量的值时，我遇到了一个问题-我不知道为什么。我猜这是因为变量是静态的。但它是一个静态方法，因为它用于从文件中列出系统中的用户，所以它必须是静态的（我从main方法调用它来将文件读取到TreeMaps）。不可能从方法中重写静态类变量吗？如果可能的话，我到底做错了什么Java 更改嵌套循环中静态类变量的值,java,loops,object,static,instance,Java,Loops,Object,Static,Instance,在嵌套循环中更改类变量的值时，我遇到了一个问题-我不知道为什么。我猜这是因为变量是静态的。但它是一个静态方法，因为它用于从文件中列出系统中的用户，所以它必须是静态的（我从main方法调用它来将文件读取到TreeMaps）。不可能从方法中重写静态类变量吗？如果可能的话，我到底做错了什么 public class Loan{ protected int noOfLoans; protected int noOfReturns; protected User user=new User(); pro
public class Loan{
protected int noOfLoans;
protected int noOfReturns;
protected User user=new User();
protected static Book book= new Book();
protected Map <Integer, Book> currentLoans=new TreeMap <Integer, Book>();
protected Map <Integer, Book> returned=new TreeMap <Integer, Book>();
protected static Map<Integer, Loan> loanList=new TreeMap<Integer, Loan>();
public static void main(String[] args){
readLoans();
}
public static void readLoans(){
loanList.clear();
BufferedReader reader = null;
try {
reader=new BufferedReader(new FileReader("loans.txt"));
} catch (FileNotFoundException e) {
e.printStackTrace();
}
String line = null;
try {
line = reader.readLine();
} catch (IOException e) {
e.printStackTrace();
}
while (line!=null) {
String[] splitOut=line.split("-");
String[] loan_User=splitOut[0].split(",");
String[] loan_CurrentLoans=splitOut[2].split(",");
String[] loan_Returned=splitOut[4].split(",");
Loan loan = new Loan();
loan.user.setFirstName(loan_User[0]);
loan.user.setSurname(loan_User[1]);
loan.user.setPersonalID(loan_User[2]);
for (int i = 1; i <= Integer.parseInt(splitOut[1]); i++) {
book.setName(loan_CurrentLoans[((Integer.parseInt
(splitOut[1])-1)*4)]);
book.setAuthorFirstname(loan_CurrentLoans[((Integer.parseInt
(splitOut[1])-1)*4)+1]);
book.setAuthorSurname(loan_CurrentLoans[((Integer.parseInt
(splitOut[1])-1)*4)+2]);
book.setISBN(loan_CurrentLoans[((Integer.parseInt
(splitOut[1])-1)*4)+3]);
loan.currentLoans.put(i, book);
}
for (int i = 1; i <= Integer.parseInt(splitOut[3]); i++) {
book.setName(loan_Returned[((Integer.parseInt
(splitOut[3])-1)*4)]);
book.setAuthorFirstname(loan_Returned[((Integer.parseInt
(splitOut[3])-1)*4)+1]);
book.setAuthorSurname(loan_Returned[((Integer.parseInt
(splitOut[3])-1)*4)+2]);
book.setISBN(loan_Returned[((Integer.parseInt
(splitOut[3])-1)*4)+3]);
loan.returned.put(i, book);
}
loan.setNoOfLoans(Integer.parseInt(splitOut[1]));
loan.setNoOfReturns(Integer.parseInt(splitOut[3]));
loanList.put(loanList.size()+1, loan);
try {
line=reader.readLine();
} catch (IOException e) {
e.printStackTrace();
}
}
try {
reader.close();
} catch (IOException e) {
e.printStackTrace();
}
}
}
当前贷款：
1.作者的书（1234567890123）
2.作者的另一本书（2345678901234）
归还贷款：
1.作者归还的书（3456789012345）
当前贷款：
1.作者的书（1234567890123）
2.作者的书（1234567890123）
归还贷款：
1.作者的书（1234567890123）
作者归还的书（3456789012345）
作者归还的书（3456789012345）
这让我相信我实际上无法创建静态Book对象的实例，但必须使其非静态，并尝试在方法中创建对象的实例。如果是这样的话——我该怎么做呢？从这里开始，很难理解你怎么能像现在这样理解，同时又会如此困惑。我不是想侮辱你，只是想说我根本不知道你在哪里
使用
new新的书本分配给它，然后设置字段。问题不在于您的书本是静态的，而更简单的是，每次在循环中更改它时，它都是同一个对象。之所以会发生这种情况，是因为您将其声明为静态场，但您的想法是错误的
让我们把问题简化，用这本书来说明：
class AnObject {
int aValue;
}
而不是IO，只需循环几次并将其添加到列表中：
class PersistenceOfChangesDemo {
static List<AnObject> theList = new ArrayList<AnObject>();
public static void main(String[] args) {
AnObject theObject = new AnObject();
for(int i = 1; i <= 3; i++) {
/* reassign the object's value */
theObject.aValue = i;
/* adds the same object each time */
theList.add(theObject);
}
/* theList is now of size 3
* but all its elements refer to the same object (theObject) */
for(AnObject anObject : theList) {
/* prints '3' every time
* because that was the last value assigned */
System.out.println(anObject.aValue);
/* prints 'true' every time */
System.out.println(anObject == theObject);
}
}
}
ChangesDemo的类持久性{
静态列表列表=新的ArrayList（）；
公共静态void main（字符串[]args）{
AnObject theObject=新的AnObject（）；
对于（inti=1；i这只是参考问题。这三个都是指同一个对象静态书籍
，因此表示最后插入的相同细节
更改只是创建Book（）
的新对象，而不是对不同的细节使用相同的对象
试试下面的代码
Loan loan = new Loan();
loan.user.setFirstName(loan_User[0]);
loan.user.setSurname(loan_User[1]);
loan.user.setPersonalID(loan_User[2]);
for (int i = 1; i <= Integer.parseInt(splitOut[1]); i++) {
book = new Book(); // added this line
book.setName(loan_CurrentLoans[((Integer.parseInt
(splitOut[1])-1)*4)]);
book.setAuthorFirstname(loan_CurrentLoans[((Integer.parseInt
(splitOut[1])-1)*4)+1]);
book.setAuthorSurname(loan_CurrentLoans[((Integer.parseInt
(splitOut[1])-1)*4)+2]);
book.setISBN(loan_CurrentLoans[((Integer.parseInt
(splitOut[1])-1)*4)+3]);
loan.currentLoans.put(i, book);
}
for (int i = 1; i <= Integer.parseInt(splitOut[3]); i++) {
book = new Book(); // added this line
book.setName(loan_Returned[((Integer.parseInt
(splitOut[3])-1)*4)]);
book.setAuthorFirstname(loan_Returned[((Integer.parseInt
(splitOut[3])-1)*4)+1]);
贷款=新贷款（）；
loan.user.setFirstName（loan_user[0]）；
loan.user.setSurname（loan_user[1]）；
loan.user.setPersonalID（loan_user[2]）；
对于（inti=1；i我是如何解决的
public static void readLoans(){
// Reads the bookList and userList.
readBooks();
readUsers();
// Creates a new BufferedReader and tries to read "loans.txt"
BufferedReader reader = null;
try {
reader=new BufferedReader(new FileReader("loans.txt"));
}
// Catches exception if "books.txt" does not exist.
catch (FileNotFoundException e) {
e.printStackTrace();
}
String line = null;
// tries to read the first line and interpret it as a String.
try {
line = reader.readLine();
}
// Catches IOexception if any is thrown when trying to read line.
catch (IOException e) {
e.printStackTrace();
}
// Loop as long as "line" is not empty, i.e. as long as a Loan is read.
while (line!=null) {
// split the String "line" at every RegEx "-"
String[] splitOut=line.split("-");
// Create a String from the first index of the first split.
String user = splitOut[0];
/* Split the second and third index of the first split and create
* new Stringarrays from them.*/
String[] loans = splitOut[1].split(",");
String[] returns = splitOut[2].split(",");
User aUser = new User();
/* Find the user in the userList whose personal ID matches the
* String "user" that we created. This is the user that we want to
* create (a) loan/s and/or (a) returned loan/s for.*/
for (int i = 1; i < userList.size()+1; i++) {
if (userList.get(i).getPersonalID().equals(user)) {
/*Set the variables for the User.*/
aUser.setFirstname(userList.get(i).getFirstname());
aUser.setSurname(userList.get(i).getSurname());
aUser.setPersonalID(userList.get(i).getPersonalID());
aUser.setTelephone(userList.get(i).getTelephone());
aUser.setLoans(userList.get(i).getLoans());
aUser.setReturns(userList.get(i).getReturns());
// Create an ArrayList for Loans and Returns for every user
ArrayList<Loan> listOfloans = new ArrayList<Loan>();
ArrayList<Loan> listOfreturns = new ArrayList<Loan>();
// if the new user has any loans...
for (int j = 0; j < aUser.getLoans(); j++) {
for (int k = 1; k < bookList.size()+1; k++) {
/* ... find the "Book" object with the
* corresponding ISBN...*/
if (bookList.get(k).getIsbn().equals(loans[j*3])) {
// ...then create a new loan object for each...
Loan loan = new Loan();
// ...and set the variables of each loan...
loan.setTitle(bookList.get(k).getTitle());
loan.setAuthor_firstname(bookList.get(k).
getAuthor_firstname());
loan.setAuthor_surname(bookList.get(k).
getAuthor_surname());
try {
loan.setIsbn(bookList.get(k).getIsbn());
} catch (Exception e) {
e.printStackTrace();
}
loan.setMaxLoan(bookList.get(k).getMaxLoan());
loan.setOnLoan(bookList.get(k).getOnLoan());
loan.setAvailable(bookList.get(k).
getAvailable());
loan.setSignature(loans[j*3+1]);
loan.setTimestamp(loans[j*3+2]);
/* ...then add each one to the "listOfloans"
* ArrayList.*/
listOfloans.add(loan);
}
}
}
/* if the "listOfloans" ArrayList is not empty,
* add the loan to loanList with User as Key.*/
if (!listOfloans.isEmpty()) {
loanList.put(aUser, listOfloans);
}
// if the new user has any returned loans...
for (int j = 0; j < aUser.getReturns(); j++) {
for (int k = 1; k < bookList.size()+1; k++) {
/* ... find the "Book" object with the
* corresponding ISBN...*/
if(bookList.get(k).getIsbn().equals(returns[j*4])){
// ...then create a new loan object for each...
Loan loan = new Loan();
// ...and set the variables of each loan...
loan.setTitle(bookList.get(k).getTitle());
loan.setAuthor_firstname(bookList.get(k).
getAuthor_firstname());
loan.setAuthor_surname(bookList.get(k).
getAuthor_surname());
try {
loan.setIsbn(bookList.get(k).getIsbn());
} catch (Exception e) {
e.printStackTrace();
}
loan.setMaxLoan(bookList.get(k).getMaxLoan());
loan.setOnLoan(bookList.get(k).getOnLoan());
loan.setAvailable(bookList.get(k)
.getAvailable());
loan.setSignature(returns[j*4+1]);
loan.setTimestamp(returns[j*4+2]);
loan.setReturndate(returns[j*4+3]);
/* ...then add each one to the "listOfreturns"
* ArrayList.*/
listOfreturns.add(loan);
}
}
}
/* if the "listOfreturns" ArrayList is not empty,
* add the returned loan to returnList with User as Key.*/
if (!listOfreturns.isEmpty()) {
returnList.put(aUser, listOfreturns);
}
}
}
// tries to read the next line and interpret it as a String.
try {
line=reader.readLine();
}
// Catches IOexception if any is thrown when trying to read line.
catch (IOException e) {
e.printStackTrace();
}
}
// try to close the BufferedReader.
try {
reader.close();
}
// Catches IOexception if any is thrown when trying to close.
catch (IOException e) {
e.printStackTrace();
}
}
publicstaticvoidreadloans（）{
//读取bookList和userList。
阅读书籍（）；
readUsers（）；
//创建新的BufferedReader并尝试读取“loans.txt”
BufferedReader reader=null；
试一试{
reader=newbufferedreader（newfilereader（“loans.txt”）；
}
//如果“books.txt”不存在，则捕获异常。
catch（filenotfounde异常）{
e、 printStackTrace（）；
}
字符串行=null；
//尝试读取第一行并将其解释为字符串。
试一试{
line=reader.readLine（）；
}
//捕获IOexception，如果在尝试读取行时抛出任何异常。
捕获（IOE异常）{
e、 printStackTrace（）；
}
//循环，只要“行”不为空，即只要读取贷款。
while（行！=null）{
//在每个正则表达式“-”处拆分字符串“line”
String[]splitOut=line.split（“-”）；
//从第一次拆分的第一个索引创建字符串。
字符串user=splitOut[0]；
/*拆分第一次拆分的第二个和第三个索引并创建
*新的StringArray将从中删除*/
String[]loans=splitOut[1]。split（“，”）；
字符串[]返回=splitOut[2]。split（，“”）；
用户aUser=新用户（）；
/*在userList中查找其个人ID与
*我们创建的字符串“user”。这是我们要创建的用户
*为创建（a）贷款和/或（a）退回的贷款*/
对于（int i=1；ifor (int i = 1; i <= Integer.parseInt(splitOut[1]); i++) {
Book newBook = new Book();
newBook.setName(loan_CurrentLoans[((Integer.parseInt
(splitOut[1])-1)*4)]);
newBook.setAuthorFirstname(loan_CurrentLoans[((Integer.parseInt
(splitOut[1])-1)*4)+1]);
newBook.setAuthorSurname(loan_CurrentLoans[((Integer.parseInt
(splitOut[1])-1)*4)+2]);
newBook.setISBN(loan_CurrentLoans[((Integer.parseInt
(splitOut[1])-1)*4)+3]);
loan.currentLoans.put(i, newBook);
}
Loan loan = new Loan();
loan.user.setFirstName(loan_User[0]);
loan.user.setSurname(loan_User[1]);
loan.user.setPersonalID(loan_User[2]);
for (int i = 1; i <= Integer.parseInt(splitOut[1]); i++) {
book = new Book(); // added this line
book.setName(loan_CurrentLoans[((Integer.parseInt
(splitOut[1])-1)*4)]);
book.setAuthorFirstname(loan_CurrentLoans[((Integer.parseInt
(splitOut[1])-1)*4)+1]);
book.setAuthorSurname(loan_CurrentLoans[((Integer.parseInt
(splitOut[1])-1)*4)+2]);
book.setISBN(loan_CurrentLoans[((Integer.parseInt
(splitOut[1])-1)*4)+3]);
loan.currentLoans.put(i, book);
}
for (int i = 1; i <= Integer.parseInt(splitOut[3]); i++) {
book = new Book(); // added this line
book.setName(loan_Returned[((Integer.parseInt
(splitOut[3])-1)*4)]);
book.setAuthorFirstname(loan_Returned[((Integer.parseInt
(splitOut[3])-1)*4)+1]);
public static void readLoans(){
// Reads the bookList and userList.
readBooks();
readUsers();
// Creates a new BufferedReader and tries to read "loans.txt"
BufferedReader reader = null;
try {
reader=new BufferedReader(new FileReader("loans.txt"));
}
// Catches exception if "books.txt" does not exist.
catch (FileNotFoundException e) {
e.printStackTrace();
}
String line = null;
// tries to read the first line and interpret it as a String.
try {
line = reader.readLine();
}
// Catches IOexception if any is thrown when trying to read line.
catch (IOException e) {
e.printStackTrace();
}
// Loop as long as "line" is not empty, i.e. as long as a Loan is read.
while (line!=null) {
// split the String "line" at every RegEx "-"
String[] splitOut=line.split("-");
// Create a String from the first index of the first split.
String user = splitOut[0];
/* Split the second and third index of the first split and create
* new Stringarrays from them.*/
String[] loans = splitOut[1].split(",");
String[] returns = splitOut[2].split(",");
User aUser = new User();
/* Find the user in the userList whose personal ID matches the
* String "user" that we created. This is the user that we want to
* create (a) loan/s and/or (a) returned loan/s for.*/
for (int i = 1; i < userList.size()+1; i++) {
if (userList.get(i).getPersonalID().equals(user)) {
/*Set the variables for the User.*/
aUser.setFirstname(userList.get(i).getFirstname());
aUser.setSurname(userList.get(i).getSurname());
aUser.setPersonalID(userList.get(i).getPersonalID());
aUser.setTelephone(userList.get(i).getTelephone());
aUser.setLoans(userList.get(i).getLoans());
aUser.setReturns(userList.get(i).getReturns());
// Create an ArrayList for Loans and Returns for every user
ArrayList<Loan> listOfloans = new ArrayList<Loan>();
ArrayList<Loan> listOfreturns = new ArrayList<Loan>();
// if the new user has any loans...
for (int j = 0; j < aUser.getLoans(); j++) {
for (int k = 1; k < bookList.size()+1; k++) {
/* ... find the "Book" object with the
* corresponding ISBN...*/
if (bookList.get(k).getIsbn().equals(loans[j*3])) {
// ...then create a new loan object for each...
Loan loan = new Loan();
// ...and set the variables of each loan...
loan.setTitle(bookList.get(k).getTitle());
loan.setAuthor_firstname(bookList.get(k).
getAuthor_firstname());
loan.setAuthor_surname(bookList.get(k).
getAuthor_surname());
try {
loan.setIsbn(bookList.get(k).getIsbn());
} catch (Exception e) {
e.printStackTrace();
}
loan.setMaxLoan(bookList.get(k).getMaxLoan());
loan.setOnLoan(bookList.get(k).getOnLoan());
loan.setAvailable(bookList.get(k).
getAvailable());
loan.setSignature(loans[j*3+1]);
loan.setTimestamp(loans[j*3+2]);
/* ...then add each one to the "listOfloans"
* ArrayList.*/
listOfloans.add(loan);
}
}
}
/* if the "listOfloans" ArrayList is not empty,
* add the loan to loanList with User as Key.*/
if (!listOfloans.isEmpty()) {
loanList.put(aUser, listOfloans);
}
// if the new user has any returned loans...
for (int j = 0; j < aUser.getReturns(); j++) {
for (int k = 1; k < bookList.size()+1; k++) {
/* ... find the "Book" object with the
* corresponding ISBN...*/
if(bookList.get(k).getIsbn().equals(returns[j*4])){
// ...then create a new loan object for each...
Loan loan = new Loan();
// ...and set the variables of each loan...
loan.setTitle(bookList.get(k).getTitle());
loan.setAuthor_firstname(bookList.get(k).
getAuthor_firstname());
loan.setAuthor_surname(bookList.get(k).
getAuthor_surname());
try {
loan.setIsbn(bookList.get(k).getIsbn());
} catch (Exception e) {
e.printStackTrace();
}
loan.setMaxLoan(bookList.get(k).getMaxLoan());
loan.setOnLoan(bookList.get(k).getOnLoan());
loan.setAvailable(bookList.get(k)
.getAvailable());
loan.setSignature(returns[j*4+1]);
loan.setTimestamp(returns[j*4+2]);
loan.setReturndate(returns[j*4+3]);
/* ...then add each one to the "listOfreturns"
* ArrayList.*/
listOfreturns.add(loan);
}
}
}
/* if the "listOfreturns" ArrayList is not empty,
* add the returned loan to returnList with User as Key.*/
if (!listOfreturns.isEmpty()) {
returnList.put(aUser, listOfreturns);
}
}
}
// tries to read the next line and interpret it as a String.
try {
line=reader.readLine();
}
// Catches IOexception if any is thrown when trying to read line.
catch (IOException e) {
e.printStackTrace();
}
}
// try to close the BufferedReader.
try {
reader.close();
}
// Catches IOexception if any is thrown when trying to close.
catch (IOException e) {
e.printStackTrace();
}
}