我无法理解 ArrayList 如何将 Class 对象作为数据类型
I cannot understand how an ArrayList can have Class object as a datatype
import java.time.LocalDate;
import java.util.ArrayList;
class Books{
String bookName, authorName;
public Books(String bName, String aName){
this.authorName = aName;
this.bookName = bName;
}
@Override
public String toString(){
return "Book Details{Book name: "+bookName+", Author: "+authorName+"}";
}
}
public class Ex7_LibraryManagementSystem {
这是怎么回事？我是 java 的新手，所以我不太了解 ArrayList。我们是否正在创建一个具有 class 数据类型的 ArrayList？？？这部分是提前 Java 还是我需要再次修改我的基础知识？我对所有这些 class 作为参数传递的书感到困惑
ArrayList<Books> booksList;
Ex7_LibraryManagementSystem(ArrayList<Books> bookName){
this.booksList = bookName;
}
public void addBooks(Books b){
this.booksList.add(b);
System.out.println("Book Added Successfully");
}
public void issuedBooks(Books b,String issuedTo,String issuedOn){
if(booksList.isEmpty()){
System.out.println("All Books are issued.No books are available right now");
}
else{
if(booksList.contains(b))
{
this.booksList.remove(b);
System.out.println("Book "+b.bookName+" is issued successfully to "+issuedTo+" on "+issuedOn);
}
else{
System.out.println("Sorry! The Book "+b.bookName+" is already been issued to someone");
}
}
}
public void returnBooks(Books b, String returnFrom){
this.booksList.add(b);
System.out.println("Book is returned successfully from "+returnFrom+" to the Library");
}
public static void main(String[] args) {
另外请解释为什么我们要在下面创建 ArrayList
ArrayList<Books> book1 = new ArrayList<>();
LocalDate ldt = LocalDate.now();
Books b1 = new Books("Naruto","Kisishima");
book1.add(b1);
Books b2 = new Books("Naruto Shippuden","Kisishima");
book1.add(b2);
Books b3 = new Books("Attack On Titan","Bhaluche");
book1.add(b3);
Books b4 = new Books("Akame Ga Kill","Killer bee");
book1.add(b4);
Books b5 = new Books("Death Note","Light");
book1.add(b5);
Ex7_LibraryManagementSystem l = new Ex7_LibraryManagementSystem(book1);
// l.addBooks(new Books("Boruto","Naruto"));
l.issuedBooks(b3,"Sanan",ldt.getDayOfMonth()+"/"+ldt.getMonthValue()+"/"+ldt.getYear());
l.issuedBooks(b1,"Sandy",ldt.getDayOfMonth()+"/"+ldt.getMonthValue()+"/"+ldt.getYear());
// l.issuedBooks(b2,"Suleman",ldt.getDayOfMonth()+"/"+ldt.getMonthValue()+"/"+ldt.getYear());
// l.issuedBooks(b4,"Sanju",ldt.getDayOfMonth()+"/"+ldt.getMonthValue()+"/"+ldt.getYear());
// l.issuedBooks(b5,"Thor",ldt.getDayOfMonth()+"/"+ldt.getMonthValue()+"/"+ldt.getYear());
l.issuedBooks(b1,"anuj",ldt.getDayOfMonth()+"/"+ldt.getMonthValue()+"/"+ldt.getYear());
}
}
请帮助我...谢谢！
泛型
您问的是：
ArrayList<Books> booksList;
What is going on here? I'm new to java so I don't get the ArrayList that much. Are we creating an ArrayList with a class datatype???
您需要了解 Generics in Java。
ArrayList是集合，一种存放对象的数据结构。<Book>（修正你的用词不当后 Books）告诉编译器我们打算在这个特定集合中只存储 Book class 的对象。
如果我们错误地尝试将 Dog 对象或 Invoice 对象放入该集合，编译器会报错。您将在 compile-time 处收到一条错误消息，说明只有 Book class 的对象可以放入该集合。
此外，您可以放置来自 class 的对象，该 class 是 Book 的 subclass。想象一下，你有 HardCoverBook 和 SoftCoverBook class 都是从 Book class 扩展而来的。这些子 class 的对象也可以进入 Book 对象的集合。
其他问题
命名很重要。清晰的命名使您的代码更易于阅读和理解。
所以您的 class Books 描述了一本 单本 书。所以应该用单数命名，Book.
当收集一堆书籍对象时，例如 List，以复数形式命名该集合。例如 List < Book > books.
您的书 class 可以更简短地写成 record。我们可以缩短您的会员字段的名称。
record Book( String title, String author ) {}
我们可以将 Ex7_LibraryManagementSystem 缩短为 Library。
我们需要两个列表，而不是您代码中看到的那个。根据您的情况，我们希望在手头图书列表和借出图书列表之间移动图书。
更多命名：构造函数中的参数不应该是 bookName，它应该类似于 initialInventory。并且该参数的类型应该只是 Collection 而不是具体 ArrayList 甚至 List.
传入 Book 对象的集合时，将它们复制到我们的 internally-managed 列表中。我们不希望调用代码能够更改我们之间的集合。通过制作副本，我们掌控一切。
就此而言，您的书没有订购，因此不需要 List。如果书籍对象是唯一的，我们可以将它们收集为 Set 而不是 List — 但我会忽略这一点。
您的addBooks只添加了一本书，所以用单数重命名。
“问题”是个奇怪的词； “借”似乎更适合图书馆。同样，issuedBooks 方法可以使用更好的命名，包括不在 past-tense 中。使用 date-time classes 表示 date-time 值，例如 LocalDate 表示 date-only 值（没有 time-of-day，也没有时区或偏移量).标记这些参数 final 以避免在您的方法中意外更改它们。
loanBook ( final Book book , final String borrower , final LocalDate dateLoaned ) { … }
我建议检查不应该发生的情况，以确保一切正常。因此，与其假设一本书是借出的，不如核实一下。如果事情看起来不对劲，请报告。
经过这些更改后，我们有这样的东西。
package work.basil.example.lib;
import java.time.Instant;
import java.time.LocalDate;
import java.util.ArrayList;
import java.util.Collection;
import java.util.List;
import java.util.Set;
record Book( String title , String author )
{
}
public class Library
{
private List < Book > booksOnHand, booksOnLoan;
Library ( Collection < Book > initialInventory )
{
this.booksOnHand = new ArrayList <>( initialInventory );
this.booksOnLoan = new ArrayList <>( this.booksOnHand.size() );
}
public void addBook ( Book b )
{
this.booksOnHand.add( b );
System.out.println( "Book added successfully." );
}
public void loanBook ( final Book book , final String borrower , final LocalDate dateLoaned )
{
if ( this.booksOnHand.isEmpty() )
{
System.out.println( "All Books are issued. No books are available right now." );
}
else
{
if ( this.booksOnHand.contains( book ) )
{
this.booksOnHand.remove( book );
this.booksOnLoan.add( book );
System.out.println( "Book " + book.title() + " by " + book.author() + " is loaned to " + borrower + " on " + dateLoaned );
}
else if ( this.booksOnLoan.contains( book ) )
{
System.out.println( "Sorry! The Book " + book.title() + " by " + book.author() + " is out on loan." );
}
else
{
System.out.println( "ERROR – We should never have reached this point in the code. " );
}
}
}
public void returnBook ( Book book , String returnFrom )
{
if ( this.booksOnLoan.contains( book ) )
{
this.booksOnLoan.remove( book );
this.booksOnHand.add( book );
System.out.println( "The Book " + book.title() + " by " + book.author() + " has been returned to the Library." );
}
else
{
System.out.println( "The Book " + book.title() + " by " + book.author() + " is not out on loan, so it cannot be returned to the Library." );
}
}
public String reportInventory ( )
{
StringBuilder report = new StringBuilder();
report.append( "On hand: " + this.booksOnHand );
report.append( "\n" );
report.append( "On load: " + this.booksOnLoan );
return report.toString();
}
public static void main ( String[] args )
{
List < Book > stockOfBooks =
List.of(
new Book( "Naruto" , "Kisishima" ) ,
new Book( "Naruto Shippuden" , "Kisishima" ) ,
new Book( "Attack On Titan" , "Bhaluche" ) ,
new Book( "Akame Ga Kill" , "Killer bee" ) ,
new Book( "Death Note" , "Light" )
);
Book b1 = stockOfBooks.get( 0 ), b2 = stockOfBooks.get( 1 ), b3 = stockOfBooks.get( 2 );
Library library = new Library( stockOfBooks );
library.loanBook( b3 , "Sanan" , LocalDate.now() );
library.loanBook( b1 , "Sandy" , LocalDate.now().plusDays( 1 ) );
library.loanBook( b2 , "anuj" , LocalDate.now().plusDays( 2 ) );
library.returnBook( b1 , "Sandy" );
System.out.println( library.reportInventory() );
}
}
当运行.
Book Attack On Titan by Bhaluche is loaned to Sanan on 2022-04-19
Book Naruto by Kisishima is loaned to Sandy on 2022-04-20
Book Naruto Shippuden by Kisishima is loaned to anuj on 2022-04-21
The Book Naruto by Kisishima has been returned to the Library.
On hand: [Book[title=Akame Ga Kill, author=Killer bee], Book[title=Death Note, author=Light], Book[title=Naruto, author=Kisishima]]
On load: [Book[title=Attack On Titan, author=Bhaluche], Book[title=Naruto Shippuden, author=Kisishima]]
import java.time.LocalDate;
import java.util.ArrayList;
class Books{
String bookName, authorName;
public Books(String bName, String aName){
this.authorName = aName;
this.bookName = bName;
}
@Override
public String toString(){
return "Book Details{Book name: "+bookName+", Author: "+authorName+"}";
}
}
public class Ex7_LibraryManagementSystem {
这是怎么回事？我是 java 的新手，所以我不太了解 ArrayList。我们是否正在创建一个具有 class 数据类型的 ArrayList？？？这部分是提前 Java 还是我需要再次修改我的基础知识？我对所有这些 class 作为参数传递的书感到困惑
ArrayList<Books> booksList;
Ex7_LibraryManagementSystem(ArrayList<Books> bookName){
this.booksList = bookName;
}
public void addBooks(Books b){
this.booksList.add(b);
System.out.println("Book Added Successfully");
}
public void issuedBooks(Books b,String issuedTo,String issuedOn){
if(booksList.isEmpty()){
System.out.println("All Books are issued.No books are available right now");
}
else{
if(booksList.contains(b))
{
this.booksList.remove(b);
System.out.println("Book "+b.bookName+" is issued successfully to "+issuedTo+" on "+issuedOn);
}
else{
System.out.println("Sorry! The Book "+b.bookName+" is already been issued to someone");
}
}
}
public void returnBooks(Books b, String returnFrom){
this.booksList.add(b);
System.out.println("Book is returned successfully from "+returnFrom+" to the Library");
}
public static void main(String[] args) {
另外请解释为什么我们要在下面创建 ArrayList
ArrayList<Books> book1 = new ArrayList<>();
LocalDate ldt = LocalDate.now();
Books b1 = new Books("Naruto","Kisishima");
book1.add(b1);
Books b2 = new Books("Naruto Shippuden","Kisishima");
book1.add(b2);
Books b3 = new Books("Attack On Titan","Bhaluche");
book1.add(b3);
Books b4 = new Books("Akame Ga Kill","Killer bee");
book1.add(b4);
Books b5 = new Books("Death Note","Light");
book1.add(b5);
Ex7_LibraryManagementSystem l = new Ex7_LibraryManagementSystem(book1);
// l.addBooks(new Books("Boruto","Naruto"));
l.issuedBooks(b3,"Sanan",ldt.getDayOfMonth()+"/"+ldt.getMonthValue()+"/"+ldt.getYear());
l.issuedBooks(b1,"Sandy",ldt.getDayOfMonth()+"/"+ldt.getMonthValue()+"/"+ldt.getYear());
// l.issuedBooks(b2,"Suleman",ldt.getDayOfMonth()+"/"+ldt.getMonthValue()+"/"+ldt.getYear());
// l.issuedBooks(b4,"Sanju",ldt.getDayOfMonth()+"/"+ldt.getMonthValue()+"/"+ldt.getYear());
// l.issuedBooks(b5,"Thor",ldt.getDayOfMonth()+"/"+ldt.getMonthValue()+"/"+ldt.getYear());
l.issuedBooks(b1,"anuj",ldt.getDayOfMonth()+"/"+ldt.getMonthValue()+"/"+ldt.getYear());
}
}
请帮助我...谢谢！
泛型
您问的是：
ArrayList<Books> booksList;What is going on here? I'm new to java so I don't get the ArrayList that much. Are we creating an ArrayList with a class datatype???
您需要了解 Generics in Java。
ArrayList是集合，一种存放对象的数据结构。<Book>（修正你的用词不当后Books）告诉编译器我们打算在这个特定集合中只存储Bookclass 的对象。
如果我们错误地尝试将 Dog 对象或 Invoice 对象放入该集合，编译器会报错。您将在 compile-time 处收到一条错误消息，说明只有 Book class 的对象可以放入该集合。
此外，您可以放置来自 class 的对象，该 class 是 Book 的 subclass。想象一下，你有 HardCoverBook 和 SoftCoverBook class 都是从 Book class 扩展而来的。这些子 class 的对象也可以进入 Book 对象的集合。
其他问题
命名很重要。清晰的命名使您的代码更易于阅读和理解。
所以您的 class Books 描述了一本 单本 书。所以应该用单数命名，Book.
当收集一堆书籍对象时，例如 List，以复数形式命名该集合。例如 List < Book > books.
您的书 class 可以更简短地写成 record。我们可以缩短您的会员字段的名称。
record Book( String title, String author ) {}
我们可以将 Ex7_LibraryManagementSystem 缩短为 Library。
我们需要两个列表，而不是您代码中看到的那个。根据您的情况，我们希望在手头图书列表和借出图书列表之间移动图书。
更多命名：构造函数中的参数不应该是 bookName，它应该类似于 initialInventory。并且该参数的类型应该只是 Collection 而不是具体 ArrayList 甚至 List.
传入 Book 对象的集合时，将它们复制到我们的 internally-managed 列表中。我们不希望调用代码能够更改我们之间的集合。通过制作副本，我们掌控一切。
就此而言，您的书没有订购，因此不需要 List。如果书籍对象是唯一的，我们可以将它们收集为 Set 而不是 List — 但我会忽略这一点。
您的addBooks只添加了一本书，所以用单数重命名。
“问题”是个奇怪的词； “借”似乎更适合图书馆。同样，issuedBooks 方法可以使用更好的命名，包括不在 past-tense 中。使用 date-time classes 表示 date-time 值，例如 LocalDate 表示 date-only 值（没有 time-of-day，也没有时区或偏移量).标记这些参数 final 以避免在您的方法中意外更改它们。
loanBook ( final Book book , final String borrower , final LocalDate dateLoaned ) { … }
我建议检查不应该发生的情况，以确保一切正常。因此，与其假设一本书是借出的，不如核实一下。如果事情看起来不对劲，请报告。
经过这些更改后，我们有这样的东西。
package work.basil.example.lib;
import java.time.Instant;
import java.time.LocalDate;
import java.util.ArrayList;
import java.util.Collection;
import java.util.List;
import java.util.Set;
record Book( String title , String author )
{
}
public class Library
{
private List < Book > booksOnHand, booksOnLoan;
Library ( Collection < Book > initialInventory )
{
this.booksOnHand = new ArrayList <>( initialInventory );
this.booksOnLoan = new ArrayList <>( this.booksOnHand.size() );
}
public void addBook ( Book b )
{
this.booksOnHand.add( b );
System.out.println( "Book added successfully." );
}
public void loanBook ( final Book book , final String borrower , final LocalDate dateLoaned )
{
if ( this.booksOnHand.isEmpty() )
{
System.out.println( "All Books are issued. No books are available right now." );
}
else
{
if ( this.booksOnHand.contains( book ) )
{
this.booksOnHand.remove( book );
this.booksOnLoan.add( book );
System.out.println( "Book " + book.title() + " by " + book.author() + " is loaned to " + borrower + " on " + dateLoaned );
}
else if ( this.booksOnLoan.contains( book ) )
{
System.out.println( "Sorry! The Book " + book.title() + " by " + book.author() + " is out on loan." );
}
else
{
System.out.println( "ERROR – We should never have reached this point in the code. " );
}
}
}
public void returnBook ( Book book , String returnFrom )
{
if ( this.booksOnLoan.contains( book ) )
{
this.booksOnLoan.remove( book );
this.booksOnHand.add( book );
System.out.println( "The Book " + book.title() + " by " + book.author() + " has been returned to the Library." );
}
else
{
System.out.println( "The Book " + book.title() + " by " + book.author() + " is not out on loan, so it cannot be returned to the Library." );
}
}
public String reportInventory ( )
{
StringBuilder report = new StringBuilder();
report.append( "On hand: " + this.booksOnHand );
report.append( "\n" );
report.append( "On load: " + this.booksOnLoan );
return report.toString();
}
public static void main ( String[] args )
{
List < Book > stockOfBooks =
List.of(
new Book( "Naruto" , "Kisishima" ) ,
new Book( "Naruto Shippuden" , "Kisishima" ) ,
new Book( "Attack On Titan" , "Bhaluche" ) ,
new Book( "Akame Ga Kill" , "Killer bee" ) ,
new Book( "Death Note" , "Light" )
);
Book b1 = stockOfBooks.get( 0 ), b2 = stockOfBooks.get( 1 ), b3 = stockOfBooks.get( 2 );
Library library = new Library( stockOfBooks );
library.loanBook( b3 , "Sanan" , LocalDate.now() );
library.loanBook( b1 , "Sandy" , LocalDate.now().plusDays( 1 ) );
library.loanBook( b2 , "anuj" , LocalDate.now().plusDays( 2 ) );
library.returnBook( b1 , "Sandy" );
System.out.println( library.reportInventory() );
}
}
当运行.
Book Attack On Titan by Bhaluche is loaned to Sanan on 2022-04-19
Book Naruto by Kisishima is loaned to Sandy on 2022-04-20
Book Naruto Shippuden by Kisishima is loaned to anuj on 2022-04-21
The Book Naruto by Kisishima has been returned to the Library.
On hand: [Book[title=Akame Ga Kill, author=Killer bee], Book[title=Death Note, author=Light], Book[title=Naruto, author=Kisishima]]
On load: [Book[title=Attack On Titan, author=Bhaluche], Book[title=Naruto Shippuden, author=Kisishima]]