I cannot understand how an ArrayList can have Class object as a datatype

Question

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 {

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??? Does this part falls in Advance Java or do I need to revise my basics again? I'm confused with all these class Books passing as an argument thing

    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) {

Also please Explain why are we creating this below 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());




    }
}

Please Help Me...Thank you!

Answer 1

Generics

You asked:

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???

You need to learn about Generics in Java.

• ArrayList is collection, a data structure for holding objects.
• <Book> (after fixing your misnomer Books) is telling the compiler that we intend to store only objects of the Book class in this particular collection.

If we mistakenly try to put a Dog object or an Invoice object into that collection, the compiler will complain. You will get an error message at compile-time explaining that only objects of the Book class can be put into that collection.

Also, you can put objects that are from a class that is a subclass of Book. Imagine you had HardCoverBook and SoftCoverBook classes that both extend from the Book class. Objects of those subclasses can also go into a collection of Book objects.

Other issues

Naming is important. Clear naming makes your code easier to read and comprehend.

So your class Books describes a single book. So it should be named in the singular, Book.

When collecting a bunch of book objects, such as a List, name that collection in the plural. For example List < Book > books.

Your book class could be more briefly written as a record. And we could shorten the names of your member fields.

record Book( String title, String author ) {}

We could shorten Ex7_LibraryManagementSystem to Library.

We need two lists rather than the one seen in your code. Given your scenario, we want to move books between a list for books on hand and a list of books loaned out.

More naming: The argument in your constructor should not be bookName, it should be something like initialInventory. And the type of that parameter should be simply Collection rather than specifically ArrayList or even List.

When passing in a collection of Book objects, copy them into our internally-managed lists. We don’t want the calling code to be able to change the collection between our back. By making a copy, we take control.

For that matter, your books are not ordered, so no need for List. If the book objects are meant to be unique, we can collect them as Set rather than List — but I'll ignore that point.

Your addBooks adds only a single book, so rename in the singular.

"Issue" is an odd term; "loan" seems more appropriate to a library. Similarly, the issuedBooks method could use better naming, including not being in past-tense. Use date-time classes to represent date-time values, such as LocalDate for a date-only value (without time-of-day, and without time zone or offset). Mark those arguments final to avoid accidentally changing them in your method.

loanBook ( final Book book , final String borrower , final LocalDate dateLoaned ) { … }

I recommend checking for conditions that should never happen, to make sure all is well. So rather than assume a book is on loan, verify. If things seem amiss, report.

After those changes, we have something like this.

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() );
    }
}

When run.

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]]