How to remove an item from an ArrayList based in parameter

Question

I am learning java and have been asked to create a 'Library' that stores books. Books should have a title, author and id. Which gets added using the method below addBook().

After the book is added, I want to be able to remove any book in the list that has a matching ID. If the ID matches it should be remove the title, author and ID from the array, not just the ID.

I have tried an number of different ways, including the solution below and looking at iterators. However, I only seem to be able to remove the ID of the book, or all the books in the lists!

I am learning, so I would be grateful for any solution to also explain why - I have been racking my head around it for days and I am super stuck!

Here is my code....

import java.util.ArrayList;
import java.util.Iterator;

/**
 * Library class that represents a library 
 * where books will be stored.
 */
 
public class Library
{
    // The list os books that is owned by the library
    private ArrayList<Book> books;
    // The name of the library
    private String name;
    

    /**
     * Constructor for Library objects. This 
     * constructor sets the name of the library.
     */
     
    public Library(String aName)
    {
        // Gives the library its name
        name = aName;
        // Initialises book field to an empty collection
        books = new ArrayList<>();
    }  
    
    /* 
     * Add a book to the library by adding its
     * author, its title and and id.
     */
    public void addBook(String author, String title, String id)
    {
        /** Add a new book to the Book list that
         *  includes its author, title and id
         */
        books.add(new Book(author,title,id));
    }

    // Remove book from library
    public void removeBook(String id)
    {
         for(Book book : books){
            // When the ID given, matches an ID
            // getId() is a getter method created in a seperate book class
            if (book.getId() == id) {
                // If that book is available
                books.remove(book);
                }
            // If statements don't match, do nothing
            else {
                // Do nothing
            }
        }
        

    }

}

"I only seem to be able to remove the ID of the book, or all the books in the lists!" < That honestly doesn't make much sense. Does your Book class contain any static fields? — OH GOD SPIDERS, Nov 19 '21 at 16:08
Also, `book.getId() == id` will not work to compare Strings. You should never compare String with `==`, see [How do I compare strings in Java?](https://stackoverflow.com/questions/513832/how-do-i-compare-strings-in-java) — OH GOD SPIDERS, Nov 19 '21 at 16:12
And the way you remove list element will throw `ConcurrentModificationException`. — PM 77-1, Nov 19 '21 at 16:16
Does this answer your question? [Remove elements from collection while iterating](https://stackoverflow.com/questions/10431981/remove-elements-from-collection-while-iterating) — PM 77-1, Nov 19 '21 at 16:17
Thanks everyone, that has really helped! The articles on .equals() and iteration really helped clear things up. — Yound Puddle Duck, Nov 24 '21 at 14:12

score 0 · Accepted Answer · answered Nov 20 '21 at 12:19

0

Use Collection::removeIf method and make sure to use equals/equalsIgnoreCase or even Objects::equals when comparing String objects:

public void removeBook(String id) {
    books.removeIf(book -> Objects.equals(id, book.getId()));
}

answered Nov 20 '21 at 12:19

Nowhere Man

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Thanks Alex, that helped solved the problem! – Yound Puddle Duck Nov 24 '21 at 14:11

How to remove an item from an ArrayList based in parameter

1 Answers1