Method to add unique object instance into an ArrayList is not working

Seems you got confused with what the expression you're evaluating in the IF statement is doing. You're if expression effectively says: "if bookTitle matches one in bookList (ie. is a duplicate)" Do X, else Y.

The "do X" branch says "Add the book" booklist.add(bookTitle) - when its a duplicate - which is wrong.

The "do Y" branch of your if says (corrected syntax): System.out.println("This book is not unique"); which has no effect on the bookList

Instead I've swapped the two branches X and Y -> Y and X So you should have:

    public void addBook(Book bookTitle){
      for(Book bookId : bookList) {
        if (bookTitle.equals(bookId)) {
          System.out.println("This book is not unique");
          break; // breaks out of the surrounding for-loop
          // stop checking books if we enter this branch
        } else {
          bookList.add(bookTitle);
          System.out.println("Book registered."); 
          // keep checking more books if we enter this branch
        }                
      } 
    }

Research tells us the more nested our code is the more likely it is for bugs to occur as the logic is harder to digest.

I've had a go at breaking it all apart and came up with:

import java.util.ArrayList;
import java.util.List;
import java.util.Objects;

class LibraryApplication {

    static Library library;

    public static void main(String[] args) {
        library = new Library();
        addBook(new Book("Title 1"));
        addBook(new Book("Title 1"));
        addBook(new Book("Title 2"));
        library.printAllBooks();
    }

    public static void addBook(Book bookTitle){
        if (library.isBookInLibrary(bookTitle)) {
            System.out.println("This book (" + bookTitle.getTitle() + ") is not unique");
        } else {
            library.add(bookTitle);
            System.out.println("Book " + bookTitle.getTitle() + " registered.");
        }
    }
}

class Book {

    private String title;

    public Book(String title) {
        this.title = title;
    }

    public String getTitle() {
        return title;
    }

    // Called by Library's printAllBooks method (library.toString())
    @Override
    public String toString() {
        return "Book{" +
                "title='" + title + '\'' +
                '}';
    }

    // auto generated by IDE (important for checking whether a collection contains an equal object)
    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (!(o instanceof Book)) return false;
        Book book = (Book) o;
        return title.equals(book.title);
    }

    // auto generated by IDE (important for checking whether a collection contains an equal object)
    @Override
    public int hashCode() {
        return Objects.hash(title);
    }

}

class Library {
    List<Book> library = new ArrayList<Book>();

    public boolean isBookInLibrary(Book book){
        return library.contains(book);
    }

    public void add(Book book) {
        library.add(book);
    }

    public void printAllBooks() {
        System.out.println(library.toString());
    }
}

Output:

Book Title 1 registered.
This book (Title 1) is not unique
Book Title 2 registered.
[Book{title='Title 1'}, Book{title='Title 2'}]


Process finished with exit code 0