基本Java库，搜索方法不起作用

Question

As a part of my Odysee on learning Java, we got a Task in which we have to implement the class Book and the class Library. You should store up to 10 books in one library also you should have the possibility to search for books in the library. Now my problem is that my search method doesn't work. Maybe someone here has an idea, this is my code:

public class Library {
    int Capacity = 10;
    int Volume = 0;
    Book[] storage = new Book[10];

    public Library() {
        System.out.println("Hello, I am a library, which can store up to 10 books!");
        this.storage = new Book[10];
    }

    public void add(Book book) {
        if (Volume < Capacity) {
            this.storage[Volume] = book;
            System.out.println("I added the book " + book + ".");
            Volume++;
        } else if (Volume >= Capacity) System.out.println("The library is full!");

    }

    public Book search(String title) {
        String result = new String();
        for (int i = 0; i < this.Volume; i++) {
            if (title.equals(this.storage[i].toString())) {
                System.out.println("The book with the title " + title + " exists in the library!");
                result = this.storage[i].toString();
            } else {
                System.out.println("The book with the title " + title + " does not exist in the library!");
                return null;
            }
        }
        Book retBook = new Book(result);
        return retBook;
    }
}


public class Book {
    String title;

    public Book(String title){
        this.title = title;
        System.out.println("Book " + title + " created.");}

    public String toString(){
        return this.title;
    };
}

Thanks for any help!

Answer 1

Your problem is here:

   for (int i = 0; i < this.Volume; i++) {
        ...
        } else {
            System.out.println("The book with the title " + title + " does not exist in the library!");
            // *** THIS LINE IS WRONG ***
            return null;
        }

Here you are trying to loop through all of your books to find the one that matches (that is what the loop is for isn't it?). Sadly, what this actually does is it returns null on the first book that doesn't match.

You need something like this:

public Book search(String title) {
    Book found = null;
    for (int i = 0; i < this.Volume && book == null; i++) {
        if (title.equals(this.storage[i].toString())) {
            System.out.println("The book with the title " + title + " exists in the library!");
            found = this.storage[i];
        }
    }
    // Check if we found it AFTER the loop completes.
    if (found == null) {
        System.out.println("The book with the title " + title + " does not exist in the library!");
    }
    return found;
}

Note here how we check to see if we've found the book after we've looked at all of the books (or we found it).

Answer 2

You must return the book when it's found, and null only if none is found:

public Book search(String title) {
    for (int i = 0; i < this.Volume; i++) {
        if (title.equals(this.storage[i].toString())) {
            System.out.println("The book with the title " + title + " exists in the library!");
            return this.storage[i];
        }
    }
    System.out.println("The book with the title " + title + " does not exist in the library!");
    return null;
}