调用内部带有 for 循环的方法，而不是更新变量

Question

I am creating a bookstore for a lab in my computer science class. It uses three classes that interact with each other to create a bookstore that adds books, sells books, displays the library, and so on. I am using an array of objects. My issue comes when I'm adding multiple books to the library, then I try to access them to sell them. I think the issue lies with the "inStock" method that I use to determine if we have a specific book to sell. I'm not sure how to access all the books that I added to sell them, and I'm not sure if my method is the best way to do this.

Whenever I try to sell a book that isn't the first one that I added, the program claims that they do not have that book in stock, even though I can display all the books with a different method.

How could I get this method to detect all the listed books that I added with the inStock method?

        // Search for the book...if found, adjust the quantity.      
        // otherwise, Book not in the BookStore.
        for(int i = 0; i < totalbooks; i++) {
            if(title.equals(books[i].getTitle())) {
                if(quantity <= books[i].getQuantity()) {
                    return true;

                }
                else
                    return false;
            }
            else return false;

        }
        return false;
    }

//this is the inStock method^
public boolean sellBook(String title, int quantity) {
        // Checks to see if the books are in stock.
        for(int i = 0; i < totalbooks; i++) {
            if(title.equals(books[i].getTitle())) {
                if(quantity <= books[i].getQuantity()) {
                    gross = gross + books[i].getQuantity()*books[i].getPrice();
                    books[i].subtractQuantity(quantity);

                    return true;
                }
                else
                    return false;
            }
            else
                return false;
        }
        return false;
    }
//this is the method I use to sell the books

case 2: System.out.println("Please enter the book title.");
                title = input.next();
                System.out.println();

                //input.hasNext();
                if(!(bookstore.inStock(title, quant))) {
                    System.out.println("I'm sorry this book is not in stock.");
                }
                else {
                    System.out.println("How many copies would you like to buy?");
                    quant = input.nextInt();
                    if(bookstore.inStock(title, quant)) {
                    bookstore.sellBook(title, quant);
                    System.out.println("You just bought " + quant +" copies of " + title);
                    }
                    else
                        System.out.println("Error: Not enough copies in stock for your purchase."); break;
//this is a part of the demo class that I use to try to sell the book.

Answer 1

Check you conditions:

if(title.equals(books[i].getTitle())) {
    if(quantity <= books[i].getQuantity()) {
       return true;

    }
    else
       return false;
}

else return false; // <-- exits method after first instance of non-matching titles

It's typically bad practice to have multiple return statements in a method for reasons like this.

Answer 2

The issue is indeed in your inStock method - return statement immediately stops the execution of a method. There is no chance for other books apart from the first one to be checked for availability. In fact, you only need two return statements:

for (int i = 0; i < totalbooks; i++) {
    if(title.equals(books[i].getTitle())) {
        return quantity <= books[i].getQuantity();
    }
}
return false;