Trouble with object array

Question

I am geting a null pointer exception when i try to run this method, the goal is to populate the booklist object array but not over 3 objects. The error occurs when i set booklist[0] = b

private Book [] booklist;
public boolean borrowBook(Book b)
{
    if(booklist == null)
    {
        booklist[0] = b;
        System.out.println(this.name+" has successfully borrowed "+b);
        return true;
    }
    if(booklist.length < 3)
    {
        booklist[booklist.length] = b;
        System.out.println(this.name+" has successfully borrowed "+b);
        return true;
    }
    System.out.println(this.name+" has reached the borrowing limit! Return those books "+this.name);
    return false;

I am not that knowledgeable with object arrays, can i initialize it and have my borrowBook method work the way it is? — Tolkingdom
– Tolkingdom, Commented Oct 1, 2014 at 2:08
of course if booklist == null you cannot access booklist[0] — Baby
– Baby, Commented Oct 1, 2014 at 2:08

DnR · Accepted Answer · 2014-10-01 02:18:39Z

4

You need ArrayList instead of array

ArrayList<Book> booklist = new ArrayList<Book>();

public boolean borrowBook(Book b){
    if(booklist.size() == 0){
        booklist.add(b);
        System.out.println(this.name+" has successfully borrowed "+b);
        return true;
    }
    if(booklist.size() < 3){ //I'm not sure what you are trying to achieve here
        booklist.add(booklist.size(), b);
        System.out.println(this.name+" has successfully borrowed "+b);
        return true;
    }

    System.out.println(this.name+" has reached the borrowing limit! Return those books "+this.name);
    return false;
}

answered Oct 1, 2014 at 2:18

DnR

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Comments

wittakarn · Accepted Answer · 2014-10-01 03:10:15Z

3

Try this.

private Book [] booklist;

public boolean borrowBook(Book b) {
    if (booklist == null) {
        booklist = new Book[3];
        booklist[0] = b;
        return true;
    }
    for (int i = 0; i < booklist.length; i++) {
        if (booklist[i] == null) {
            booklist[i] = b;
            return true;
        }
    }
    return false;
}

public static void main(String[] args) {
    Book caller = new Book();
    System.out.println(caller.borrowBook(new Book()));
    System.out.println(caller.borrowBook(new Book()));
    System.out.println(caller.borrowBook(new Book()));
    System.out.println(caller.borrowBook(new Book()));
}

edited Oct 1, 2014 at 3:10

answered Oct 1, 2014 at 2:25

wittakarn

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1 Comment

Tolkingdom Over a year ago

Thanks for this! , Other answers were all right , but my teacher expects me to use just a normal array for this project. Thanks everyone , and thank you wittakarn

wittakarn · Accepted Answer · 2014-10-01 02:09:45Z

1

Likewise, @August said. Before line booklist[0] = b;

You must initial by using booklist = new Book[3];

answered Oct 1, 2014 at 2:09

wittakarn

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3 Comments

Tolkingdom Over a year ago

Would my method still work at not letting it store more than 3 books into this array?

Tolkingdom Over a year ago

how can i check how many non-null objects are in booklist. By initializing it to length 3 , if(booklist.length < 3) will not work. what way can i check

wittakarn Over a year ago

I post my new answer, you can entertain. Good luck.

Elliott Frisch · Accepted Answer · 2014-10-01 02:17:49Z

Java arrays are not dynamically sized, it's possible to write a method to do it. For example,

private static Book[] addBook(Book[] arr, Book b) {
    int newLen = (arr == null ? 1 : arr.length + 1);
    Book[] dest = new Book[newLen];
    System.arraycopy(arr, 0, dest, 0, arr == null ? 0 : arr.length);
    dest[newLen - 1] = b;
    return dest;
}

Then you could use it with something like

public boolean borrowBook(Book b) {
    if (booklist == null || booklist.length < 3) {
        booklist = addBook(booklist, b);
        System.out.println(this.name + " has successfully borrowed " + b);
        return true;
    }
    System.out.println(this.name + " has reached the borrowing limit! "
            + "Return those books " + this.name);
    return false;
}

Alternatively, you could just allocate an array of size n or use a collection like an ArrayList.

August · Accepted Answer · 2014-10-01 02:18:35Z

1

Initializing booklist will fix the NullPointerException, but your method's logic will still be flawed. It appears that you want to append a new book to the array of books checked out, whereas your method will only set the last element in the array.

To fix this, you could use a counter that stores the index of the last book inserted. Or, I suggest using a growable array such as ArrayList, which supports appending new elements with ArrayList#add.

answered Oct 1, 2014 at 2:18

August

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