删除和排序 LinkedList 中的项目 - Java

Question

I have a LinkedList data structure I am using to store some items. This question is part of a course which requires me to use the standard java.utils LinkedList. The basic scenario is to be able to add in Books and Dictionaries to a LinkedList and then remove them. The question I am stuck on now requires sorting the LinkedList so that all the Dictionaries are at the end of the LinkedList and the Books are at the start.

Here is my current code for the sorting of items:

public static void moveDictionaries(Bookshelf b){
    int size = b.bookshelf.size();
    for(int i = 0; i<b.size(); i++) {
        Book temp = b.bookshelf.get(i);
        if(temp instanceof Dictionary){
            b.bookshelf.remove(temp);
            b.bookshelf.add(size - 1, temp);
        }
    }
}

A Dictionary is a subclass of Book. What I do is use a for loop to iterate through the list. I check whether the current item is a Dictionary, and if so, I add it to the end of the LinkedList, using the size as the position to add it into. I then remove the item from the LinkedList. Here is my Bookshelf class:

public class Bookshelf {

    List<Book> bookshelf = new LinkedList<Book>();

    public int size(){
        return bookshelf.size();
    }

    public void addBookOnLeftSide(Book b){
        bookshelf.add(b);
    }

    public void addBook(int i, Book b){
        bookshelf.add(i, b);
    }

    public Book remove(int i){
        Book temp = bookshelf.get(i);
        bookshelf.remove(i);
        return temp;
    }

    public void printLR(){
        for(int i = 0; i < bookshelf.size(); i++){
            System.out.println(bookshelf.get(i).toString());
        }
    }

    public void printRL(){
        for(int i = bookshelf.size() - 1; i>=0; i--){
            System.out.println(bookshelf.get(i).toString());
        }
    }
}

And lastly my Main class where i am testing it:

 public static void main(String[] args) {

        Book bookOne = new Book(101, "Hello");
        Book bookTwo = new Book(102, "Bye");
        Book bookThree = new Book(103, "Morning");
        Dictionary dictionaryOne = new Dictionary("104", "Afternoon", 10, "English", "Spanish", 10);
        Dictionary dictionaryTwo = new Dictionary("105", "Evening", 10, "English", "Spanish", 10);

        Bookshelf bookshelf = new Bookshelf();

        bookshelf.addBookOnLeftSide(dictionaryOne);
        bookshelf.addBookOnLeftSide(bookOne);
        bookshelf.addBookOnLeftSide(bookThree);
        bookshelf.addBookOnLeftSide(dictionaryTwo);
        bookshelf.addBookOnLeftSide(bookTwo);

        moveDictionaries(bookshelf);
        bookshelf.printLR();
    }

The problem I am having is that when I run this method, it does not work as expected. It only moves one of the Dictionaries and the other one somehow goes missing. I am not sure why this is happening. Any help would be great.

Answer 1

Consider your code:

for(int i = 0; i<b.size(); i++) {
    Book temp = b.bookshelf.get(i);
    if(temp instanceof Dictionary){
        b.bookshelf.remove(temp);
        b.bookshelf.add(size - 1, temp);
    }
}

Consider the cases where you have a bookshelf: B1, D1, D2, B3 and i = 1

In this situation D1 will be moved to the end and i will be incremented leaving a bookshelf of B1, D2, B3, D1 and i = 2

On the next iteration B3 will be considered, skipping D2 altogether.

Your code should look something like

int start = 0;
int end = bookshelf.size();
while (start < end) {
    if (bookshelf.get(start) instanceof Dictionary) {
        bookshelf.moveToEnd(start);
        end--;
    } else {
        start++;
    }
}