如何将新数据从数组列表写入文件底部？

Question

I'm trying to write data to a .txt file (stock.txt) from an array list. My program (a till system) loads the existing data perfectly (bar incrementing the initial value which tells the program how many IDs are in the file, still working on that) but I've ran into a problem.

The program loads the data into the program without a problem. When I call the save() method, it writes any new data to the top of stock.txt and deletes the increment number entirely, causing runtime errors on the next run. I have to manually delete the new data, add the increment number in and then run it for the program to work.

Here's the code for the save() method:

    public void save() throws IOException {
    // IMPLEMENTATION
    PrintWriter outfileStock = new PrintWriter(new FileWriter(SHOP_STOCK_DATA_FILE));
    outfileStock.println(identifier);
    outfileStock.println(name);
    outfileStock.println(cost);
    outfileStock.println(quanity);
    for (Item item : product) {
        outfileStock.println(item.getIdentifier());
        outfileStock.println(item.getName());
        outfileStock.println(item.getCost());
        outfileStock.println(item.getQuanity());
    }


    outfileStock.close();

And the code for the load() method:

    public void load() throws FileNotFoundException {

    //Load data for product (array list for stock)


    product.clear(); //clear arraylist to load data from stock.txt into it.
    Scanner infileStock = new Scanner(new FileReader(SHOP_STOCK_DATA_FILE));

        int num = infileStock.nextInt();
        infileStock.nextLine();
        for (int i = 0; i < num; i++) {
            String id = infileStock.nextLine();
            String n = infileStock.nextLine();
            int c = infileStock.nextInt();
            infileStock.nextLine();
            int q = infileStock.nextInt();

            //infileStock.nextLine(); deleted due to NoElementException error, error still occurring
            //infileStock.next(); throwing same error, same line. pls based stackoverflow bless me with answers

            infileStock.nextLine(); 

            //Place data from for loop into 1D array "p" via their identifiers from Item.java
            Item p = new Item(id, n, c, q);

            //Add products from 1D array above to a 2D array "product" (ArrayList) to store them temporarily for the shop to load
            product.add(p);

            //Human check to see where the load(); method runs to if an error occurs
            System.out.println("Stock Item Loaded");
            System.out.println(p);
        }

        //No more stock items to load? AWESOME. Close the file, you weren't born in a barn.
        infileStock.close();

The printed result, when loading the program, shows this:

Stock Item Loaded
1234, Steak pie, 399, 4
Stock Item Loaded
1235, Steak and kidney pie, 450, 2
Stock Item Loaded
1236, Chicken and mushroom pie, 389, 4
Stock Item Loaded
1237, Testpilot pie, 199, 9

and after adding in new data, it throws the error "NoSuchElementException" and the stock file looks like this (notice no increment number at the top, which is causing the error):

Stock Item Loaded
runtimeError pie, 299, 55, 1234
Stock Item Loaded
Steak pie, 399, 4, 1235
Stock Item Loaded
Steak and kidney pie, 450, 2, 1236
Stock Item Loaded
Chicken and mushroom pie, 389, 4, 1237

With the new pie being added to the top of the stock.txt file instead of the bottom as I'd like.

Answer 1

You need to use the FileWriter constructor that takes a boolean argument indicating whether to append to the file or not.

public FileWriter(String fileName, boolean append) throws IOException

Constructs a FileWriter object given a file name with a boolean indicating whether or not to append the data written.