CODEWITHSUNDEEP
javaarraylist
Phan Khuong
Phan Khuong

Reputation: 51

ArrayList just add the last element for all positions

This is my whole code, it's quite complex but please help me. It's taken me for 2 days but I failed:

public static ArrayList<DocGia> XuatDocGia() throws IOException {
    ArrayList<DocGia> listDocGia = new ArrayList<>();
    File fileDocGia = new File("fileDocGia.txt");
    if(fileDocGia.exists() == false) {
        System.out.println("Chưa có đọc giả nào trong thư viện");
    } else {
        BufferedReader br = new BufferedReader(new FileReader("fileDocGia.txt"));     
        if (br.readLine() == null) {
            System.out.println("Chưa có đọc giả nào trong thư viện");
        } else {
            int soDong = DemSoDong("fileDocGia.txt");
            int dongHienTai = 0;
            Scanner fileScanner = new Scanner(fileDocGia);
            for(int i = 0, z = 0;;i++, z++) {
                DocGia docGia = null;
                System.out.println("***Đọc giả thứ: " + (i+1));
                docGia.tendocgia = fileScanner.nextLine();
                if(i >= 1) {
                    docGia.tendocgia = fileScanner.nextLine();
                }
                docGia.maDocGia = fileScanner.nextLine();
                docGia.soSachmuon = fileScanner.nextInt();
                docGia.thoiGianMuonSach = fileScanner.nextInt();
                listDocGia.add(docGia);
                docGia.XuatDocGia();
                dongHienTai += 4;
                if(dongHienTai == soDong) {
                    fileScanner.close();
                    break;
                }
            }
        }

    for(DocGia docGia: listDocGia) {
        docGia.XuatDocGia();
    }
  }
  return listDocGia;
}

look at my code, when i run:

docGia.XuatDocGia();

-> the value of every single element is right at debug. it also means the value of the variable inside is right. but at the end of this function. i run

for(DocGia docGia: listDocGia) {
    docGia.XuatDocGia();
}

this is XuatDocGia funtion:

    public static void XuatDocGia(){
    System.out.println(tendocgia);
    System.out.println(maDocGia);
    System.out.println(soSachmuon);
    System.out.println(thoiGianMuonSach);
}

It just shows for me the last element in this ArrayList, repeat in 3 times( equal the number of elements).

I think a problem come from adding process of listDocGia.add(docGia);

You guys no need to bother everything else in my code, because i know it's really complex. I have tested carefully, just focus on my problem.

I'm so sorry because i'm Vietnamese and beginner at Java. The next time everything will be English. Thank you so much.

Upvotes: 0

Views: 99

Answers (1)

Eran
Eran

Reputation: 393801

If this is the actual code, you are adding null references to your List, but since you are using a static method to print the values, you don't get a NullPointerException. Assuming your code passes compilation, this means all the members of the DocGia class are static, which explains why you get the same values in each iteration of your loop.

You should change

DocGia docGia = null;

to

DocGia docGia = new DocGia ();

and change all the members of DocGia (including the XuatDocGia method that prints them) to be non static.

Upvotes: 1

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