ArrayList的ArrayList

Question

for(int i=0;i<Stockhouse.total_stocks;i++){
        buyers.clear();

        for(int j=0; j<buy.size();j++){
        if(buy.get(j).stock_Id.equals("Stock"+i)){
            System.out.println("Agent "+buy.get(j).ID+",stock"+buy.get(j).stock_Id+",price "+buy.get(j).price +",quant "+buy.get(j).quantity);
            Auctionhouse buypr= new Auctionhouse(buy.get(j).price,buy.get(j).quantity);
            buyers.add(buypr);
        }
    }
        total_buy.add(buyers);
        System.out.println("buy size "+total_buy.get(i).size());
    }

here the buy size shows me correctly the size of each ArrayList as an elements of total_buy ArrayList.

My problem is that after when i need these sizes again,i only get the size of the last ArrayList.

  total_buy.get(0).size()=3
  total_buy.get(1).size()=3
  total_buy.get(2).size()=3.......

but only total_buy.get(2).size()=3 is actually 3!

Answer 1

You are reusing the same object, and the reference is getting added to the total_buy list. Instead of

buyers.clear();

do

buyers = new ArrayList<Auctionhouse>();

This will create a new object for buyers every time, and therefore keep updated references in the total_buy list.