将项添加到ArrayList的LinkedList大小错误

Question

Okey, in my program I created LinkedList of ArrayLists:

 List<Marker> list1 = new ArrayList<Marker>();
 LinkedList<List<Marker>> list2 = new LinkedList<List<Marker>>();

Everything works fine while adding items to list1: list1.add(mark);

But when I try to add my list1 to list2 ( list2.add(list1) ), I am getting strange things: each list size in list2 becomes equals to last added list size+1, and i can't even read these lists items with (for example) list2.getFirst().get(0); - doing this I got error;

For example I am doing this:

mark.addSomeInfo();
list1.add(mark);
list2.add(list1);
mark.addSomeInfo();
list1.add(mark);
list2.add(list1);
mark.addSomeInfo();
list1.add(mark);
list2.add(list1);

And I need to get this:

list2.get(0).size() = 1;
list2.get(1).size() = 2;
list2.get(2).size() = 3;

But instead I am getting this:

list2.get(0).size() = 4;
list2.get(1).size() = 4;
list2.get(2).size() = 4;

Why? What I am doing wrong?

Answer 1

The LinkedList<List<Marker>> list2 add method does not create a copy of the list you added into it. It only adds a reference of list1 to list2 . So in essence your list2 holds 3 references to the same list.

What you want to do is create the copies yourself as you are placing them into list2

mark.addSomeInfo();
list1.add(mark);
list2.add(new ArrayList<Mark>(list1));
mark.addSomeInfo();
list1.add(mark);
list2.add(new ArrayList<Mark>(list1));
mark.addSomeInfo();
list1.add(mark);
list2.add(new ArrayList<Mark>(list1));

Answer 2

All the reference of LinkedList refers to same List Object which is list1 so it prints size 4.

As you used ArrayList which supports duplicate element so every time you add mark object it will make a duplicate entry.

Answer 3

List2中的引用是针对List1的。这就是为什么所有相​​同的SOP给出相同的结果4

Answer 4

list1 is just a reference in your code.So in essence, you are adding the same arraylist object to the linkedlist again and again. And in the end the same arraylist is copied 3 times in your example and has size 4.

Answer 5

At 0th,1st and 2nd position of list2, you have added same list1 object. which is being changed by

 list1.add(mark);

As the above function has been called four times in your code.Hence list1 reference now points to the object on heap which has 4 items, and hence size of all 3 items of list1 has 4 elements.

Answer 6

mark.addSomeInfo();
list1.add(mark);  //this gets added to list each time you add it --1
list2.add(list1); //this gets added too as many times as you add list1
mark.addSomeInfo();
list1.add(mark);  // 2
list2.add(list1);  //2
mark.addSomeInfo();
list1.add(mark);  // 3
list2.add(list1);  // 3

So after this you will have 3 elements in list1 and 3 lists in list2