Unable to understand the behavior of following program ( Using java collection)

Question

I have following program in which I am adding few number to set and list and then removing them, Could some one please explain why Set and list have different behavior.

public class SetList {
public static void main(String[] args){
    Set<Integer> set = new TreeSet<Integer>();
    List<Integer> list = new ArrayList<Integer>();
    for(int i=-3;i<3;i++){
        set.add(i);
        list.add(i);
    }
    for(int i=0;i<3;i++){
        set.remove(i);
        list.remove(i);
    }
    System.out.println(set+"            "+list);
}

}

and output is

[-3, -2, -1]            [-2, 0, 2]

I am able to understand the behavior of Set but unable to understand the behavior of List output. All help really appreciated.

Answer 1

Set and List are different types of collections . Set is an associative collection, thus Set.remove(i) will remove the element having the value of i . While List is an indexed collection, so List.remove(i) removes the element at the i th position in the list.

So after removing the elements 0 to 3 from a Set containing elements of -3 ... 3, your Set will predictably contain the values -3 to -1.

With the List, the outcome of the same sequence of removals may be a bit more surprising, but it is actually logical. Originally your list contains:

Index  0  1  2  3  4  5  6
Value -3 -2 -1  0  1  2  3

list.remove(0) removes the element at index 0, resulting in

Index  0  1  2  3  4  5
Value -2 -1  0  1  2  3

Notice that all elements after the (removed) first have shifted one position forward! Thus, when list.remove(1) removes the element at index 1, it "hops over" the element -2. The result is

Index  0  1  2  3  4
Value -2  0  1  2  3

Similarly the next operation, list.remove(2) "hops over" the element 0, resulting in

Index  0  1  2  3
Value -2  0  2  3

And last, list.remove(3) removes the last element, giving the end result:

Index  0  1  2
Value -2  0  2

Answer 2

调用Set.remove（int）时，java会将参数自动装箱为Integer，但是存在一个List.remove（int），该列表按其索引删除值。