我的ArrayList会发生什么

Question

I am trying to understand Java.

Let's assume I have an ArrayList of size 50 and pre-populated with some names.

Let's assume I remove 3rd and 4th element from the array list. What will happen to my array list? Will it get rearranged? Will it return null if I try to access the now-deleted 3rd and 4th elements?

Answer 1

No, the elements after the one you are going to delete will be shifted to the left (expensive operation), so you won't have any hole.

As as side note: if you remove the 3rd element then the 5th element will be shifted to the left, so if afterwards you remove the 4th, you are instead removing the fifth of the starting collection. To remove two consecutive elements you should provide the same index twice.

Answer 2

They will be rearranged and shifted.

If you want them to return null instead, just set those elements you want to remove to null explicitly rather than removing them.

Answer 3

Why didn't you try it yourself?

List<String> list = new ArrayList<String>();
list.add("A");
list.add("B");
list.add("C");
list.add("D");
list.add("E");
list.add("F");
list.add("G");

for(int i = 0; i < list.size(); i++) System.out.println("index " + i + ": " + list.get(i));

System.out.println();
list.remove(0); // remove "A"

for(int i = 0; i < list.size(); i++) System.out.println("index " + i + ": " + list.get(i));

OUTPUT:

index 0: A
index 1: B
index 2: C
index 3: D
index 4: E
index 5: F
index 6: G

index 0: B
index 1: C
index 2: D
index 3: E
index 4: F
index 5: G

Answer 4

You actually have both options available:

final List<Character> x = new ArrayList<Character>(asList('a', 'b', 'c', 'd'));
x.set(1, null); // removes an element without shifting
x.remove(0);    // removes an element with shifting
System.out.println(x);

Prints

[null, c, d]

Answer 5

数组列表元素将重新排列

Answer 6

An ArrayList is a consecutive list of items that can be referenced by an index. So when you delete an item, all following items will be shifted.

Answer 7

The elements will be shifted.

See the javadoc for ArrayList remove:

java.util.ArrayList
public E remove(int index)
Removes the element at the specified position in this list. Shifts any subsequent elements to the left (subtracts one from their indices).
Specified by:
remove in interface List
Overrides:
remove in class AbstractList
Parameters:
index - the index of the element to be removed
Returns:
the element that was removed from the list
Throws:
IndexOutOfBoundsException -

Answer 8

根据remove方法的javadoc，剩下的条目将向后移动，因此没有间隙。

Answer 9

Usually you use java collections as a dynamic storage, without defining the size.

List<String> list = new ArrayList<String>(); //dynamic

For static pre-defined collections you use java arrays .

String[] array = new String[50]; //static