Arraylist 到数组复制 Java

Question

 ArrayList<ArrayList<Integer>> L1=new ArrayList<>();
 ArrayList<Integer> L2 = new ArrayList<>();
 ArrayList<Integer> L3 = new ArrayList<>();
 ArrayList<Integer> L4 = new ArrayList<>();
 L2.add(5); L3.add(6); L4.add(9);
 L2.add(2); L3.add(1); L4.add(3);
 L2.add(1); L3.add(1); L4.add(2);
 L1.add(L2); L1.add(L3); L1.add(L4);
 L1.remove(L1.size() - 1);

Now I want to save the element removed from L1 in an array. As this list contains three elements, I want to create an array like int ar[] = new int[3]; and save this removed value into this array.

Answer 1

If you're ok with Integer[] array, then just use toArray method:

ArrayList<Integer> removed = L1.remove(L1.size() - 1);
Integer[] integers = removed.toArray(new Integer[removed.size()]);

If you want int[] array, you could use streams:

int[] ints = L1.remove(L1.size() - 1).stream().mapToInt(i -> i).toArray();

.mapToInt(i -> i) is used to unbox Integer values to int