List 转化为Array

1、循环遍历集合，给数组赋值

       ArrayList<Integer> resList = new ArrayList<>();
       Collections.addAll(resList,666,888,999);
       
       int[] res = new int[resList.size()];
       
       for (int i = 0; i < resList.size(); i++) {
          res[i] = resList.get(i);
       }

       System.out.println(Arrays.toString(res));

2、调用toArray无参方法。
  返回类型为Object[]类型的数组，无法强制转换为其他类型（如 int[] 、String[]等）。

       ArrayList<Integer> resArr = new ArrayList<>();
       Collections.addAll(resArr,666,888,999);
       
       Object[] objectArr = resList.toArray();
       
       System.out.println(Arrays.toString(objectArr));

3、调用toArray(T[] a)有参方法，转换为指定类型的数组。

       //转换为integer类型数组
       ArrayList<Integer> resArr = new ArrayList<>();
       Collections.addAll(resArr,666,888,999);
       Integer[] integersArr = resList.toArray(new Integer[resList.size()]);
       System.out.println(Arrays.toString(integersArr));
       
       //转换为character类型数组
       ArrayList<Character> cList = new ArrayList<>();
       Collections.addAll(cList,'s','d','b');
       Character[] charactersArr = cList.toArray(new Character[cList.size()]);
       System.out.println(Arrays.toString(charactersArr));
       
       //转换为String类型数组
       ArrayList<String> sList = new ArrayList<>();
       Collections.addAll(sList,"hello","world");
       String[] StringArr = sList.toArray(new String[sList.size()]);
       System.out.println(Arrays.toString(StringArr));

4、调用list的 stream().mapToInt(x->x).toArray() 方法将Integer类型的list集合转化为int类型的数组
   list<Integer> --> int[]

       ArrayList<Integer> resArr = new ArrayList<>();
       Collections.addAll(resArr,666,888,999);
       
       int[] iArr = resList.stream().mapToInt(x -> x).toArray();
       
       System.out.println(Arrays.toString(iArr));

Array 转化为 List

1、遍历数组，向集合中添加元素

       String[] arr = {"hello","world"};
       ArrayList<String> rsList = new ArrayList<>();
       for (String s : arr) {
           rsList.add(s);
      }
      System.out.println("rsList = " + rsList);

2、直接调用Arrays的 asList() 方法直接转化

       String[] arr = {"hello","world"};
       List<String> iList = Arrays.asList(arr);
       System.out.println("iList = " + iList);

3、使用Collections的addAll(Collection<? super T> c, T… elements)方法

       String[] arr = {"hello","world"};
       ArrayList<String> itList = new ArrayList<>();
       boolean b = Collections.addAll(itList, arr);
       System.out.println("itList = " + itList);