0. Utils

• 字符串数组的排序：

  Set<String> set = new TreeSet<String>();
  Collections.addAll(set, args);	// args 为 String[] 类型
  

• 数组复制：

  • system.arraycopy：必须明确自行建立一个数组对象，

  int[] arr1 = {1, 2, 3, 4, 5}; 
  int[] arr2 = new int[5];
  
  System.arraycopy(arr1, 0, arr2, 0, arr1.length);
  

  • Arrays.copyOf：可直接传回一个数组对象：

  int[] arr1 = {1, 2, 3, 4, 5}; 
  int[] arr2 = Arrays.copyOf(arr1, arr1.length);
  // 如果新数组的长度超过原数组的长度，则保留数组默认值
  int[] arr3 = Arrays.copyOf(arr1, 10);
  		// 1 2 3 4 5 0 0 0 0 0
  

1. List <–> Array

• list 实例转换为 String[]，使用 list 的实例方法：toArray

  List<String> list = new ArrayList<String>();  
  list.add("China");  
  list.add("Switzerland");  
  list.add("Italy");  
  list.add("France");  
  String [] countries = list.toArray(new String[list.size()]);  
  

• String[] => list，使用 Arrays 的静态方法 asList

  String[] countries = {"China", "Switzerland", "Italy", "France"};  
  List list = Arrays.asList(countries);  
  

2. Array => list => Set

• 其实是先将 array 转换为 list 再转换为 set：

  String [] countries = {"India", "Switzerland", "Italy"};        
  Set<String> set = new HashSet<String>(Arrays.asList(countries));  
  

3. Map => Set/List

• Map => List

  List<Value> list = new ArrayList<Value>(map.values());
  

• Map => Set

  Set<String> targetSet = new HashSet<>(sourceMap.values());  
  

4. Collections

• shuffle：置乱一个数组（list）

  public static void shuffle(List<?> list, Random rnd) {
          int size = list.size();
          if (size < SHUFFLE_THRESHOLD || list instanceof RandomAccess) {
              for (int i=size; i>1; i--)
                  swap(list, i-1, rnd.nextInt(i));
          } else {
              Object arr[] = list.toArray();
  
              // Shuffle array
              for (int i=size; i>1; i--)
                  swap(arr, i-1, rnd.nextInt(i));
  
              ListIterator it = list.listIterator();
              for (int i=0; i<arr.length; i++) {
                  it.next();
                  it.set(arr[i]);
              }
          }
      }